JPH04196368A - Led drive circuit - Google Patents

Abstract

PURPOSE:To prevent an LED from decreasing in output even if it increases in temperature by a method wherein a circuit structure which compensates the change of the LED in output with temperature change is provided. CONSTITUTION:An NPN type transistor 2 is connected at its collector to a power supply through a protective resistor 10 and at its emitter to the anode of an LED 1. The cathode of the LED is connected to a resistor 3, whose one end is connected to the cathode of the LED 1 and other end grounded, A first and a second potential dividing resistors 4 and 5, whose ends are connected to the anode and the cathode of the LED 1, respectively and other ends are connected to each other, and an operational amplifier 6, whose non-reverse input terminal, reverse input terminal, and output terminal are connected to a low voltage power supply (voltage Vconst), a joint between the resistors 4 and 5, and the base of the transistor 2, are provided. That is, an LED is provided with a circuit which compensates the change of the LED in output with temperature change. By this setup, the LED 1 can thus be prevented from decreasing in optical output even if it increases in temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an LED drive circuit used in a photoelectric rotary encoder or the like.

[Conventional technology]

Figure 5 shows a conventional LED frame rotation circuit used in photoelectric rotary encoders, etc.
This is a circuit diagram of

This LED drive circuit consists of an NPN transistor 2 whose collector is connected to the cathode of the LED 1, and one end of which is connected to the cathode of the LED 1.
A resistor 3 is connected to the emitter of the N-type transistor 2 and the other end is grounded, and the non-inverting input terminal, inverting input terminal, and output terminal are respectively constant voltage sources, and one end of the resistor 3 on the non-grounded side is NPN. The operational amplifier 6 is connected to the base of a type transistor 2, and the anode of the LED 1 is connected to a power source.

Since one end of the resistor 3 is connected to the inverting input terminal of the operational amplifier 6, the voltage across the resistor 3 is equal to the voltage V of the constant voltage power supply.
confft. Here, if the resistance value of the resistor 3 is R1 and the current flowing through the resistor 3 is I, then the current I is always a constant value, and the LED 1 connected to the collector of the transistor 2 has a constant current I (base current can be ignored).

[Invention or problem to be solved]

However, even if a constant current flows through LED 1, the LED
When the temperature of the LED 1 increases by ΔT [deg], the light output P of the LED 1 decreases. The optical output P is expressed as P = l 1 (1+2ΔT) [mW] - (
2) can be expressed as the approximate equation (2). Here, K1: LED luminous efficiency at reference temperature [mW/m]:
In the temperature change rate [deg-'] of the LED output, R2 is a negative value.

SUMMARY OF THE INVENTION An object of the present invention is to provide an LED drive circuit with a function of compensating for a decrease in LED output due to temperature changes.

[Means to solve the problem]

The LED driving circuit of the present invention is connected to a power source via a collector, a power source, or a resistor,
An NPN type transistor connected to the emitter or the anode of the LED or a PN transistor whose collector is grounded or grounded through a resistor and connected to the emitter or the cathode of the LED.
In the case of a PN upper type transistor and an NPN type transistor, one end is connected to the cathode of the LED and the other end is grounded, and in the case of a PNP type transistor, one end is connected to the anode of the LED and the other end is connected to the power source. a resistor connected to the anode and cathode of the LED, one end of which is connected to the anode and cathode of the LED, and the other end of which is connected to each other, a first voltage dividing resistor closer to the transistor and a second voltage dividing resistor further away from the transistor; The input terminal, the inverting input terminal, and the output terminal each include a constant voltage source, a connection point between the two voltage dividing resistors, and an operational amplifier connected to the NPN type or PNP type transistor base, and the voltage of the constant voltage source is set to VCOn3t+. The forward voltage of the LED at the reference temperature is vF. , the temperature change rate of the output of the LED is , the temperature change rate of the forward voltage VFO is 3,
Assuming that the resistance values of the first and second voltage dividing resistors are R1 and R2, and the power supply voltage is VCC, in the case of an NPN type transistor, R, +R2VFO (R2 in 3), and in the case of a PNP type)/Rancissistor, , R, +R2VFO (R20 to 3), voltage vCo□1, resistance value R+, R2
are determined, and the resistance values R1 and R2 are large enough to ignore the current flowing through both voltage dividing resistors.

[Effect]

The LED drive circuit (in the case of an NPN transistor) of the present invention will be explained.

The LED forward voltage V1 is determined by the LED temperature ΔT [deg]
As it rises, it decreases. Expressed in the formula, VF = VFO(1
+ K3ΔT ) [VF = · (3) This can be expressed by the approximate equation (3). Here, VFO is the forward voltage at the reference temperature, and K3 is the temperature change rate [de
g-']. K3 is a negative value because it decreases by v or 6 guns. The voltage V12 at the connection point between the first and second voltage dividing resistors is set to the voltage V of a constant voltage source by an operational amplifier. onst
be equivalent to.

V,2=V,. □, ...(4)
Assume that both voltage dividing resistors have high resistance and the current flowing through both voltage dividing resistors is ignored. Further, the resistance values of the first and second voltage dividing resistors are set to R+ and R2, and the voltage dividing ratio X of the voltage across the LED is set to 2 and 2, respectively.

Then, the voltage v+2 at the connection point between both voltage dividing resistors can also be expressed as follows.

! +2 = ROI +VFO (3ΔT to 1+) x
.=(6) Here, R is the resistance value of the resistor connected to the cathode of the LED, I is the current flowing through the LED and this resistor, and the second term on the right side is based on equation (3).

From equations (4) and (6), Vconst=ROI +V. (1+KaΔT)x
-(7) By moving ■ to the left side, the term 1- and this current I determine the light output of the LED according to equation (2).

...(9) When formula (9) is expanded, 1 P" (■confft-vFOX VrOX 8
3△to+■constK2ΔT −2ΔT to VFO x
-V, o xK2 to 3ΔT2) = 1
10) The term after @ in parentheses on the right side of equation (lO) is K2
Since it is raised to the power of 3, it is small and can be ignored.

Since the partial pressure ratio x (0<x<1) is -V,, x K3ΔT”Vconst is 2ΔT−VF
ox K2ΔT=0...
-(12) That is, the sum of the third to fifth terms regarding the six guns in the parentheses of equation (10) is zero. Therefore, L.E.
The optical output P of D does not change with temperature.

Note that the same applies to the case of a PNP type transistor.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows L of an embodiment of the present invention (corresponding to claim 1).
It is a circuit diagram of an ED drive circuit.

In the LED drive circuit of this embodiment, the collector or protective resistor 1
An NPN transistor 2 is connected to the power supply through the emitter terminal 0 and connected to the anode of the LED 1, and a resistor 3 whose one end is connected to the cathode of the LED 1 and the other end is grounded. The anode of LED 1 is connected to can-1 and the other end of can-1 is connected to each other. Second
The voltage dividing resistor 4゜5, the non-inverting input terminal, the inverting input terminal, and the output terminal are each a constant voltage source (voltage v0゜.5.),
It consists of an operational amplifier 6 connected to the connection point of the voltage dividing resistor 4.5 and the base of the transistor 2.

Here, the forward voltage VF of LED 1, the voltage V can of the constant voltage source, the temperature change rate of the output of LEDI, and the temperature change rate of the forward voltage VF are 3, respectively. vFO = 1.7 [VF V Cfl. st =], 2[VF K2 = -0,007[deg-']K3 =
-0,001 [deg-'], the voltage division ratio
380 [kΩ] R2=620 [kΩ]. In order to reduce the current flowing through the voltage dividing resistor 4.5, the resistance values R, , R2 are set large.

Therefore, in the LED drive circuit of this embodiment, the LED
The optical output P of 1 does not change with temperature.

FIGS. 2 and 3 are circuit diagrams of LED drive circuits according to other embodiments of the present invention (each corresponding to claim 2.3).

In the embodiment shown in FIG. 2, an LED 7 is connected between the collector of the NPN transistor 2 and the power supply. In the embodiment of FIG. 3, between the can-1'' of LED 1 and resistor 3,
D8 is connected. In the embodiments shown in FIGS. 2 and 3, the values of R1 and R2 are determined in the same manner as in the first embodiment, and the light output P of the LEDI, 7.8, is determined by the temperature. does not change depending on

Note that the protective resistor 10 is provided to prevent rush current when the power is turned on.

FIG. 4 is a circuit diagram of an LED driving circuit according to another embodiment of the present invention.

This embodiment corresponds to claim 1, and the NPN transistor 2
0 instead of PNP fF', an l-run lister 9 is used, the collector is grounded, the emitter is connected to the cathode of LED 1, the emitter is connected to the anode of LED 1, and the other end is connected to the anode of LED 1. A resistor 3 is connected at one end to the power source, and a first .
The second voltage dividing resistor 4.5, the non-inverting input terminal, the inverting input terminal, and the output terminal are each connected to a constant voltage source (voltage V.onst
), the connection point of the voltage dividing resistor 4.5, and the operational amplifier 6 connected to the base of the transistor 2.

Naturally, an embodiment in which a PNP transistor 9 is used in place of the NPN transistor 2 in the embodiments shown in FIGS. 2 and 3 is also conceivable. Then, by determining each value of R1°R2 in the same manner as in the embodiments shown in FIGS. 1 to 4,
The light output P of LEDl, 7.8 does not change with temperature.

[Effects of the Invention] As described above, the present invention has the effect that the pressure of the LED does not decrease even if the temperature rises by providing a circuit configuration that compensates for temperature changes in the light output of the LED.

[Brief explanation of the drawing]

FIG. 1 corresponds to claim 1 of one embodiment of the present invention, and NP
Circuit diagram of LED drive circuit (in case of N-type transistor),
FIGS. 2 and 3 show L of other embodiments of the present invention (corresponding to claim 2.3, respectively, in the case of an NPN transistor).
The circuit diagram of the ED drive circuit, FIG. 4, is another embodiment of the present invention (corresponding to claim 1, in the case of a PNPN upper type transistor).
Circuit diagram: FIG. 5 is a circuit diagram of a conventional example of an LED driving circuit. 1.7, 8...LED 2-...NPN type transistor 3...Resistor 4.5...Voltage dividing resistor 6...Operation amplifier 9...PNPN upper type transistor 10...Protection resistor

Claims (1)

[Claims] 1. An NPN transistor whose collector is connected to a power source or a power source via a resistor, whose emitter is connected to the anode of the LED, or whose collector is grounded or grounded via a resistor, and whose emitter is connected to the cathode of the LED. In the case of an NPN transistor, one end is connected to the cathode of the LED and the other end is grounded, and in the case of a PNP transistor, one end is connected to the anode of the LED, and the other end is connected to the anode of the LED. a resistor whose end is connected to a power source; one end connected to the anode and cathode of the LED, and the other ends connected to each other, for a first voltage divider on the side closer to the transistor and a second voltage divider on the side farther from the transistor. a resistor, and an operational amplifier whose non-inverting input terminal, inverting input terminal, and output terminal are each connected to a constant voltage source, a connection point between the voltage dividing resistors, and a base of the NPN or PNP transistor, and the constant voltage The source voltage is V_c_o_n_s_t, the L
The forward voltage at the reference temperature of ED is V_F_O, and the LE
If the temperature change rate of the output of D is, the temperature change rate of forward voltage V_F_O is K_3, the resistance values of the first and second voltage dividing resistors are R_1 and R_2, and the power supply voltage is V_C_C, then N
In case of PN type transistor, R_2/(R_1+R_2)=V_c_o_n_s_t
K_2/{V_F_O(K_2+K_3)} In case of PNP transistor, R_2/(R_1+R_2)={(V_C_C-V_c
_o_n_s_t)K_2}/{V_F_O(K_2+
K_3)}, the voltage V_c_o_n_s_t
, resistance values R_1 and R_2 are determined, and resistance value R_1
, R_2 is an LED drive circuit so large that the current flowing through both voltage dividing resistors can be ignored. 2. The LED according to claim 1, wherein the LED is connected between the collector of the NPN transistor or a resistor via the power supply and the power supply, and between the collector of the PNP transistor or the resistor via the ground and the ground. drive circuit. 3. The LED drive circuit according to claim 1, wherein the LED comprises two or more LEDs connected in series.