TRIAC Delay Angle and Power Dissipation
TRIAC’s delay angle (also called the firing angle
α) controls power by “cutting” part of the AC sine wave
before allowing current to flow.
In AC mains, the voltage waveform is sinusoidal:
A TRIAC stays OFF at the start of each half-cycle.
After a delay angle α, it is triggered ON and conducts
for the rest of that half-cycle.
Effect of Delay Angle
- Small delay angle → more of the sine wave passes → more RMS voltage → more power
- Large delay angle → less of the sine wave passes → lower RMS voltage → less power
The output waveform becomes a chopped sine wave.
Waveform Examples
- α = 0° → full sine wave passes
- α = 90° → first half of each half-cycle removed
- α = 150° → only a small tail passes
Power Equation
For a resistive load:
Because phase control reduces RMS voltage, average power decreases.
RMS Voltage Formula
Where:
- Vm = peak AC voltage
- α = firing angle in radians
Example
Suppose:
- 230V AC heater
- Resistive load
Delay angle = 0°
- Full waveform applied
- Vrms ≈ 230V
- Maximum heating power
Delay angle = 90°
- Only later half of waveform conducts
- RMS voltage drops
- Power becomes roughly 50%
Delay angle near 180°
- Very little conduction
- Tiny RMS voltage
- Almost no power
TRIAC Internal Power Dissipation
The TRIAC itself also dissipates heat.
When ON, it has a voltage drop of about 1–2V:
As firing angle increases:
- Load power decreases
- Load current decreases
- TRIAC heating usually decreases
Side Effects of Phase Cutting
- Harmonics
- EMI / electrical noise
- Poor power factor (especially with motors)
Applications
- Lamp dimmers
- Heater controllers
- Fan regulators
These devices work by phase-cutting the AC waveform using a TRIAC.