|A thyristor is used to drive a load. It is switched on by applying a positive voltage to its input pin (the ‘gate’).
How does it operate?
A thyristor is a controllable diode. It is triggered to conduct a forward current (in the direction of the diode arrow – from the anode to the cathode) if the voltage of the gate is about +1V relative to the cathode.
After the thyristor starts to conduct, current continues to flow until the voltage between the anode and cathode pins is reduced to zero.
A thyristor behaves rather like a combination of a transistor driver and a latch.
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit.
The circuit on the left will switch the current through the Output device on if the switch SW1 is closed and the Input signal voltage (applied to the gate) rises above about 1V.
The current will continue to flow through the Output device, even if the Input signal voltage falls below 1V.
The current through the thyristor can be turned off by opening the switch SW1.
To turn the current back on, the switch needs to be closed and an Input signal voltage above 1V needs to be applied again.
The ‘Output’ box shown on the circuit diagram would need to be deleted and replaced by the actual Output device to produce a working circuit.
It is important to select a thyristor which can provide a maximum current greater than the current needed by the Output device. Otherwise the thyristor could be damaged. A variety of thyristors are available. Some low cost examples are:
||Rapid Electronics Order Code
||Maximum safe current
||Very low cost. |
||Slightly more expensive. Much higher current (suitable for e.g. motors). RoHS compliant.|
Thyristors are also known as Silicon Controlled Rectifiers (SCRs).
- A ‘smart’ steady hand game or other game of skill that sounds a buzzer if the wire is touched and keeps sounding it until a push-to-break switch is pressed.
Pins of 2N5060 thyristor
Pins of C106D1 thyristor
How part of the PCB (using a 2N5060) might look
The switch SW1 is a push-to-break switch.
The pin arrangement would be different for the C106D1 thyristor.
The PCB shows the output device connected via a 2-way PCB terminal.
Before adding the components, make sure that the signal coming in (on the blue PCB track) changes from high to low.
If there is a fault, check that:
- The voltage on the cathode pin is low (0V)
- The input signal voltage on the gate is the same as the signal from the previous subsystem
- The thyristor is the right way round
If there is a fault, check the tracks and solder joints.
- A latch, followed by a driver can be used to give a similar action, but this produces a more complicated circuit.
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