BRIGHT

DIM

R1MAX

250 k:

DIAC

R2

3.3 k:

C1

100 nF

LOAD

AC

MAINS

TRIAC

FORWARD PHASE DIMMER

LM3450

SNVS681D – NOVEMBER 2010 – REVISED MAY 2013

www.ti.com

PHASE DIMMER OPERATION

A simplified schematic of a phase dimmer is shown in Figure 25. An RC network consisting of R1, R2, and C1

delay the turn-on of the triac until the voltage on C1 reaches the trigger voltage of the diac. Increasing the

resistance of the potentiometer (wiper moving downward) increases the turn-on delay which decreases the on-

time or “conduction angle” of the triac (

θ). This reduces the average power delivered to the load.

Figure 25. Basic Forward Phase Dimmer

Phase dimmer voltage waveforms are shown in Figure 24.

Figure 24a shows the full sinusoid of the input voltage. Even when set to full brightness; few dimmers will provide

100% conduction angle.

Figure 24b shows a waveform from a forward phase dimmer. The off-time can be referred to as the firing angle

and is simply 180° –

θ.

Figure 24c shows the waveform of a reverse phase dimmer (also called an electronic dimmer in the lighting

industry). These typically or more expensive, microcontroller based dimmers that use switching devices other

than triacs. Note that the conduction angle starts from the zero-crossing, and terminates some time later. This

method of control reduces the noise spike at the transition.

Any form of phase dimming modulates the incoming AC waveform by chopping part of the sinusoid, reducing the

average power to the load. These dimmers work very well with standard incandescent bulbs, but not with power

converters. A converter attempts to regulate the load in with presence of any input, effectively ignoring the phase

angle. To implement a dimmable converter, the angle must be sensed at the input, decoded and used to properly

control the LED current regulator.

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