Power output modulation / dimming

AC

Thyristor (aka SCR):

  • current flow only in one direction (only positive or only negative)
  • The voltage on the other half waveform stays to ~ zero.
  • Max duty cycle 50%

TRIAC

  • current flows in both directions
  • Better to use opto TRIAC
  • Max duty cycle 100%

Thyristor / TRIAC control methods

  • zero cross trigger
  • random turn ON

Zero cross trigger

  • switch ON / OFF only occurs when zero crossing
  • When zero crossing:
    • Signal on gate is ON: output is ON until the next zero cross
    • Signal on gate is OFF: output is OFF until the next zero cross

Random turn ON trigger

  • switch ON occurs any time
  • switch OFF occurs when zero crossing

GTO (gate turn off) thyristor

  • Random turn ON AND turn OFF
  • It can be switched off with a trigger on the gate.

Modulating power on AC

  • Random turn ON TRIAC: any duty cycle from 0 - 100%
  • zero cross detector: when is the right time to send signal on the gate?
  • timer capture compare:
  • start on zero cross
  • set comparator to the duty cycle desired (AC frequency is well known, 50 or 60Hz)
  • Automatically turns ON signal on gate when reaching the desired D.C.

DC

MOSFET (e.g. IRF540N) + PWM