A thyristor is a power semiconductor that continues conducting after receiving a trigger until the current falls below its holding current value.
So far, to turn loads on and off we used transistors (BJT or MOSFET). They are very common in direct current.
To control alternating current it is convenient to use bidirectional devices like the TRIAC. The thyristor family acts like an electronic latch.
Transistors vs Thyristors Comparison
| Characteristic | Transistor (BJT/MOSFET) | Thyristor (SCR/TRIAC) |
|---|---|---|
| Current | Direct (DC) | Alternating (AC) or pulsating DC |
| Control | Linear (Amplifier) or ON/OFF | Only ON/OFF (Latching) |
| Turn-off | By removing control signal | By cutting off main current (Zero Crossing) |
| Primary use | Digital electronics, power supplies | Dimmers, AC Motors, Ovens |
The SCR (Silicon Controlled Rectifier)
The SCR (Silicon Controlled Rectifier) is the grandfather of the family. Imagine a normal Diode (which only allows current to flow in one direction) but with a third leg called the Gate.
Operation: The Latch
The SCR has a very particular personality:
Off State: Even if there is a positive voltage at the Anode, it does not conduct. It is an open circuit.
Triggering: If we apply a small current pulse to the Gate, the SCR “fires” and begins to conduct at full power.
Memory (Latch): Once turned on, you can remove the Gate current. The SCR remains on indefinitely (it has “latched”).
How is it turned off?
Unlike a transistor, you cannot turn off an SCR from the Gate. It doesn’t matter if you set the gate to 0V, it continues conducting.
The only way to turn it off is to cut the main current (Anode-Cathode) or reduce it below a minimum (
Typical use: controlled rectification, power controls, and crowbar overvoltage protection circuits.
The TRIAC (Triode for Alternating Current)
The SCR is unidirectional (like a diode), so it only utilizes half the AC waveform. To control lights and motors with alternating current, we need something that conducts in both directions.
If we put two SCRs in anti-parallel (one facing each direction) and connect their gates, the TRIAC is born.
TRIAC Characteristics
- Terminals: MT1 (Main Terminal 1), MT2, and Gate. (There is no anode or cathode because current flows in both directions).
- Bidirectional: It can conduct current whether the AC wave is positive or negative.
- Control: It is triggered by applying a pulse to the Gate (referenced to MT1).
- Turn-off: Like the SCR, it stays on until the main current disappears.
And when does the current disappear in a household outlet? At the Zero Crossing. Since the sine wave passes through 0V every 10ms (at 50Hz), the TRIAC turns off by itself 100 times per second. If we want it to stay on, we must trigger it again in each half-cycle.
Phase Control (The Dimmer)
This property of turning off when the current falls below the holding current and turning on via the gate is the basis for many dimmers and speed controls for universal motors.
We don’t lower the voltage with a resistor (it would burn up). What we do is chop off pieces of the sine wave.
Zero Crossing: The wave passes through 0V. The TRIAC turns off. The bulb is off.
Waiting (Delay): The control circuit waits for a time (e.g., 5 milliseconds).
Triggering: We send a pulse to the Gate. The TRIAC turns on instantly.
Conduction: The bulb receives electricity for the rest of the cycle until the next zero crossing.
- If we trigger early, almost the entire wave passes → Light at 100%.
- If we trigger late, only a small final portion passes → Light at 10%.
The Danger of dV/dt and the Snubber Network
TRIACs are robust, but they hate surprises. If the voltage rises too quickly (a voltage spike on the mains), the TRIAC can “get scared” and turn on by itself without anyone touching the Gate. This is dangerous if it controls a power saw.
When the load and the
- A Resistor in series with a Capacitor.
- This network absorbs fast spikes and protects the component.
NEVER connect the Gate of a TRIAC directly to a microcontroller pin! If the TRIAC fails internally, 220V could enter the pin, fry the board, pass to the USB and potentially destroy your PC.
We use an Opto-Triac (like the MOC3021 or MOC3041). This is an optocoupler (seen in Module 3) that has an LED on the input side and a small TRIAC on the output side.
- Arduino turns on the internal LED.
- The light triggers the small TRIAC.
- The small TRIAC triggers the large power TRIAC (BTA16, BT137, etc).
This keeps the logic and the mains separated within the isolation limits of the component and the board.
