JP2603215Y2 - Rotary encoder output protection circuit - Google Patents

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an output protection circuit for a rotary encoder, and more particularly to a protection circuit for a push-pull output circuit used as the output circuit.

[0002]

FIG. 4 is a circuit diagram showing a push-pull output circuit and its equivalent circuit. As shown, two transistors Tr are provided at the output stage.
11 and Tr12 are connected in series, and the transistors are turned on alternately, so that the output current flows in both directions of flowing in and flowing out. Therefore, when the output terminal is short-circuited to another output or short-circuited to the positive terminal side or the negative terminal side of the power supply, there is a problem that the transistor in the output stage is destroyed.

The present invention has been made to solve such a problem, and it is an object of the present invention to provide an output protection circuit for a rotary encoder in which the output stage is not destroyed even if a short circuit occurs as described above. With the goal.

[0004]

An output protection circuit for a rotary encoder according to the present invention comprises a PNP-type first output transistor (Tr1) and a circuit between a base of the first output transistor and a positive terminal of a power supply. A first overcurrent detection transistor (Tr3) connected to the power supply, a first resistor (R1) connected between the emitter of the first output transistor and the positive terminal of the power supply, and a first overcurrent. A second resistor (R3) connected to the base of the detection transistor and the positive terminal of the power supply; an emitter of the first output transistor;
A third resistor (R4) connected between the base of the overcurrent detection transistor and a fourth resistor connected between the collector of the first output transistor and the base of the first overcurrent detection transistor; And a resistor (R5). Further, the output protection circuit of the rotary encoder is connected in series to the first output transistor, and an npn-type second output transistor (Tr2) whose output is guided from the connection point, and a base of the second output transistor. A second overcurrent detection transistor (Tr
4) a fifth resistor (R7) connected between the emitter of the second output transistor and the negative terminal of the power supply; and a fifth resistor (R7) connected between the base of the second overcurrent detection transistor and the negative terminal of the power supply. A sixth resistor (R9) connected therebetween, and a seventh resistor (R10) connected between the emitter of the second output transistor and the base of the second overcurrent detection transistor.
And an eighth resistor (R11) connected between the collector of the second output transistor and the base of the second overcurrent detection transistor. Further, the output protection circuit of the rotary encoder according to the present invention has two diodes (D1, D2) connected in series in a forward direction between the first output transistor and the second output transistor, An output is derived from a connection point between the two diodes.

[0005]

In the present invention, for example, when the second output transistor is driven, its emitter current increases according to the load. Then, for example, when the voltage across the fifth resistor rises and the second overcurrent detection transistor starts to drive, the base current of the second output transistor decreases, and further, the collector potential of the second output transistor decreases. As a result, the positive current returns to the base of the second overcurrent output transistor via the eighth resistor, and the collector current of the second output transistor is further restricted. Therefore, even if the output terminal is short-circuited with the power supply, the overcurrent is prevented from flowing. Also, in the present invention, when the power supply is connected to the opposite polarity,
Two diodes (D1, D2) protect the output transistor and the like.

[0006]

1 is a circuit diagram of an output protection circuit of a rotary encoder according to an embodiment of the present invention. In this output protection circuit, the transistors Tr1 and Tr2 are connected in series via diodes D1 and D2. An output terminal OUT is led from a connection point between the diodes D1 and D2. The emitter of the transistor Tr1 is connected to the positive electrode side of the power supply via the resistor R1.
The emitter 2 is connected to the ground side of the power supply via a resistor R7. A resistor R2 is connected between the emitter and the base of the transistor Tr1, and this emitter is connected to the base of the transistor Tr3 via the resistor R4. A resistor R3 and a capacitor C1 are connected in parallel between the positive terminal of the power supply and the base of the transistor Tr3. A resistor R5 is connected between the base of the transistor Tr3 and the collector of the transistor Tr1. Transistor T
The collector of r3 is connected to the collector of transistor Tr5 via a resistor R6, and the emitter of transistor Tr5 is grounded.

A resistor R8 is connected between the emitter and the base of the transistor Tr2.
Is connected to the base of the transistor Tr4. A resistor R9 and a capacitor C2 are connected in parallel between the ground side and the base of the transistor Tr4. The resistor R11 is connected between the base of the transistor Tr4 and the collector of the transistor Tr2. Transistor T
The emitter of r4 is grounded, and the collector of the transistor Tr4 is connected to the collector of the transistor Tr6 and to the positive terminal of the power supply via the resistor R12. Further, a Zener diode ZD is connected between the collector of the transistor Tr1 and the positive terminal of the power supply.
1 is connected, and a Zener diode ZD2 is connected between the collector of the transistor Tr2 and the ground side.

FIG. 2 is a circuit diagram in which a circuit on the transistor Tr2 side of the circuit of FIG. 1 is extracted. For example, the operation will be described focusing on a state where the base current is supplied to the transistor Tr2 via the resistor R12 while the transistor Tr6 of FIG. 1 is being driven. It is assumed that the transistor Tr2 is on, the collector-emitter thereof is in a saturated state, and the voltage VCE is "0" V. At this time, the emitter current i of the transistor Tr2 has a magnitude corresponding to the load (not shown) connected to the output terminal and the voltage applied thereto.

FIG. 3 is a characteristic diagram showing the relationship between the output remaining voltage Vi (collector voltage) and the output current i (emitter current) of the circuit of FIG. 2, and shows the load resistance between the output terminal OUT and the power supply. It shows the locus of the output current when the resistors are connected and their resistance values are sequentially reduced. As shown, as the emitter current i of the transistor Tr2 increases, the potential Vi of the collector thereof increases. When the transistor A reaches the point A (more precisely, immediately before reaching the point A), the base-emitter voltage VBE of the transistor Tr4 is 0.6 V, and the base current is not flowing yet. Assuming that the collector-base-emitter voltage of Tr2 is 0 V, the emitter current i of transistor Tr2 is approximated by the following equation (1-4). That is, assuming that the current flowing through the resistor R11 is i2, it is expressed by the following equation (1-1). When the transistor Tr2 is turned on (collector-emitter voltage is 0 V), this is equivalent to connecting the resistors R9 and R10 in parallel, and the current i2 flows through the resistors R9 and R10 connected in parallel. The voltage V9 is represented by the following equation (1-2). The voltage obtained by adding the base potential of the transistor Tr4 and the voltage V9 across the resistors R9 and R10 connected in parallel is equal to the voltage across the resistor R7. Can be By substituting equation (1-1) into equation (1-3) and rearranging, equation (1-4) is an approximate equation of emitter current i of transistor Tr2.
An expression is obtained.

(Equation 1) Here, if R7 = 15Ω R9 = 82KΩ R11 = 4.7KΩ R10 = 2.2KΩ, then i = 58.2mA. That is, when the emitter current i reaches the point A, the transistor Tr4 starts to turn on. By the start of the ON operation of the transistor Tr4, the base current of the transistor Tr2 decreases, and the emitter current i does not flow any more even if the load resistance decreases.

[0010]

(Equation 1) Then, i = 58.7 mA. That is, when the emitter current i reaches the point A, the transistor Tr4 starts to turn on. By the start of the ON operation of the transistor Tr4, the base current of the transistor Tr2 decreases, and the emitter current i does not flow any more even if the load resistance decreases.

The point B in FIG. 3 is an operating point when the output terminal OUT is short-circuited to the power supply.
The magnitude of the emitter current i varies depending on the power supply voltage at that time, and is approximated by the following equation.

[0012]

(Equation 2)

For example, when the 24V power supply is short-circuited to the output terminal OUT, V1 = 23.2V (the forward voltage of the diode D2 is 0.8V), and the emitter current i is 18.3 mA according to the above equation. That is, the output terminal OU
Even when T is short-circuited to the power supply, the emitter current i can be suppressed to such a small value. Note that the base current flowing through the transistor Tr4 is neglected in Equation 2, for the following reason. At the point B, the collector current flowing through the transistor Tr2 is about 10 mA or more.
The current is about several tens mA, and the base current of the transistor Tr2 is the collector current / transistor amplification factor, which is about several hundred μA. Therefore, the collector current of the transistor Tr4 becomes smaller than several hundred μA, and its base current becomes several μA, which is within the allowable range even if the base current is ignored in Equation 2.

The above operation is described with reference to the transistor Tr.
The transistor Tr1 side corresponds to the transistor Tr2, the transistor Tr3 corresponds to the transistor Tr4, the resistor R5 corresponds to the resistor R11, and the resistor R3 corresponds to the resistor R9.
The resistor R4 corresponds to the resistor R10, and the resistor R1 corresponds to the resistor R7. By the way, the diodes D1 and D2 in FIG. 1 are connected when the polarity of the power supply is erroneously reversed (V in FIG. 1).
c has a function of preventing the transistors Tr1 and Tr2 from being destroyed, and has a function of preventing the transistors Tr1 and Tr2 from being destroyed.
Has a function to prevent noise and to prevent breakdown of the transistors Tr1 and Tr2. Further, the resistors R2 and R8 have a function of accelerating the OFF operation of the transistors Tr1 and Tr2.

[0015]

As described above, according to the present invention, when an overcurrent flows, the overcurrent detection transistor operates, and the collector potential of the output transistor is fed back to the base of the overcurrent detection transistor via a resistor. As a result, the collector current of the output transistor is limited, so that an overcurrent can be prevented from flowing. When the power supply is connected in reverse polarity, the diodes connected in series protect the output transistor and the like.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an output protection circuit of a rotary encoder according to an embodiment of the present invention.

FIG. 2 is a circuit diagram in which a part of FIG. 1 is extracted.

FIG. 3 is a Vi-i characteristic diagram showing the operation characteristics of the circuit diagram of FIG. 2;

FIG. 4 is a push-pull output circuit and its equivalent circuit diagram.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-60-148221 (JP, A) JP-A-2-86313 (JP, A) JP-A-61-293185 (JP, A) JP-A-4- 251515 (JP, A) JP-A-4-358413 (JP, A)

Claims (2)

(57) [Scope of request for utility model registration] 1. A PNP-type first output transistor (T
r1), a first overcurrent detection transistor (Tr3) connected between the base of the first output transistor and the positive terminal of the power supply, an emitter of the first output transistor, and a positive terminal of the power supply And a second resistor (R1) connected between the base of the first overcurrent detection transistor and the positive terminal of the power supply.
3), a third resistor (R4) connected between the emitter of the first output transistor and the base of the first overcurrent detection transistor, the collector of the first output transistor and the third resistor (R4). A fourth resistor (R5) connected between the first output transistor and a base of the overcurrent detection transistor, and an npn connected in series with the first output transistor.
A second output transistor (Tr2), a second overcurrent detection transistor (Tr4) connected between the base of the second output transistor and the negative terminal side of the power supply,
A fifth resistor (R7) connected between the emitter of the second output transistor and the negative terminal of the power supply, and a fifth resistor (R7) connected between the base of the second overcurrent detection transistor and the negative terminal of the power supply; A sixth resistor (R9) connected to the second
A seventh resistor (R10) connected between the emitter of the output transistor and the base of the second overcurrent detection transistor, the collector of the second output transistor and the second overcurrent detection transistor A rotary encoder having an eighth resistor (R11) connected between the output transistor and the base, wherein the first output transistor and the second transistor are alternately driven, and an output is guided between both output transistors Output protection circuit. 2. The first output transistor and the second output transistor.
2. The output of the rotary encoder according to claim 1, further comprising: two diodes (D1, D2) connected in series in a forward direction between the output transistors and the output transistors. Protection circuit.