Analog of a thyristor with a controlled switching threshold
Thyristors are a semiconductor device with an S-shaped current-voltage characteristic. They are a kind of managed keys with a state memory that are turned on when a control signal is applied to the input of the thyristor. When working on direct current, you can turn off the thyristor by removing the supply voltage. A big and unavoidable disadvantage of thyristors is that they have a low input impedance and, most importantly, they do not have the ability to adjust the switching threshold.
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Figure 1 shows a simple circuit of thyristor analogues that have a high input resistance and the ability to adjust the switching voltage. Both devices contain comparators U1.1 of the LM339 chip as an active component.
Figure 1 A comparator analog of a thyristor with an adjustable switching threshold.
It works as follows. A control signal is applied to the input of the device (the inverting input of the comparator), the voltage of which exceeds the threshold of switching the comparator. This threshold is set by adjusting the potentiometer R2 in the range from 0 to 4 V. If the voltage at the comparator output is close to the supply voltage of the device (the level of the logical unit) before the input signal is supplied, then after switching the voltage at the comparator output drops to zero, the transistor Q1 2N7000 closes. The voltage on its drain is switched from zero to the supply voltage. This voltage is applied through the resistor R7 to the input of the comparator and snaps its state. You can return the thyristor analog to its original state by briefly disconnecting the supply voltage with the S1 button.
Figure 2 shows a simplified version of the comparator analog of the thyristor. The pulse of the input control signal through the Schottky diode D1 1N5817 is fed to the non-inverting input of the comparator U1.1. In the initial state, before the control signal is supplied, the voltage at the output of the comparator is zero. If the input voltage exceeds the switching voltage of the comparator set by adjusting the potentiometer R2 in the range from 0 to 1 V, the comparator will switch. A high-level voltage will appear at its output, which enters the comparator input through the resistor R5 and snaps its state.
Figure 2 A variant of the comparator analog of the thyristor.
You can return the device to its original off state by pressing the S1 button.
Michael A. Shustov is a doctor of technical sciences, candidate of chemical sciences and the author of over 800 printed works in the field of electronics, chemistry, physics, geology, medicine, and history.
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