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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 ...
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like a normal diode, the anode must be positive relative to the cathode.
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the device will not conduct until there is a short gate current pulse to "turn the device on"
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once conducting, the device "self latches" and will continue to conduct with or without a gate current
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the only way to turn off the device is to cause the anode current to drop close to zero
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in the simulation this is done by momentarily opening S1
For triggering ...
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In the simulation momentarily close S2
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The gate current must exceed the minimum gate current, typically up to 20mA.
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The gate voltage must exceed the minimum gate voltage, about 700mV (also like a normal diode)
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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
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VR is the voltage across the resistor
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VS is the power supply voltage
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VG is the gate voltage during triggering ~ 700mV (see device data sheet)
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IG is the minimum gate current ~ 0.5 mA (see device data sheet)
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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
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P = I V A heatsink may well be needed for higher switching currents
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V is the voltage across the thyristor ~ 1 Volt is typical (Many MOSFETs can do better than this)
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I is the curernt through the thyristor
Commutating Capacitor
This is a method of shutting off a thyristor. In the simulation below,
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Drag the border to enlarge the oscilloscopes.
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Trigger the thyristor.
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The oscilloscope traces show the potential on each side of the capacitor.
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The voltage across a capacitor changes slowly unless there are hugh currents.
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When you close the "Shut Off" switch, the potential on the right side drops by 12 volts from 12 to zero.
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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
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This negative potential reverse biases the thyristor so it shuts off.
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A few milliseconds later, the capacitor charges up again.
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The thyristor will not re-trigger until there is another pulse of gate current.
Light Dimmer
Adjust the duty cycle or mark space ratio of the 400Hz source.