JPH0355232Y2 - - Google Patents

Description

[Detailed Description of the Invention] A. Field of Industrial Application The present invention relates to a relay drive circuit, and more specifically to a circuit that drives a relay supplied with power from an unregulated DC power supply with a constant current.

B. Summary of the invention Conventionally, in order to stabilize the relay drive current, the voltage across the feedback resistor connected to the emitter of the relay drive transistor is compared with the signal voltage using a voltage comparison circuit, and the output of this voltage comparison circuit is used to perform switching. In addition to controlling the transistor, the relay driving transistor was also controlled by the output of the switching transistor, but in this invention, the voltage of the feedback resistor is added to the charging/discharging circuit consisting of the resistor and capacitor via the diode, and this charging/discharging By using the voltage as a feedback voltage, the switching transistor can be omitted and the relay driving transistor can be directly controlled by the output of the voltage comparison circuit.

C. Prior Art Figure 3 shows a conventional circuit, in which the signal voltage divided by resistors R1 and R2 is applied to the positive input of voltage comparator A, and the voltage across feedback resistor R8 is applied to the negative input. has been added. The output voltage of the voltage comparator A is fed back to the positive input by a hysteresis resistor R3, and the input voltage has a hysteresis width between when the output is at the H level and when the output is at the L level. Also, the output of voltage comparator A is connected to switching transistor Q via resistor R4.
1, and the output of the switching transistor Q1 is applied to the base of a PNP transistor Q2 for driving the relay, and the relay coil RL is excited by the transistor Q2 for driving the relay.
A feedback resistor R8 for detecting the relay current and feeding it back to the negative input terminal of the voltage comparator A is connected in series to the relay coil RL, and a capacitor C1 is connected in parallel to the feedback resistor R8.

In the above configuration, when a signal is input now, the positive input voltage of voltage comparator A changes from O volts to V
When rising to 1 volt, the output of voltage comparator A is low
to H, switching transistor Q1
is turned on, and at the same time, a part of the output voltage is fed back through the hysteresis resistor R3 to raise the potential at the positive input end to V2 volts.
In addition, the relay driving PNP transistor Q2 is turned on by the output of the switching transistor Q1,
Current begins to flow through relay coil RL. This relay current increases and the voltage across feedback resistor R8 becomes V2.
volts, the output of voltage comparator A inverts from H to L, turning off switching transistor Q1 and relay driving transistor Q2,
Cut off the current supply to relay coil RL. However, the discharge current of the capacitor C1 flows through the feedback resistor R8, and when this current decreases and the voltage across the feedback resistor R8 reaches V1 volts, the output of the voltage comparator A is reversed from L to H again. In this way the circuit performs a relaxation oscillation. Figure 4 shows the output of the voltage comparator and the relay current or the voltage across the feedback resistor, that is, the waveform of the relay current.As shown above, the relay current oscillates within a certain range, and its upper and lower limits are Since it does not depend on unregulated DC power supply voltage, the relay coil is always supplied with a constant current without being affected by fluctuations in the power supply voltage.

As mentioned above, the conventional circuit shown in Figure 3 has the advantage of being able to supply a constant relay current even if an unregulated power supply is used to drive the relay, but on the other hand, it uses two transistors in the relay drive circuit. Therefore, there was a problem that it was difficult to adopt it for applications where substrate area and cost were limited.

D Problems to be solved with the invention In the conventional circuit shown in Figure 3, one transistor is
In order to reduce the number of transistors, for example, as shown in Figure 5, an NPN transistor Q is connected in series with the relay coil RL.
4 by the output of the voltage comparator A, the voltage across the feedback resistor R8 proportional to the relay current is fed back to the negative input of the voltage comparator A during the ON period of the transistor Q4.
During the off period of transistor Q4, the relay coil
The relay current flowing through the loop between RL and diode D1 is not detected by feedback resistor R8, and therefore the period during which the output of voltage comparator A is L becomes virtually zero, and the function of constant current is not achieved. It disappears.

E. Means to Solve the Problem Therefore, the present invention applies the voltage across the feedback resistor to the CR time constant circuit through the reverse blocking diode during the ON period of the relay driving transistor, and turns off the relay driving transistor. During this period, the discharge voltage waveform of this time constant circuit is approximated to the feedback voltage due to the relay current, and as a result, it has the same constant current function as the conventional circuit shown in Figure 3, and also has a switch of the relay drive circuit. This makes it possible to omit a switching transistor.

Embodiment FIG. 1 shows an embodiment of the present invention. The difference from the conventional circuit shown in FIG. 3 is that the output of the voltage comparator A directly controls the relay driving transistor Q3, and the voltage across the feedback resistor R8 connected in series to the emitter of the transistor Q3 is applied via the diode D2. The voltage is applied to the negative input terminal of the voltage comparator A, and a charging/discharging circuit consisting of a resistor R9 and a capacitor C3 is connected between the negative input terminal and the ground.

In the above configuration, when the voltage of the unregulated DC power supply increases, the relay current increases and the rate of increase in the voltage across the feedback resistor R8 increases, and the on period of voltage comparator A becomes shorter. The relay current is always constant when the output of device A is inverted and relay drive transistor Q3 is turned off. By adjusting the constant, it is possible to perform the same operation as when the relay current is fed back even during the OFF period of the transistor Q3.

In the embodiment of FIG. 2, transistor Q3 in FIG.
Instead of feeding back the voltage at the emitter of transistor Q3, the voltage at the base of transistor Q3 is fed back. With this configuration, the value of resistor R8 is sufficiently small compared to resistor R5. , the base voltage can be considered as the voltage across the feedback resistor R8 plus the base-emitter voltage (approximately 0.6 volts), so the operation is the same as in FIG. In this embodiment, the hysteresis resistor R3 in Fig. 1 is omitted, and instead, an operational amplifier with a built-in phase correction circuit is used as the voltage comparator A, and the hysteresis characteristics are improved by actively utilizing the delay of the operational amplifier itself. I am getting .

Effects of the invention As described above, according to the invention, the ON period of the relay driving transistor is controlled by applying a voltage proportional to the actual relay current to the negative input of the voltage comparator via the diode. During the off-period of the transistor, the transistor connected to the negative input terminal
Since the CR time constant circuit applies a voltage with a waveform similar to the relay current waveform, there is no need to pass relay current through the feedback resistor during the off period of the relay drive transistor, and therefore the relay drive By connecting a feedback resistor to the emitter side using an NPN as a relay transistor, it became possible to directly control the relay drive transistor with a voltage comparator, thereby reducing the size of the printed circuit board. This made it possible to reduce costs.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram showing one embodiment of the present invention;
The figure is a circuit diagram showing another embodiment, FIG. 3 is a circuit diagram showing a conventional example, and FIG. 4 is a waveform diagram showing the same operation as above.
FIG. 5 is an explanatory diagram showing the problems of the conventional example. Q3... Relay drive transistor, RL...
Relay coil, R8... Feedback resistor, A... Voltage comparator, D1, D2... Diode, R1, R2,
~, R9...Resistor, C1, C2...Capacitor,
Q1, Q2, ~, Q5...transistors.

Claims (1)

[Scope of utility model registration request] A relay coil is connected to the collector of the relay driving transistor, a feedback resistor is connected to the emitter, and the output terminal of a voltage comparator with a hysteresis voltage provided between the output voltage and the positive input voltage is connected to the transistor through the resistor. At the same time, the emitter or base voltage of the transistor is applied to the negative input terminal of the voltage comparator through a diode, and a parallel circuit of a resistor and a capacitor is connected between the negative input terminal and ground. A relay drive circuit consisting of connecting.