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A thyristor is a semi-conductor device that can be used to latch an output. It has three connections, two of which, the cathode and the anode are connected as a normal diode. The third, the gate, is connected to the control voltage. If the thyristor recognises a voltage at the gate, the thyristor is said to be 'latched' and will stay latched even if the voltage at the gate is removed. It then has to be reset by removing the voltage supply to the anode. In this exercise we will investigate the use of thyristors in latching circuits.

Circuit symbol of a thyristor

Task 1: The thyristor as a latch

Model 1

  1. Open Yenka file Model 1.
  2. Look at circuit A. Press the push-to-make switch and then release. Describe what happens.
    The buzzer remains on as long as the switch is pressed.
  3. Look at circuit B. Press the push-to-make switch. Describe what happens in the circuit.
    When the switch is pressed, the buzzer turns on and stays on even after the push-to-make switch is released.
  4. Why do you think this happens in circuit B?
    When the switch is pressed, the thyristor receives a signal at the gate, this completes the circuit and the buzzer stays on.
  5. Why do you think there is a push to make switch in parallel with the thyristor?
    This switch enables the thyristor to reset.


A thyristor can be used as a latch in a circuit.

Task 2: Thyristor and sensing circuits

Model 2

  1. Open Yenka file Model 2.
  2. Label the legs of the transistor and the thyristor. Draw it below.


  3. Increase the light falling on the LDR. Describe what is happening in the circuit.
    As the LDR receives more light, the transistor obtains a threshold voltage of more than 0.6 V. The thyristor also receives a voltage at the gate, so the bulb turns on and stays on.
  4. Decrease the light falling on the LDR. Describe what is happening in the circuit. Can you give an explanation for this?
    Because the thyristor is latched, the bulb stays on. It needs to be reset.
  5. Interact with the model and add a component so that the thyristor can be reset. Draw the new circuit below.



A push-to-make switch is placed in parallel with the thyristor in order to reset it.

Teacher Summary