Controlled Rectifiers (SCR)
In the previous topic you saw that switching beats linear for efficient DC regulation. But most power starts as AC from the mains — before you can regulate, you must first rectify. A basic diode bridge gives you fixed DC, but what if you want to control how much power reaches the load?
That is where the controlled rectifier comes in. By replacing diodes with SCRs (Silicon Controlled Rectifiers), you choose exactly when each half-cycle begins conducting. The SCR is the oldest power semiconductor switch — a thyristor that latches on with a gate pulse and turns off only when current reaches zero. Like a switching regulator, it controls power by switching rather than burning it as heat.
How an SCR Works
The SCR behaves like a diode that stays off until you tell it to conduct. A short gate pulse triggers it, and the device latches on until the current naturally falls to zero.
- An SCR (Silicon Controlled Rectifier) is a diode with a gate. It blocks current in both directions until a gate pulse triggers it.
- Once triggered, it latches ON — current flows from anode to cathode. It turns off only when the current drops to zero (natural commutation).
Firing Angle α
The firing angle α is the delay (in degrees) from the zero-crossing of the AC input to the moment the gate pulse fires. It directly controls how much of each cycle reaches the load.
- At α = 0°, maximum output. At α = 180°, zero output.