RU2595536C1 - Galvanically isolated voltage comparator supplied by input signal - Google Patents

Abstract

FIELD: control equipment.

SUBSTANCE: present invention relates to automatic control systems. Arrangement ensures a constant current of a comparator load to an input signal while the input voltage considerably exceeds the preset value of standardization. Level of standardization of input voltage is set by a variable resistor, connected at the input side of the comparator. Comparator comprises two diode optical pairs, p-n-p and n-p-n transistors, variable and four permanent resistors, Schmitt trigger of CMOS microcircuit.

EFFECT: technical result: galvanically isolated voltage comparator that does not require a separate galvanically isolated voltage supply source with minimum standardization level of not more than 2 V and variable load current in the whole range of input voltage.

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Description

The invention relates to automatic control and monitoring systems, in particular, for comparing the input signal at several voltage levels with the transmission of the result of the comparison through an optocoupler.

The closest in technical essence is a galvanically isolated voltage comparator powered by an input signal according to patent No. 2540808, containing an adjustable zener diode, diode optocoupler, a CMOS chip with a Schmitt trigger and five resistors, providing galvanic isolation from the input signal and not requiring an isolated power source.

The disadvantage of this voltage comparator is the large value of the minimum voltage of the comparing, determined by the voltage drop at the adjustable zener diode (2.5 V), the optocoupler LED (about 1.5 V) and the current-sensing resistor of the optocoupler LED.

When the voltage comparing levels are close to the minimum, a small value of the resistance of the current-sensing resistor of the optocoupler LED when the input voltage exceeds the set comparison threshold causes a significant increase in the load current to the input signal.

These shortcomings do not allow the use of this comparator as a low-level voltage comparator.

The proposed galvanically isolated voltage comparator powered by the input signal, consisting of two diode optocouplers, pn-p and npn transistors, one variable and four constant resistors and a Schmitt trigger CMOS chip, provides a minimum level of comparing voltage of not more than 2 V and does not increase the load current to the input signal when the input voltage significantly exceeds the set threshold of comparing.

The circuit of a galvanically isolated voltage comparator powered by an input signal is shown in the drawing.

The drawing shows:

1 - pnp transistor;

2 - variable resistor;

3 - current-carrying resistor pnp transistor;

4 - current-sensing resistor n-p-n transistor;

5 - load resistor of the photodiode of the second diode optocoupler;

6 - high resistance resistor;

7 - the first diode optocoupler;

8 - n-p-n transistor;

9 - second diode optocoupler;

10 - Schmitt trigger CMOS chips;

+ Uin - input terminal of a galvanically isolated voltage comparator;

OV is the common output signal of a galvanically isolated voltage comparator;

Output - output terminal of a galvanically isolated voltage comparator;

+ U - output for connecting the supply voltage of the CMOS chip.

The comparator input voltage of the comparator is determined by the sum of the voltage drops on the LED of the second diode optocoupler 9, the emitter-collector of the open pnp transistor 1 and resistors 3 and 2.

The maximum voltage of the input signal is limited by the lowest limit voltage of the collector-emitter of transistors 1 or 8.

The voltage comparator operates as follows. The LEDs of the diode optocouplers 7 and 9 are used as zener diodes with a stabilization voltage of approximately 1.5 V and are used to stabilize the current of the transistors 1 and 8. When the input signal voltage is less than the comparing level, no current flows through the LEDs of the diode optocouplers 7 and 9. The collector load of the pnp transistor 1 is only the base current of the npn transistor 8, and the collector load of the npn transistor 8 is only the base current of the pnp transistor 1, so transistors 1 and 8 are in saturation mode and have the minimum collector-emitter voltage. High-resistance resistor 6 is necessary to turn on transistors 1 and 8 until the input voltage reaches the set level of comparing. When the input voltage is increased and the second diode optocoupler 9 reaches a sufficient voltage to turn on, a current begins to flow through the LED of the second optocoupler 9, which will increase the current through the photodiode of the second diode optocoupler 9 and, therefore, increase the voltage across the load resistor of the photodiode of the second diode optocoupler 5. The excess voltage at the load resistor of the photodiode of the second diode optocoupler 5 threshold switching Schmitt trigger CMOS chip 10 will lead to its switching. The Schmitt trigger CMOS chip 10 is used to generate the output signal of a galvanically isolated voltage comparator.

The LED current of the second diode optocoupler 9, necessary to switch the Schmitt trigger of the CMOS chip 10, is set by the resistance of the pickup resistor pn-p of the transistor 3. The resistance value of the pickup resistor npn of the transistor 4 should provide sufficient base current pnp of the transistor 1 to find it saturated when the input voltage is equal to the comparing level. The stability of the comparing level is determined by the stability of the voltage of the on LED of the second diode optocoupler 9, the stability of the saturation voltage of the collector-emitter of the pnp transistor 1 and the stability of the current-carrying resistor pnp transistor 3.

With a significant excess of the voltage of the input signal of the comparing level, transistors 1 and 8 exit the saturation mode. The stabilization of their currents allows you to keep unchanged the load current of the comparator to the input signal, not counting a slight increase in the load current of the comparator from the voltage increase on the high-resistance resistor 6.

A variable resistor 2 sets the comparing level of a galvanically isolated voltage comparator powered by the input signal.

Claims (1)

A galvanically isolated voltage comparator powered by an input signal, containing two diode optocouplers, pnp and npn transistors, one variable and four constant resistors, a Schmitt CMOS chip, the input signal connected to the first output of the variable resistor, the second output of the variable resistor connected to the LED anode the first diode optocoupler and the first output of the pickup resistor pnp transistor, the second output of the pickup resistor pnp transistor connected to the emitter of the pnp transistor, the cathode of the LEDs The ode of the first diode optocoupler is connected to the base of the pnp transistor, the collector of the npn transistor and the first output of the high-resistance resistor, the collector of the pnp transistor is connected to the base of the npn transistor and the anode of the LED of the second diode optocoupler, the emitter of the npn transistor is connected to the first output of the transistor connected to the second terminal of the high-resistance resistor, the second terminal of the current-sensing resistor npn of the transistor and the cathode of the LED of the second diode optocoupler, the cathode of the photodiode of the second diode opt the dots are connected to the supply voltage of the CMOS chip, the anode of the photodiode of the second diode optocoupler is connected to the input of the Schmitt trigger CMOS circuits and the first output of the load resistor of the photodiode of the second diode optocoupler, the second output of the load resistor of the photodiode of the second optocoupler is connected to the common output of the supply voltage of the CMOS chip, the output is three microcircuit is the output of a galvanically isolated voltage comparator powered by the input signal.

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