A thyristor acts as a self latching switch with no moving parts.
It has high reliability and fast switching.
It's triggered by a short gate current pulse.
Thyristors are also called Silicon Controlled Rectifiers because they are like diodes but with added switching behaviour.
For conduction to take place ...
like a normal diode, the anode must be positive relative to the cathode.
the device will not conduct until there is a short gate current pulse to "turn the device on"
once conducting, the device "self latches" and will continue to conduct with or without a gate current
the only way to turn off the device is to cause the anode current to drop close to zero
in the simulation this is done by momentarily opening S1
For triggering ...
In the simulation momentarily close S2
The gate current must exceed the minimum gate current, typically up to 20mA.
The gate voltage must exceed the minimum gate voltage, about 700mV (also like a normal diode)
The device stays triggered until the anode current drops below the holding current of about 10mA.
Gate Resistor Value
R = VR / IG VR = VS - VG
VR is the voltage across the resistor
VS is the power supply voltage
VG is the gate voltage during triggering ~ 700mV (see device data sheet)
IG is the minimum gate current ~ 0.5 mA (see device data sheet)
R = ( 12 - 0.7 ) / 0.005 = 2.2kΩ A resistor smaller than this should be used without exceeding the maximum gate current.
Thyristor Power Dissipation
P = I V A heatsink may well be needed for higher switching currents
V is the voltage across the thyristor ~ 1 Volt is typical (Many MOSFETs can do better than this)
I is the curernt through the thyristor
This is a method of shutting off a thyristor. In the simulation below,
Drag the border to enlarge the oscilloscopes.
Trigger the thyristor.
The oscilloscope traces show the potential on each side of the capacitor.
The voltage across a capacitor changes slowly unless there are hugh currents.
When you close the "Shut Off" switch, the potential on the right side drops by 12 volts from 12 to zero.
When you close the "Shut Off" switch, the potential on the left side drops by 12 volts from 1.2 to 1.2 - 12 = minus 10.8 V
This negative potential reverse biases the thyristor so it shuts off.
A few milliseconds later, the capacitor charges up again.
The thyristor will not re-trigger until there is another pulse of gate current.
Adjust the duty cycle or mark space ratio of the 400Hz source.