• Also known as a silicon controlled rectifier (SCR).
  • Its like a transistor:
    • When a small current flows into the Gate (G) it allows a larger current to flow from the Anode (A) to the Cathode (C).
  • Like the FET, the pinout of the thyristor is dependent on the shape of its casing.
  • Can be used to create a latching circuit:
    • Such a circuit is sometimes known as a flip-flop but its actual name is a bistable:
      • This is a circuit that stays ON after a momentary signal is received to the input and is otherwise OFF.
      • It has two stable states; ON and OFF.
  • Very useful in burglar alarms:
    • Even a single break in a light beam or a short signal from a pressure pad is enough to make the system latch and stay on until reset.
  • However, unlike transistors, even if the current through the gate switches off, the larger current flowing from the Anode to the Cathode remains active:
    • In essence it is like a transistor which latches ON.
    • Once the thyristor is activated it can only be turned OFF (un-latched) by shutting down all the power flowing to it.
      • This type of circuit is a latching circuit.
      • The word un-latched is speculation.

How Do They Work?

  • A thyristor can be used as a latch using the circuit below (learn it):


  • The thyristor will activate if a voltage of more than 2V is applied to the gate pin even with a very small current:
    • After this the thyristor will stay ON until reset unlike an FET which would turn OFF when the voltage at its Gate pole disappears.
    • The fact that they need very little current is a significant advantage:
      • This means that even a small piezo transducer will be enough to trigger the thyristor as in the diagram alongside:
        • Piezoelectric material produces a small electric charge when squeezed.
        • The capacitor in this circuit diagram smoothes the supply of electricity going into the thyristor:
          • Without it the pulsing current of the buzzer may reset the thyristor:
            • Remember that buzzers make sound and to make sound there must be an alternating current (speculation).
  • It can be configured to activate according to light intensity by using the circuit alongside:
    • This circuit would activate as soon as the light intensity reaches a level determined by the variable resistor (provided that SW1 is closed):
      • However, I would connect a second resistor in series to the variable resistor
        • If the variable resistor was adjusted to, or near to, zero resistance then the current would probably flow through the variable resistor and then directly into the 0V rail. Thus the thyristor would be bypassed:
          • This is speculation.
  • It can be turned OFF by interrupting the current flow through it or by turning OFF the circuit completely.
  • Thyristors often have a metal backing:
    • This can act as a heat sink independently.
    • Or it can allow a larger heat sink to be attached.
    • Thyristors get very hot when handling large currents.
      • This is also an advantage of thyristors in that they are able to drive components that require large amounts of current.

Steady-Hand Game:

  • The circuit below uses a thyristor to create a circuit for a steady hand game.
    • The Reset button is a push-to-break switch which, when pressed, disconnects the entire circuit for the fraction of a second necessary to turn the thyristor OFF.
    • The Contact Wires are represented by the push-to-make switch but in reality they will simply be two copper wires arranged as necessary.
    • The 10kΩ resistor and the 1kΩ resistor are both probably for circuit protection purposes.
    • The resistor R1 is likely a pull down resistor that prevents the thyristor from activating unless the contact wires touch.



  • The above circuit could also be used to make a burglar alarm with the push-to-make switch representing a pressure pad or an equivalent detection device.
  • But it could also be made into a burglar alarm by attaching any other sensor’s output to the gate pole of the thyristor.
  • It could also be made into a door-opening alarm by rearranging the push-to-make switch. In this case the push-to-make switch would be placed in the door jamb where it would be closed when the door is closed. However, when the door opens the push-to-make switch opens, forcing the current to flow to the gate terminal of the thyristor which in turn switches the thyristor ON. It is possible to activate/deactivate this circuit using an SPST switch that is placed in series to the push-to-break reset switch.

Alarm Thyristor

  • It is possible to make the alarm so that it is very difficult to turn OFF:
    • This can be done by using two push-to-break switches connected in parallel as the Reset.
    • This would mean that the circuit could only be switched OFF if both switches are pressed simultaneously:
      • If the two switches were placed far apart it would therefore take two people to turn the alarm OFF if it were tripped:
        • This, of course, could prove rather bothersome to the owner though.
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