DELAY

DELAY

(a)

(b)

(c)

?

?

LM3445

SNVS570L – JANUARY 2009 – REVISED MAY 2013

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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. Voltage waveforms for a simple triac dimmer are shown in Figure 13. Figure 13a shows the

full sinusoid of the input voltage. Even when set to full brightness, few dimmers will provide 100% on-time, i.e.,

the full sinusoid.

Figure 13. Line Voltage and Dimming Waveforms

Figure 13b shows a theoretical waveform from a dimmer. The on-time is often referred to as the "conduction

angle" and may be stated in degrees or radians. The off-time represents the delay caused by the RC circuit

feeding the triac. The off-time be referred to as the "firing angle" and is simply 180° -

θ.

Figure 13c shows a waveform from a so-called reverse phase dimmer, sometimes referred to as an electronic

dimmer. These typically are more expensive, microcontroller based dimmers that use switching elements other

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

of control reduces the noise spike at the transition.

Since the LM3445 has been designed to assess the relative on-time and control the LED current accordingly,

most phase-control dimmers, both forward and reverse phase, may be used with success.

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