JPH03260816A - Constant voltage circuit - Google Patents

Abstract

PURPOSE:To save the extra electric power by discriminating whether the load is kept in a high or low load state, and switching a specific constant voltage circuit. CONSTITUTION:A constant voltage circuit 16 consisting of a Zener diode 11 and a resistance 12 is provided together with a constant voltage circuit 26 consisting of a Zener diode and a resistance 22 having the larger resistance value than the resistance 12. Furthermore the state discriminating means 30 and 31 are added to detect the currents flowing to the load together with the constant circuit switch means 32, 13, 14, 23 and 24 which use the circuits 16 and 26 as the load power supplies in the high and low load states respectively. In such a constitution, a sufficient current is supplied in the high load state while the Zener voltage is applied to the load. At the same time, the currents flowing to the load and the Zener diode are reduced in the low load state while the Zener voltage is applied to the load. Thus the extra electric power can be saved as an entire constant voltage circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a constant voltage circuit using a Zener diode.

<Prior Art> When a voltage higher than the Zener voltage is applied to a Zener diode, the voltage value hardly changes even if the current flowing through the Zener diode changes significantly. A constant voltage circuit that utilizes this characteristic to supply a stable voltage to an electronic circuit is known. Like this 1. An example of a constant voltage circuit is shown in FIG. In the figure, 201 is a Zener diode. The anode of the Zener diode 201 is connected to ground, and the cathode is connected to one end of the resistor 202. The other end of the resistor 202 is the power supply voltage vDDK.
It is connected. 205 is a load such as an electronic circuit,
It is connected between the output terminal 206 connected to the cathode of the Zener diode 201 and the ground.

In the circuit shown in FIG. 9, the Zener voltage of the Zener diode 201 is set to a constant voltage, so the voltage VOUT at the output terminal 206 is always constant. Therefore,
A constant voltage is applied to the load 205.

Here, the resistance value R of the resistor 202 is such that even when the load 205 is in a high load state and the load 205 requires a large amount of current, a sufficient current can be supplied to the load 205, and the Zener diode 201 can maintain the Zener voltage. It is set to a value that can supply the generated current. Therefore, the resistance value R
is the value obtained by dividing the potential difference across the resistor 2020 by the current value flowing through the resistor 202, that is, the difference between the power supply voltage VDD and the Zener voltage VZ, divided by the sum of the maximum value ILmaz of the load current and the minimum value IZeaim of the Zener current. will be the value.

Expressing this in the formula, R= (VDD Vz) /
(ILa+az+IZm1m).

<Problems to be Solved by the Invention> In the above circuit, when the load 205 is a microcomputer, for example, when the microcomputer enters a low load state such as a standby mode, the current flowing through the load 205 decreases. The current value flowing through the Zener diode 201 increases by a current value equal to this decreased current. Therefore, in the entire circuit shown in FIG. 9, the same current flows even when the load is in a low load state as when it is in a high load state. That is, there is a problem in that during a low load state, more power than necessary is consumed to generate the Zener voltage.

The present invention has been made in view of the above problems, and it is an object of the present invention to provide a constant voltage circuit that can save excess power.

Means for Solving the Problems> The present invention provides a first Zener diode cathode.
a first constant voltage circuit that generates a Zener voltage by connecting to a power supply through a resistor and having an anode grounded; A second constant voltage circuit that is connected to a power supply through a resistor and whose anode is grounded to generate a Zener voltage of the same voltage as the Zener voltage of the first constant voltage circuit, and a second constant voltage circuit that can be driven by being supplied with a power supply voltage. and 1. a load, a state determining means for detecting the current flowing through the load or the voltage applied to the load and determining whether the load is in a high load state or a low load state; and a state determining means for determining whether the load is in a high load state by the state determining means When it is determined that the load is in a low load state, the Zener voltage of the first constant voltage circuit is set as the power supply voltage of the load, and when the state determining means determines that the load is in a low load state, the second constant voltage is set. The present invention is characterized by comprising constant voltage circuit switching means for setting the Zener voltage of the circuit to the power supply voltage of the load.

<Operation> According to the present invention, the current flowing through the load or the voltage applied to the load is detected to determine whether the load is in a high load state or a low load state. And if the load is high,
The first constant voltage circuit is activated and applies Zener voltage to the load. Furthermore, when the load becomes low, the second constant voltage circuit is activated and applies Zener voltage to the load. In this case, since the resistance value of the second resistor of the second constant voltage circuit is larger than the resistance value of the first resistor of the first constant voltage circuit, the mA flowing to the Zener diode decreases. , - less current flows across the path.

Embodiment> A first embodiment will be described with reference to FIG. 3
5 is a microcomputer as a load, which is connected between the output terminal 36 and the ground. The microcomputer 35 handles two operating modes: a normal mode that requires a large amount of current, and a standby mode that allows only a small amount of current to flow. When the microcomputer 35 is in the normal mode, a large amount of current flows through it, which corresponds to a high load state, and when it is in the standby mode, only a small amount of current flows through the microcomputer 35, so it corresponds to a low load state.

11 is a Zener diode, and the Zener voltage is Vz
r [V ). The anode of the Zener diode 11 is connected to ground, and the cathode is connected to one end of the resistor 12. The other end of the resistor 12 is connected to the voltage VI) via the drain and source of a P-channel MO8 field effect transistor (hereinafter referred to as P-MOS) 13.
I) K connected.

The resistance value of the resistor 12 is set to such a value that sufficient current can be supplied to the microcomputer 35 in the normal mode, and the Zener diode 11 can supply a current that generates the Zener voltage VZI. Note that a first constant voltage circuit 16 is configured by the Zener diode 11 and the resistor 12. Furthermore, the connection point between the Zener diode 11 and the resistor 12 is P
- It is connected to the resistor 30 for load current detection via the source tray of MOSi2.

21 is a Zener diode □, and the Zener voltage is vzzcV) in the Zener diode 11. Furthermore, the Zener diode 21 is the Zener diode 1
It has a characteristic of generating the Zener voltage Vzr with a smaller current than that of the transistor No. 1.

Therefore, the Zener voltage VZI is applied to the Zener diode 21.
is occurring, the current flowing through the Zener diode 21 is equal to the Zener voltage VZI
The current flowing through the Zener diode 11 is smaller than the current flowing through the Zener diode 11 when the current occurs. The anode of the Zener diode 21 is connected to ground, and the cathode is connected to one end of the resistor 22. The other end of the resistor 22 is P-MO82
It is connected to the voltage boost voltage VDD through the drain and source of 3. The resistance value of the resistor 22 is set to a larger value than the resistance value of the resistor 12. Therefore, resistance 12.2
When the same voltage is applied to resistor 2, the current flowing through resistor 22 is smaller than the current flowing through resistor 12.

In addition, the resistance value of the resistor 22 can supply a current slightly larger than the sum of the current flowing from the microcomputer 35 to the standby mode microcomputer 35 and the minimum current necessary for the Zener diode 21 to generate the Zener voltage. is set to a value like Note that a second constant voltage circuit 26 is configured by the Zener diode 21 and the resistor 22.

The connection point between the Zener diode 21 and the resistor 22 is P-M
It is connected to the load current detection resistor 30 via the source and drain of O824.

The end of the resistor 30 on the constant voltage circuit side is connected to the O of the comparator 31.
The input terminal of the comparator 31 is connected to the end of the resistor 30 on the output terminal 36 side. That is, the comparator 31 detects the potential difference between both ends of the resistor 30, and outputs a voltage signal "L" if the potential difference is above a predetermined value, and outputs a voltage signal "H'" if it is smaller than the predetermined value. is set to a potential difference across the resistor 30 caused by a current slightly larger than the current flowing through the resistor 30 when the microcomputer 35 is in standby mode.

Further, the output terminal of the comparator 31 is P-MOSi2.
14 and the input terminal of the inverter 32. The output terminal of inverter 32 is P-MO823
, 24 gates. In addition, the resistance is 30. The comparator 31 constitutes a state determining means, and the inverters 32P-MOSi2, 14, 23, and 24 constitute a constant voltage circuit switching means.

Further, the above circuit includes the first. Second constant voltage circuit 16.2
6, comparator 31. The inverter 32 is provided with a main switch (not shown) that supplies or cuts off power.

In the above circuit, when the main switch is turned on (supply of power), the comparator 31. The inverter 32 is now ready for operation. Then, at the moment when the main switch is turned on, the potential difference between the source and gate of P-MOSi2.14, 23.24 is small L・, so P-MOS1
A direct current flows between the sources and drains of 3, 14, 23 and 24. The M'J, current is from the power supply to the P-MOSi
2. Path through resistance 1mP-MO814 and p-MO
823° resistance 22. The signal passes through two routes, one passing through the P-MO 824, and flows to the microcomputer 35 via the resistor 30 and the output terminal 36. When the microcomputer 35 is in the normal mode, a large amount of current flows through the microcomputer 35, and similarly, a large amount of current flows through the resistor 30. At this time, resistance 3
Since the potential difference between both ends of 00 exceeds the predetermined value of the comparator 31, the comparator 31 outputs a voltage signal "L'". This voltage signal "L"" is input to the gates of P-MOSi2 and 14, and current continues to flow between the sources and drains of P-MOSi2 and 14.
The voltage signal "L" output from the inverter 32 is input to the input terminal of the inverter 32, and the voltage signal "H'" is output from the output terminal of the inverter 32. This voltage signal "H'" is P-
Input to the gates of MO823 and 24, P-MO823
, 24, no current flows between the sources and drains. Therefore, only the first constant voltage circuit 16 continues to operate, and a constant voltage VZI equal to the Zener voltage VZI is applied to the output terminal 36. At this time, since the resistance value of the resistor 12 is set to be small, sufficient current can be supplied to the microcomputer 35.

Next, when the microcomputer 35 goes into standby mode,
The current flowing through the microcomputer 35 decreases, and the current flowing through the resistor 30 also decreases. When the potential difference across the resistor 30 becomes smaller than a predetermined value of the comparator 31, the comparator 31 outputs a voltage signal "H'". This voltage signal "H'" is input to the gate of P-MOSi 2.14, and no current flows between the source tray 7 of P-MOSi 2.14. In addition, the voltage signal “H′” is applied to the inverter 32.
The inverter 32 outputs a voltage signal "L". This voltage signal "L" is input to the gate of the P-MO 823.24 so that a current flows between the source and drain of the P-MO 823 and 24. That's it.

Therefore, only the second constant voltage circuit 26 operates, and a constant voltage VZI equal to the Zener voltage VZI is applied to the output terminal 36.
ZI is applied. Here, the resistance value of the resistor 22 is the resistance value of the resistor 1
Since the resistance value of resistor 22 is larger than that of resistor 22, the current flowing through resistor 22 is small. Further, the value of the current flowing through the Zener diode 21 is also smaller than that flowing through the Zener diode path 16.

Therefore, the current flowing through the entire circuit shown in Figure 1g1 also becomes slightly smaller.

Next, a case in which the microcomputer 35 attempts to return to the normal mode will be described. As mentioned above, the resistance value of the resistor 22 is the same as the resistance value of the resistor 22 when the microcomputer 35 is in standby mode.
The value is set so that a current slightly larger than the sum of the current flowing through the zener diode 5 and the minimum current necessary for the zener diode 21 to generate a zener voltage is set. Therefore, the current flowing through the resistor 22 to the microcomputer 35 increases slightly compared to the standby mode, and
Zener diode 21 continues to generate Zener voltage VZI. At this time, since the potential difference across the resistor 300 exceeds the predetermined value of the comparator 31, the first constant voltage circuit 16
is activated again, and the second constant voltage circuit 26 is stopped. 1st
When the constant voltage circuit 16 is activated, a large amount of current is supplied to the microcomputer 35, and the microcomputer 35 returns to the normal mode.

As described above, according to this embodiment, the Zener diode 1
1 and the resistor FL12 constitute a first constant voltage circuit 16 that can supply a large amount of current to the microcomputer 35 and generate the Zener voltage Vzx, and the Zener diode 21 and the resistor 22 control the current flowing to the Zener diode 21 and the microcomputer 35. A second constant voltage circuit 26 is configured to suppress the current flowing through the microcomputer 35 using a resistor 30 and a comparator 31. Based on the detection result, the current flowing through the microcomputer 35 is controlled by the inverter 32. The first constant voltage circuit 16 and the second constant voltage circuit 26 are switched by P-MOSi2, 14, 23.24. Therefore, when the microcomputer 35 is in the normal mode, only the first constant voltage circuit 16 operates,
While applying the Zener voltage VZI to the microcomputer 35, a sufficient current can be supplied. Also, microcontroller 3
5 is in standby mode, the second constant voltage circuit 2
6 is activated and applies Zener voltage VZI to Mico/35. Since the current flowing through the resistor 22 of the second constant voltage circuit 26 is smaller than the current flowing through the resistor 12 of the first constant voltage circuit 16, the current flowing through the Zener diode 11 is small. Therefore, it is possible to save unnecessary power for the entire circuit.

Note that the Zener voltage VZI is several [V], and the constant voltage circuit 16
．． When the current flowing through the resistor 26 is several [mA], if the resistance value of the resistor 30 is about 10 [Ω], the voltage drop caused by the resistor 30 can be slightly reduced to the extent that it hardly affects the operation of the microcomputer 35. I can do it. Furthermore, the current flowing through the load current detection circuit is on the order of several μA, which is extremely small compared to the current (several mA) saved by the circuit shown in FIG.

Next, a second embodiment will be described based on FIGS. 2 and 3. In FIG. 2, 85 represents four loads 81
, 82, 83, and 84 individually, and is connected between the output terminal 86 and the ground. The four loads 81.82, 83.84 are microcontroller 8
These are lamps and the like that are driven by the power supplied from 5.

Next, 41, 51, and 61.71 are Zener diodes, and their Zener voltages are equal and are all vz2 [v]. However, when the Zener voltage VZ2 is generated in each Zener diode, the characteristics of each Zener diode are set so that the current flowing through the Zener diode 41 is the largest and decreases in the order of Zener diodes 51, 61, and 71. The anodes of the Zener diodes 41, 51, 61.71 are connected to ground, and the candos are connected to the resistor 42.
, 52, 62, and 72, respectively.

The other ends of resistors 42.52 and 62.72 are P-MO843,
It is connected to the voltage source VDD via the drain and source of 53, 63, and 73. Each resistance value is resistance 4
The resistance value of resistance 2 is the smallest, and the resistance value is set to be greater than resistance 52.62.72. Note that the Zener diode 41 and the resistor 42 constitute a constant voltage circuit 46, and the Zener diode 51 and the resistor 52 constitute a constant voltage circuit 56. In addition, Zener diode 61
A constant voltage circuit 66 is configured by the and resistor 62, and a constant voltage circuit 76 is configured by the Zener diode 71 and the resistor 72. Also, Zener diode 41.51.6
1.71 and the resistors 42, 52, and 62.72 are connected to a resistor 90 for detecting load current through the sources and drains of the P-MOs 844, 54, 64, and 74, respectively. The end of the resistor 90 on the constant voltage circuit side is connected to the comparator 9.
1, 92, 93, and 94, and the end of the resistor 90 on the output terminal 86 side is connected to the comparator 9.
The respective ■ input terminals of 1, 92, 93, and 94 are connected. That is, comparators 91, 92, 93 .
94 detects the potential difference across the resistor 900. In Figure 3,
Comparator 91°9 for potential difference across resistor 900
2.93.94 operation is shown. In the figure, the vertical axis is resistance 90
is the potential difference [mA] between both ends of . The comparator 91 is
The potential difference between both ends of the resistor 90 is 0 (mV) or more Vx (mV)
When it is less than vs [mV], it outputs "H", and when it is more than vs [mV], it outputs L'. The comparator 92 has a voltage of 0 (m V ) or more Vz (
Outputs "H'" below vz(mV), and outputs "H'" below vz(mV).
Outputs L. Comparator 93 is 0 (mV) or more V
When it is less than a [mV], it outputs "H", and when it is more than Vx [mV], it outputs "L".

The comparator 94 is O [mV) or more Va (m V 3
When it is less than v4, it outputs “H”, and when it is more than v4 [m73, it outputs “L”. However, the potential difference across the resistor 900 is the microcontroller 8
When the current flowing through 5 is maximum, it is set to be less than Vi (mV).

The output terminals of comparators 91, 92.93 shown in FIG. 2 are connected to the input terminals of inverters 95, 96.97, respectively. The output terminals of the comparator 94 and inverters 95, 96.97 are connected to a NAND circuit 450.
Connected to the input terminal.

mater, co/parator 93, 94 and inverter 95.
The output terminal of 96 is connected to the input terminal of NAND circuit 550. The output terminals of comparators 92, 93, 94 and inverter 95 are connected to the NAND circuit 650 input terminal. Furthermore, the output terminals of the comparators 91.92, 93.94 are connected to the NAND circuit 750 input terminal. The output terminal of the NAND circuit 45 is P-MO843,
44 gates. The output terminal of the NAND circuit 55 is connected to the gates of the P-MOs 853 and 54.

The output terminal of the NAND circuit 65 is connected to the gate of the P-MO 863.64. The output terminal of the NAND circuit 75 is P
- Connected to the gates of MO873 and 74. In addition, the resistance is 90. By comparator 91.92°93.94,
The state determining means is configured, and the inverter 95.96.97
, NAND circuit 45°55.65,75. P-MO8
43, 44, 53°54.63.64.73.74 constitute a constant voltage circuit switching means.

Further, the above circuit includes constant voltage circuits 46 and 56.

66, 76, comparator 91, 92.93,9
4° inverter 95, 96, 97. A main switch (not shown) for supplying or cutting off power is provided to the NAND circuit 45°55.65.75.

In the above circuit, when the main switch is turned on (supply of power), each comparator 91, 92, 9
3.94. Inverters 95, 96.97 and NAND circuits 45, 55, 65.75 become operational. Then, the moment the main switch was turned on, P-MO84
P-M
A current flows between the source and drain of the OS. the result,
The current is from the power supply to P-MO843, 53, 63.

73, resistance 42, 52, 62, 72, P-MO
844, 54, 64, 74, and flows to the microcomputer 85 via the resistor 90 and the output terminal 86'.

When the loads 81, 82, 83.84 are all driven and the current flowing to the microcomputer 85 is the maximum, that is, the current flowing to the resistor 90 is the maximum, and the potential difference between both ends of the resistor 90 is Va (mV3 or more V+[mV)] Consider the case where it is less than At this time, the comparators 91, 92, and 93 output the voltage signal "L", and the comparator 94 outputs the voltage signal "H'".
Then, a voltage signal "H" is input to all the input terminals of the NAND circuit 45. Therefore, NAN
The D circuit 45 outputs a voltage signal "L", and the voltage signal "L" is input to the gates of the P-MOs 843 and 44.

Therefore, current continues to flow between the source trays 7 of the P-MOs 843 and 44. Also, NAND circuits 55, 65.
A voltage signal "L" is inputted to 75 from one of the input terminals.Therefore, the NAND circuits 55, 65.75 output a voltage signal "H'", and the voltage signal "H" is applied to the P-MO8.
It is input to 5 gates: 53, 54, 63°, 64, 73, 74. Therefore, P-MO853, 54, 63, 64
, 73°74, no current flows between the source and drain. As a result, only the constant voltage circuit 46 continues to operate, a constant voltage VZ2 is applied to the output terminal 86, and a large amount of current determined by the resistor 42 is applied to the microcomputer 8.
5.

Next, consider the case where one of the loads 81, 82, 83, and 84 stops, the current flowing to the microcomputer 85 decreases, and the potential difference between both ends of the resistor 90 becomes V2 (mV) or more and less than 3 [mV]. think. At this time, comparators 91 and 92
outputs a voltage signal "L", and comparators 93 and 94 output a voltage signal mH'. Therefore, the NAND circuit 55
outputs a voltage signal "L'", and current flows between the source trays 7 of the P-MOs 853 and 54. and,
The NAND circuits 45, 65, 75 output the voltage signal "Hl, and current flows between the sources and drains of the P-MOs 843, 44, 63, 64°73.74. As a result, only the constant voltage circuit 56 is activated, and a constant voltage VZ2 is applied to the output terminal 86.

Here, since the resistance value of the resistor 52 is set larger than the resistance value of the resistor 42, the current flowing through the resistor 52 is small. In addition, the current value flowing through the Zener diode 51 is also
The current is set to be smaller than the current flowing through the Zener diode 41. Therefore, the constant voltage circuit 56 includes the constant voltage circuit 4
Only less current than 6 flows.

Next, two of the loads 81.82 and 83.84 stop, and the current flowing to the microcomputer 85 becomes even smaller, tx < tx
If the potential difference between both ends of the resistor 90 is Vl (mV: ) or more, Vz
(mV, consider the case where it is less than 1. In this case, the comparator 91 outputs the voltage signal "L" 2, and the comparators 92, 93, 94 output the voltage signal "H". Therefore, the NAND circuit 65 outputs the voltage signal "L". Outputs the signal "L"" and P-
Current flows between the source and drain of MO863 and MO864. Then, the NAND circuits 45, 55, 75 output a voltage signal "H", and the P-MOs 843, 44, 53
.

No current flows between the sources and drains of 54, 73, and 74. As a result, only the constant voltage circuit 66 operates, and a constant voltage VZ2 is applied to the output terminal 86. Here, since the resistance value of the resistor 62 is set larger than the resistance values of the resistors 42 and 52, the current flowing through the resistor 62 is small. Furthermore, the value of the current flowing through the Zener diode 61 is also set to be smaller than the current flowing through the Zener diode 41.51. Therefore, less current flows through the constant voltage circuit 66 than in the constant voltage circuit 46.56.

Next, only one of the loads 81, 82, 83.84 is driven, the current flowing to the microcomputer 85 is minimized, and the resistor 9
Consider a case where the potential difference between both ends of 0 is 0 [mV] or more and less than vt (mV). At this time, comparator 91.92
, 93 and 94 all output a voltage signal "H". As a result, the NAND circuit 75 outputs a voltage signal "L",
Current begins to flow between the sources and drains of the P-MOs 873 and 74. And NAND circuits 45, 55
.

65 outputs a voltage signal "H'", and P-MO843°4
4. No current flows between the source and drain of 53, 54, and 63.64. Therefore, only the constant voltage circuit 76 operates, and a constant voltage VZ2 is applied to the output terminal 86. Here, since the resistance value of the resistor 72 is set larger than the resistance values of the resistors 42, 52, and 62, the current flowing through the resistor 72 is small.

Furthermore, the value of the current flowing through the Zener diode 71 is also set to be smaller than the current flowing through the Zener diode 41, 51, 61. Therefore, less current flows through the constant voltage circuit 76 than in the constant voltage circuits 46, 56, 66. Therefore, the current flowing through the entire circuit of FIG. 2 is also reduced.

As described above, according to this embodiment, the Zener die 1 odes 41, 51, 4F4, 71 and the resistor 42
52.62.72 constitute the constant voltage circuits 46, 56.66.76, so that the current flowing through each constant voltage circuit is maximized in the constant voltage circuit 46, and the constant voltage circuits 56, 66. The current flowing through the microcomputer 85 is detected by the resistor 90 and the comparators 91, 92, 93, 94 in the order of 76, and based on the detection results, the inverters 95, 96, 97, . NAND circuit 45
,55,65°75,P-MO843,44,53,
54, 63° 64.73.74 to switch the constant voltage circuit operated. Therefore, microcontroller 8
When all of the loads 81, 82, 83, and 84 connected to the microcomputer 85 and the load are being driven, only the constant voltage circuit 46 operates, and can supply the microcomputer 85 and the load with the sufficient voltage x[a. In addition, a load 81 connected to the microcomputer 85,
When three loads among 82, 83, and 84 are being driven, only the constant voltage circuit 56 is activated, and it is possible to supply the necessary current to the microcomputer 85 and the loads being driven, and all of the loads are being driven. Since the current flowing to the load is smaller than when the zener diode is in use, and the current flowing to the Zener diode is also smaller, excess power can be saved for the entire circuit. Furthermore, when two of the loads 81, 82, 83° 84 connected to the microcomputer 85 are being driven, only the constant voltage circuit 66 is activated, supplying sufficient current to the microcomputer 85 and the loads being driven. In addition, the current flowing to the load is even lower than when three or more loads are being driven, and the current flowing to the Zener diode is also reduced, so excess power is saved in the entire circuit. be able to. In addition, a load 81.8 connected to the microcomputer 85
When only one of the loads 2, 83, and 84 is being driven, only the constant voltage circuit 76 is activated, and can supply sufficient current to the microcomputer 85 and the load being driven. Compared to when the load is driven, both the current flowing through the load and the current flowing through the Zener diode are minimized, resulting in the advantage that excess power can be saved for the entire circuit. That is, according to this embodiment, the current flowing through the microcomputer 85 is 4.
Even when changing between stages, the Zener voltage VZ2 is always applied to the microcomputer 85, and the effect is that the necessary current can be supplied and excess power can be further saved.

However, when the Zener voltage VZ2 is several [V] and the current flowing through each constant voltage circuit is several [mA], if the resistance value of the resistor 90 is about 1O [Ω], it will hardly affect the operation of the microcomputer 85. It is possible to reduce the voltage drop caused by the resistor 90 to such an extent that it does not cause a voltage drop. Furthermore, the current flowing through the load current detection circuit is on the order of several [μA], which is extremely small compared to the current (several [mA]) saved by the circuit shown in FIG.

Next, a third example will be described based on the tig4 diagram. 125 is a microcomputer as a load, and output terminal 1
26 and ground. Microcomputer 12
5 has two operating modes, a normal mode and a standby mode, similar to the first embodiment described above.

101 is a Zener diode, and the Zener voltage is V
za [V]. The anode of Zener diode 101 is connected to ground, and the cathode is connected to one end of resistor 102 and resistor 112. The other end of the resistor 102 is connected to the power supply voltage VDDK, and the resistor 112
The other end is connected to the power supply voltage VDD via the drain and source of the P-MO8113. The resistance value of the resistor 102 is such that a current that is slightly larger than the sum of the current flowing to the microcomputer 125 when the microcomputer 125 is in standby mode and the minimum current required for the Zener diode 101 to generate Zener' voltage is supplied. It is set up so that it can be done. Also, the resistance value of the resistor 112 is such that the resistance value of the resistor 112 is 10
When combined in parallel with 2, microcontroller 12 in normal mode
The value is set so that a sufficient current can be supplied to the terminal. The resistors 102 and 112 are connected in parallel to form a combined resistor and the Zener diode 101 to form a first constant voltage circuit. Also, by the resistor 102 and Zener diode 101! Two constant voltage circuits are constructed.

A connection point between the Zener diode 101 and the resistors 102 and 112 is connected to a resistor 120 for detecting load current.

The end of the resistor 120 on the constant voltage circuit side is connected to the comparator 121.
is connected to the ○input terminal of the resistor 120, and the output terminal 126 of the resistor 120
The side end is connected to the (2) input terminal of the comparator 121. The comparator 121 determines the potential difference between both ends of the resistor 120, and if the potential difference is greater than a predetermined value, a voltage signal "L'" is output.
is output, and if it is smaller than a predetermined value, it outputs a voltage signal "H". Also, this predetermined value is the potential difference across the resistor 1200 caused by a current slightly larger than the current flowing through the resistor 120 when the microcomputer 125 is in standby mode. The output section of the comparator 121 is set to P-
1i at the gate of MO8113! It is continued. In addition, the resistance 120. The comparator 121 constitutes a state determining means, and the P-MO 8113 constitutes a constant voltage circuit switching means.

The above circuit also includes a constant voltage circuit and a comparator 12.
1 is provided with a main switch (not shown) for supplying or cutting off power.

When the main switch is turned on (supply of power) in the circuit described above, the comparator 121 becomes operable. Then, the moment the main switch is turned on, P
Since the potential difference between the source and gate of -MO8113 is small, a current flows between the source and drain of P-MO8113. As a result, the current passes through two paths: one from the power supply through the resistor 102 and the other through the P-MO 8113 and the resistor 112, and then through the resistor 120. The signal flows to the microcomputer 125 via the output terminal 126. When the microcomputer 125 is in the normal mode, a large amount of current flows through the microcomputer 125, and similarly, a large amount of current flows through the resistor 120. At this time, the potential difference between both ends of the resistor 120 is determined by the comparator 121
exceeds the predetermined value, the comparator 121 outputs a voltage signal "L'". This voltage signal “L” is P-M
Power is applied to the gate of the OS, and current continues to flow between the source and drain of the P-MO8113. Therefore, the current passes through two paths, the resistor 102 and the resistor 112, and a sufficient current can be supplied to the microcomputer 125 in the normal mode. At this time, the Zener voltage VZ3 is applied to the output terminal 126.
A constant voltage equal to is applied.

Next, when the microcomputer 125 enters standby mode, the current flowing through the microcomputer 125 becomes small and the resistor 125
The current flowing through will also decrease. Then, when the potential difference across the resistor 120 becomes smaller than a predetermined value of the comparator 121, the comparator 121 outputs a voltage signal "H'". This voltage signal "H" is input to the gate of P-MO8113, and no current flows between the source and drain of P-MO8113. Therefore, the microcomputer 125 has a resistor of 10
Current is supplied only from resistor 2, and no current is supplied from resistor 112. Therefore, less current flows through the entire circuit, and no extra power is consumed.

Next, a case will be described in which the microcomputer 125 attempts to return to the normal mode. As mentioned above, the resistance value of the resistor 102 is determined by the current flowing through the microcomputer 125 when the microcomputer 125 is in standby mode and the Zener diode 10.
1 is set so that a current slightly larger than the sum of the minimum current necessary to generate a Zener voltage can be supplied. Therefore, the current flowing to the microcomputer 125 through the resistor 102 increases slightly compared to the standby mode, and the Zener diode 101 continues to generate the Zener voltage Vzs. At this time, since the potential difference across the resistor 120 exceeds the predetermined value of the comparator 1210, f/LfL also flows through the resistor 112.

As a result, a large amount of current is supplied to the microcomputer 125,
Myco/125 is back in normal mode.

As described above, according to this embodiment, the Zener diode 1
A constant voltage circuit is constructed by connecting two resistors 1022 and 112 in parallel to 01, and resistor 120 and comparator 1
21 detects the current flowing to the microcomputer 125, and based on the detection result, the current is made to flow through the resistor 113 or is cut off. Therefore, when the microcomputer 125 is in the normal mode, current flows through the resistor 102 and the resistor 112, and a sufficient current can be supplied while applying the Zener voltage Vzs to the microcomputer 125. Furthermore, when the microcomputer 125 is in standby mode, current no longer flows through the resistor 112 and current only flows through the resistor 102.
less current flows through the As a result, excess power can be saved for the entire circuit. Furthermore, in this embodiment, only one Zener diode and P-MOS are used, and no inverter is used, so the overall circuit configuration can be made simpler than in the first embodiment. You can get the effect that you can.

Note that when the Zener voltage VZa is several [V] and the current flowing through the constant voltage circuit is several (mA), the resistance value of the resistor 120 is set to 1.
If it is about 0 [Ω], the voltage drop caused by the resistor 120 can be reduced to such an extent that it hardly affects the operation of the microcomputer 125. Further, the current flowing through the load current detection circuit is on the order of several μA, which is extremely small compared to the current (several mA) saved by the circuit shown in FIG.

Next, a 40th embodiment will be described based on FIGS. 5 and 6. In Figure 5, 175 represents four loads 1
This is a microcomputer for driving 71, 172, 173, and 174 individually, and is connected between the output terminal 176 and the ground. 4 loads 171, 172, 173
, 174 are lamps and the like driven by power supplied from the microcomputer 175.

131 is a Zener diode, and the Zener voltage is V
za (V). The anode of the Zener diode 131 is connected to ground, and the cathode is connected to the resistor 132,
142, 152, and 162. The other end of the resistor 132 is connected to the power supply voltage VDD. The other ends of the resistors 142, 152, and 162 are connected to the power supply voltage vDD via the tray/source of the P-MO 8143, 153, and 163, respectively. In addition,
A first constant voltage circuit is constituted by the Zener diode 131 and the resistor 132, 142, 152, and 162 connected in parallel. Also, resistance 132°1
42.152 are connected in parallel and synthesized, and the Zener diode 131 constitutes a second constant voltage circuit. Further, the resistors 132 and 142 are connected in parallel to form a combined resistor and the Zener diode 131 to constitute a third constant voltage circuit. Further, the resistor 132 and the Zener diode 131 constitute a fourth constant voltage circuit.

Zener diode 131 and resistor 132, 142°1
The connection point of 52.162 is the resistor 170 for load current detection.
It is connected to the. The end of the resistor 170 on the constant voltage circuit side is connected to the ■input terminal of each of the comparators 144, 154, and 164, and the end of the output terminal 176@ of the resistor 170 is connected to the ■input terminal of each comparator. .

Comparators 144, 154, and 164 detect the potential difference across resistor 170. FIG. M6 shows the operation of the comparators 144, 154, 164 with respect to the potential difference across the resistor 170. In the figure, the vertical axis represents the potential difference [mV] across the resistor 1700.

The comparator 144 indicates that the potential difference across the resistor 1700 is 0.
[mV] or more Vs [mV3 or less, outputs "Hl", v5
Outputs “L” at [mv] or more. Comparator 154
outputs "Hl" when it is 0 [mV] or more and less than Vg [mV], and outputs "L" when it is more than v6 [mv]. The comparator 164 outputs "Hl" when the voltage is greater than or equal to O[:mV] and less than VrCmVE, and outputs "L" when it is greater than or equal to V7 [mV].

The output terminal of comparator 144 is connected to the gate of P-MO8143. The output terminal of the comparator 154 is P
- Connected to the gate of MO8153.

The output terminal of the comparator 164 is connected to the gate of the P-MO 5163. Tao, resistance 170. Comparator 1
44, 154, 164 constitute a state determining means, and the P-MO8143, 153°163 constitute a constant voltage circuit switching means.

The above circuit also includes a constant voltage circuit and a comparator 144.
154 and 164 are provided with main switches (not shown) for supplying or cutting off power.

In the above circuit, when the main switch is turned on (power is supplied), the comparator W.

154.164 becomes operational. Then, the moment the main switch was turned on, P-MO8143,
Since the potential difference between the source and gate of P-MO8143 and 153°163 is small, a current flows between the source and drain of P-MO8143 and 153°163. As a result, the current flows from the source to P
-MOS 143, 153° 163, resistance 132, 14
2,152,162 and resistor 170. Output terminal 17
6 to the microcomputer 175. And load 17
1,172°173.174 are all driven and the current flowing to the microcomputer 175 is maximum, that is, the resistor 1
Consider a case where the current flowing through the resistor 70 is maximum and the potential difference across the resistor 1700 is Vy[:mV] or more.

At this time, all of the comparators 144, 154, and 164 output the voltage signal "L'", and the P-MO8143,1
It is input to gates 53 and 163.

Therefore, current flows between the sources and drains of SP-rMO8143, 153, and 163. As a result, the resistance 132,
Current flows through all of 142, 152, and 162.

Next, one of the loads 171, 172, 173, and 174 stops, and the current flowing to the microcomputer 175 decreases.
The potential difference between both ends of the resistor 1700 is V6 [m V ) or more V7
Consider the case where the voltage is less than [mV].

At this time, the comparators 144 and 154 output the voltage signal “L”.
", and this voltage signal "L" is P-MO8143,
153 gates. Therefore, SP-MO81
Current continues to flow between the sources and drains of 43 and 153. Further, the comparator 164 outputs a voltage signal "H'", and this voltage signal "H" is input to the gate of the P-MO 8163. Therefore, a current flows between the source and drain of the P-MO81630 and becomes tx. As a result, current flows through resistors 132, 142, and 152, but resistor 16
No current flows through 2, and the load 171172.173
, 174 are all driven, the current flowing through the entire circuit becomes smaller.

Next, two of the loads 171, 172, 173, and 174 stop, the current flowing to the microcomputer 175 further decreases, and the voltage across the resistor 1700 decreases to Vs [mV: 1 or more V
Consider the case where it is less than 6 (:mv). At this time, the comparator 144 outputs the voltage signal "Ll", and this voltage signal JI L I is input to the gate of the P-MO8143. Therefore, current continues to flow between the source and drain of P-MO8143. In addition, the comparator 154,
164 outputs a voltage signal "H'", and this voltage signal "H"
''' is input to P-MO8153, 163.P-
No current flows between the source and drain of MO8153 and MO8163. As a result, current flows through the resistors 132 and 142, but very little current flows through the resistors 152 and 162.
The current flowing through the entire circuit is smaller than when three or more of the loads 171, 172, 173, and 174 are being driven.

Next, only one of the loads 171, 172, 173, and 174 is driven, the current flowing to the microcomputer 175 is minimized, and the voltage across the resistor 1700 is 0 [mV] or more VB
Consider the case where it is less than [m V ].

At this time, all comparators 144, 154°16
4 outputs a voltage signal "H"", and this voltage signal "H"G
It is input to tP-MO8143, 153, '163. Therefore, no current flows between the source and drain of P-MOS I4s::1゛5゛3゜163.

As a result, current flows only through resistor 132, and resistor 142.
152 and 162, so the load 17
1. The current flowing through the entire circuit is smaller than when two or more of 172, 173, and 174 are driven.

As described above, according to this embodiment, the Zener diode 1
31 and four resistors 132, 142, 152° 162 are connected in parallel to form a constant voltage circuit, the resistor 170 and comparators 144, 154° 164 detect the current flowing to the microcomputer 175, and based on the detection result, The number of current paths supplied from the electric wire to the microcomputer 175 was changed. Therefore, while always applying the Zener voltage Vz41 to the microcomputer 175, the number of current paths supplied to the loads is determined based on the number of loads being driven among the loads 171, 172, 173, and 174 connected to the microcomputer 175. changes. As a result, the current flowing through the microcomputer 175 and the Zener diode 131 is reduced, allowing the necessary current to be supplied to the driven load and the microcomputer 175, and the effect that excess power can be further saved in the entire circuit is achieved.

Note that when the Zener voltage VZ4 is several [V] and the current flowing through the constant voltage circuit is several (mA), the resistance value of the resistor 170 is set to 1.
If it is set to about 0 [Ω], the voltage drop caused by the resistor 170 can be reduced to such an extent that the operation of the microcomputer 175 is hardly affected. Furthermore, the current flowing through the load current detection circuit is on the order of several μA, which is extremely small compared to the current (a (mA)) saved by the circuit of FIG.

A fifth embodiment will be described based on FIGS. 7 and 8. In this embodiment, the state determining means that detects the current flowing through the load in the third embodiment is changed to state determining means that detects fluctuations in the voltage applied to the load. In FIG. 7, the constant voltage circuit, load, and constant voltage circuit switching means are the same as in the third embodiment, so the same numbers as in FIG. 4 are given, and detailed explanations are omitted.

203 and 202 are resistors for dividing the Zener voltage Vzs [V) of the Zener diode 101 at a constant ratio, and the divided voltage is used as the reference voltage VRjCF.
Let it be t. Then, the # reference voltage VREFI is input to the ■ input terminal of the comparator 201 .

Furthermore, the Zener voltage Vza is
Input to input terminal. The output terminal OCI of comparator 201 is connected to the R terminal of R/S 71J block flop 220. Next, 213 and 212 are resistors for dividing the Zener voltage VZa of the Zener diode 101 at a constant ratio, and the divided voltage is set as the reference voltage VREF'2. The resistance values of the resistors 212 and 213 are set so that the reference voltage VRεF2 is higher than the reference voltage VREFI. Reference voltage vR
zrz is input to the ○ input terminal of the comparator 211. Furthermore, the Zener voltage VZ3 is
■Input to the input terminal. The output terminal OC2 of the comparator 211 is connected to the C@ terminal of the R/S flip 70 tube 220. Also, R/87 lip 70 tube 220 S
The terminal is connected to the circuit starting switch (not shown) and Q
glA is connected to the gate of P-MO8113. In addition, comparators 201, 211° resistors 202, 203
, 212, 213. The R/87 lip-flop 220 constitutes state determining means. The connection point between the Zener diode 101 and the resistor 102.112 is connected to the output terminal 126.

FIG. 8(a) shows the Zener voltage Vzs and the reference voltage V■Fl.
.

VRIiF! and the comparator voltage Vc, and the eighth
Figures (b), (c) * (d) show voltage signals output from the output terminals OCI, OC2 and Q terminal, respectively. In FIGS. 8(a) to 8(d), the horizontal axis represents time common to each figure. In FIG. 8(a), the comparator voltage Vc is the comparator voltage of the comparators 201 and 211, and the reference voltage VREFz and the reference voltage VR
EFI.

The voltage is set to be between. That is, the comparator voltage Vc and the reference voltage VIIIFI.

VRzvx (D relationship is Viu:rz > Vc >
Becomes VRICFI.

When the start switch is turned on (tl in the figure), the comparators 201 and 211 and the R/S flip-flop 22
0 becomes operational, and current flows through the resistor 102 from the power supply voltage VDD. A part of the current flowing through the resistor 102 flows to ground through the Zener diode 101, and the rest flows to the output terminal and is supplied to the microcomputer 125. At this time, as mentioned above, VREF2) Vc > VREF
There is a review that will set up I and Naruyouni. Therefore, voltage signals "L'" are output from the output terminals OCI and OC2 of the comparators 201 and 211 to the R terminal and C terminal of the R/S flip 70 tube 220, respectively. Furthermore, since the initial signal is input to the SS terminal of the R/S flip-flop 220, a voltage signal "H'" is output from the Q terminal.

Thereafter, at a time point indicated by t2, the microcomputer 125 attempts to enter the normal mode. At this time, the current flowing through the resistor 102 is constant, and the current flowing through the microcomputer 125 increases, so the current flowing through the Zener diode 101 decreases. Therefore, Zener voltage VZ3 drops. Since the comparator voltage Vc is always lower than the Zener voltage VZ3 by a fixed voltage, the comparator voltage Vc also decreases as the Zener voltage Vza decreases. Further, since the reference voltage VREFI and the reference voltage REF2 each divide the Zener voltage Vzs at a constant rate, they drop as the Zener voltage Vzs drops. At this time, compared to the voltage drop of the comparator voltage VC, the reference voltage VR1
+F1 *Since the voltage drop of the reference voltage VRgp2 is small, the comparator voltage Vc and the reference voltage VilffiFl
The voltage is reversed, and VREF2 > VBIFI > VC. As a result, the output terminal OCI of the comparator 201
A voltage signal “H′” is output from the R/S flip-flop 220 and input to the R terminal of the R/S flip-flop 220. Further, a voltage signal “L” is outputted from the output terminal OC2 of the comparator 211 and inputted to the C terminal of the R/S7 lip-flop 220. Therefore, the voltage signal "L" is output from the Q terminal of the R/S flip-flop 220.
''' is input to P-MO8113, and P-MO8l13
Current flows through the seven source trays and the resistor 112. Therefore, at the time indicated at t3, the current passes through resistors 102 and 112 to provide sufficient current to microcomputer 125 in normal mode. Then, as the current supplied to the microcomputer 125 further increases, the current flowing through the Zener diode 101 begins to increase. Therefore, the Zener voltage Vza increases, and at the same time, the comparator voltage VC
, reference voltage VRKFI, and reference voltage VREF2 also rise. At this time, since the rising voltages of the reference voltage VREFI and reference voltage VREF2 are smaller than the rising voltage of the comparison voltage VC, the comparison voltage VC and the reference voltage V
The voltage on REFI is reversed again and VREF2) Vc
) ViF, rx and fx le. Since then, Conhaleley
A voltage signal “L” is output from the output terminal OCI of the comparator 201 and inputted to the R terminal of the R/S flip-flop 220. Further, a voltage signal “L” is output from the output terminal OC2 of the comparator 211. , is input to the C terminal of the R/8 flip 70 tube 220. Therefore, the R/571
The voltage signal “L” is output from the Qjlt child of the J-tub flop 220.
''' continues to be output.

Thereafter, at a time point indicated by t4 in the figure, each voltage slightly returns to the voltage at t2, and thereafter maintains that value. Therefore, resistance 1
Current continues to flow to both 02 and 112, and the microcomputer 125
Continue to supply sufficient tl current to.

Next, at a time point indicated by ts, the microcomputer 125 attempts to enter standby mode. At this time, the current flowing through the resistor 102 was constant and was flowing through the microcomputer 125 [fi
decreases, so the current flowing through the Zener diode 101 increases. Therefore, Zener voltage Vzs increases. As the Zener voltage Vza increases, the comparator voltage Vc, reference voltage VRF:Fl, and reference voltage VR
EF2 also increases. At this time, compared to the rising voltage of the comparator voltage VC, the reference voltage vagyt and the reference voltage VR
Since the rising voltage of E F2 is small, the voltages of the comparator voltage Vc and the reference voltage VREF2 are reversed (Vc).
Vixrz > Vizrx. As a result, a voltage signal "L" is output from the output terminal OCI of the comparator 201, and R11i of the R/S flip-flop 220
input to the child. Further, a voltage signal "H'" is output from the output terminal OC2 of the comparator 211, and R, /S
It is input to the C terminal of the 7-lip flop 220. Therefore, Qj) If! of R/S7 lip-flop 220. A voltage signal "H'" is output from the child.

This voltage signal 'H''' is input to P-MO8113,
No current flows between the source and drain of P-MO8113. Therefore, at the time indicated by ts, '(frt passes only through the resistor 102, and the microcomputer 1 in standby mode
25, only the necessary current is supplied.

Then, as t/Lft supplied to the microcomputer 125 further decreases, the current flowing through the Zener diode 101 begins to decrease. Therefore, the Zener voltage Vza
falls, and at the same time, the comparator voltage Vc, reference voltage VRICFI, and reference voltage VRKF2 also fall.

At this time, the reference voltage VREFI and the reference voltage VRKF! are compared with the voltage drop of the comparator voltage vc. The voltage drop is .

Because the comparator voltage VC and the reference voltage VRI are small,
The voltage on FII is reversed again and Vnzvz >Vc >V
It becomes nzvl. As a result, the voltage signal "L" is output from the output terminal OC1 of the comparator 201, and the R/S7
It is input to the printer flop 2200R terminal.

Further, a voltage signal “L” is output from the output terminal OC2 of the comparator 211, and the R/S flip-flop 220
It is input to the Cgs child of . Therefore, the voltage signal "H" continues to be output from the 1;Ill terminal of the R/S flip-flop 220. Thereafter, each voltage returns to the value at ts at a time point indicated by t7 underground, and thereafter maintains that value.

Therefore, current flows through the resistor 102 and no current flows through the resistor 112, so that only the necessary current continues to be supplied to the microcomputer 125.

As mentioned above, according to the actual current side, the Zener voltage Vza
Comparators 201 and 211 compare the divided voltage and the comparator voltage Vc, and the microcomputer 125
Detecting the switching of the operation mode, the R/S flip-flop 220 caused current to flow through the resistor 112 or cut it off. Therefore, when the microcomputer 125 is in the normal mode, tK flows through the resistor 102 and the resistor 112, so that a sufficient current can be supplied while applying the Zener voltage VZ3 to the microcomputer 125. Also, microcomputer 125
When is in standby mode, no current flows through the resistor 112 and only through the resistor 102, so the Zener voltage Vzs is applied to the microcomputer 125, and the current flowing through the microcomputer 125 and the Zener diode 10
The current flowing through 1 is reduced. As a result, it is possible to save unnecessary power for the entire circuit.

<Effects of the Invention> According to the present invention, the first constant voltage circuit includes a Zener diode and a first resistor, and the second resistor includes a Zener diode and a second resistor having a higher resistance value than the first resistor. a constant voltage circuit; a state determining means for detecting a current flowing through the load or a voltage applied to the load; when the load is in a high load state, the first constant voltage circuit is used as a power source for the load; and when the load is in a low load state, the first constant voltage circuit is used as a power source for the load; The present invention is configured to include constant voltage circuit switching means that uses the second constant voltage circuit as a power source for the load.

Therefore, when the load is in a high load state, sufficient current can be supplied to the load while applying the Zener voltage to the load. Furthermore, when the load is in a low load state, the current flowing through the load and the current flowing through the Zener diode decreases while applying the Zener voltage to the load. As a result, it is possible to save unnecessary power for the entire circuit.

[Brief explanation of drawings]

1 is a circuit diagram showing the first embodiment, FIG. 2 is a circuit diagram showing the second embodiment, FIG. 3 is an explanatory diagram of the operation of the comparator of the second embodiment, and FIG. A circuit diagram showing an example of
FIG. 5 is a circuit diagram showing the fourth embodiment, FIG. 6 is an explanatory diagram of the operation of the comparator of the fourth embodiment, FIG. 7 is a circuit diagram showing the fifth embodiment, and FIG. 8 is a circuit diagram showing the fifth embodiment. FIG. 9 is an explanatory diagram of voltages at various parts of the embodiment No. 5, and FIG. 9 is a circuit diagram showing a conventional example. 11.21... Zener diode, 12.22...
・Resistance, 30...Resistance, 31...Comparator, 3
5... Microcomputer, 32... Inverter, 13, 14
゜23.24...P-MO80 Fig. 111.12...Soener diode 12,
22...1 degree 17'L Blade...Low (L 31...Computer L-ta 32 °°° 1> Header 13, 14.23.24., P-MO5 Fig. 3 Figure 4

Claims (1)

[Claims] A first constant voltage circuit that generates a Zener voltage by connecting the cathode of the first Zener diode to a power supply via a first resistor and grounding the anode; A cathode is connected to a power source via a second resistor having a resistance value greater than the first resistor, and an anode is grounded to generate a Zener voltage of the same voltage as the Zener voltage of the first constant voltage circuit. A second constant voltage circuit, a load that can be driven by supplying power supply voltage, and detects the current flowing through the load or the voltage applied to the load, and determines whether the load is in a high load state or a low load state. a state discriminating means for discriminating; and when the state discriminating means determines that the load is in a high load state, the Zener voltage of the first constant voltage circuit is set as the power supply voltage of the load; 1. A constant voltage circuit, comprising: constant voltage circuit switching means for setting the Zener voltage of the second constant voltage circuit to the power supply voltage of the load when it is determined that the load is in a low load state.