Introduction to the PUT

Introduction

A specific combination of 2 complementary transistors leads to an interesting self-latching behavior, that can be compared with the behavior of a thyristor.

In the figure below, the connection between the 2 complementary transistors is shown. The configuration has a similar behavior as a thyristor, but is NOT a direct replacement for a thyristor !

Therefore, the circuit is also called "TUT" : Transistorised Universal Thyristor.

Let's take a look at the behavior of this configuration and compare it with the behavior of a thyristor.

What do we know about a thyristor ? Well, it has a gate, an anode, and a cathode. When a small current is injected into the gate (milliamperes), the thyristor will start to conduct, provided that the anode-to-cathode current is above a certain threshold, called latching current (IL). Once the thyristor is conducting, the gate can be disconnected, and the thyristor will still continue conducting without any gate current.

The only way to make the thyristor stop conducting, is to make the anode-to-cathode current lower than the minimal holding current. So cutting off the power supply briefly will make the thyristor stop conducting again and return to the initial situation.

The circuit above behaves like a thyristor. We can choose between inputs that function as gates: a positive gate (meaning that we have to make this gate high to make the PUT latch) connected to momentary SWITCH2 and a negative gate (meaning we have to make this gate low to make the PUT latch) connected to momentary SWITCH1.

• At power on, SWITCH1 and SWITCH2 are open. Q1 will not conduct, because its base is pulled high by R1. Q2 will not conduct, because its base is floating and does not receive any current.

• Because Q1 does not conduct, the OUTput will be high. By putting a small capacitance in parallel to R1, the PUT can be forced into a non-conducting state at power on.

• When SWITCH2 is open and SWITCH1 is closed, the base of Q1 is pulled to towards ground and Q1 will conduct. When Q1 conducts, the OUTput will go low and the base of Q2 receives current via R2, so Q2 will also conduct.

• When Q2 conducts, the base of Q1, which was pulled low by SWITCH1, is now pulled low by Q2. Due to this self-latching effect, Q1 will continue conducting, even when SWITCH1 is opened again.

• So when SWITCH1 is closed shortly, the OUTput will go low and stay low. The only way to reset the circuit back to the initial state, is to briefly disconnect it from the power supply.

• This is the same self-latching behavior as a thyristor.

• When SWITCH1 is open and SWITCH2 is closed, the base of Q2 is pulled high and Q2 will conduct. When Q2 conducts, the base of Q1 is pulled low and Q1 will conduct. When Q1 conducts, the base of Q2, which was pulled high by SWITCH2, is now pulled high by Q1 via R2. Due to this self-latching effect, Q2 will continue conducting, even when SWITCH2 is opened again.

  • So when SWITCH2 is closed shortly, the OUTput will go low and stay low. The only way to reset the circuit back to the initial state, is to briefly disconnect it from the power supply.

  • This is the same self-latching behaviour as a thyristor.

Why the transistor combination is also called a PUT : Programmable Unijunction Transistor ? 

The difference between an ordinary unijunction transistor and a programmable unijunction transistor is the fact that the trip voltage, at which the transistor starts conducting, can be programmed by a reference voltage applied to the gate of the PUT.

In the figure below, 2 gate pins are shown because the PUT can be used either with Gate 1 or with Gate 2 as a reference voltage input. When using gate1, the PUT will conduct when the anode voltage exceeds the reference voltage on gate1 with 0.7V (base emitter voltage drop). When using gate2, the PUT will conduct when the cathode becomes 0.7V lower than the voltage at gate2.

In the figures below, practical circuits are shown illustrating the behavior of the PUT transistor combination.

The reference voltage at node 1 is half the supply voltage, so 2,5V, because R1 = R2.

The voltage at node 2 can be set with a potmeter. 

• Left figure below : When the voltage at node 2 is set to a voltage that is 0,7V lower than the voltage at node 1, then Q1 will start conducting, When Q1 starts conducting, the base of Q2 receives current via Q1, so Q2 starts conducting. When Q2 conducts, it will pull the base of Q1 to the positive rail, so Q1 will keep conducting. The transistor combination now latched itself into conduction and will continue conducting, even when lowering the voltage at node 2, so it becomes lower than the voltage of node 1. 

• The PUT will unlatch when the potmeter is set to a voltage close to the supply rail, so no current can flow through Q1 anymore, or when disconnecting the power supply briefly.

• Right figure below : When the voltage at node 2 is set to a voltage that is 0,7V higher than the voltage at node 1, then Q1 will start conducting, When Q1 starts conducting, the base of Q2 receives current via Q1, so Q2 starts conducting. When Q2 conducts, it will pull the base of Q1 towards ground, so Q1 will keep conducting. The transistor combination now latched itself into conduction and will continue conducting, even when lowering the voltage at node 2, so it becomes lower than the voltage of node 1. 

• The PUT will unlatch when the potmeter is set to a voltage close to ground, so no current can flow through Q1 anymore, or when disconnecting the power supply briefly.

.

Another example of the behavior of a PUT is shown in the circuit below. 

When the power is connected, the LED will illuminate, getting its current via R1 from the power supply. The PUT formed by Q1 and Q2 is not active because the base of Q1 is pulled down towards ground by R4 and the base of Q2 is pulled up to the voltage over the LED. So both transistors are not conducting. From the moment that the switch is pressed, the base of Q1 receives current from the power supply via R3, which limits this current.

Q1 starts conducting and pulls down the base of Q2, so current can flow through the base emitter junction of Q2 causing Q2 to conduct. Q2 will now pull up the base of Q1, so the base of Q1 receives current via Q2. This means that the PUT combination is now latched into conduction, thereby "short-circuiting" the LED, which will extinguish.

When releasing the switch, the PUT combination remains latched and the LED will stay off.

When the power supply is disconnected briefly, the PUT combination "unlatches" and the LED will illuminate again.