I am confused by zero crossing detection of opto-triac. I know what is the zero crossing detection or switch. (ZCS)

Zero crossing detects what is the point that wave is going to positive to negative or negative to positive.

But, I cant explain function of below circuit with help of zero crossing.

Can it control the load using PWM signal at terminal named signal? And explain How can dim the light bulb using PWM signal.

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Devices such as the MOC3041 will only turn on as the supply goes through zero - this is desirable for controlling loads such as heaters or motors as it can reduce the interference they cause and avoid getting partial cycles. The load will be either fully on or fully off. Any PWM that is desired can only be performed at a very slow rate - this is acceptable for heaters where the response time may be many minutes.

The load can only be controlled for complete cycles (or half-cycles) of the AC supply - this is not what is required for controlling the intensity of a lamp by using phase control where a fraction of a cycle is desired. The lamp would flicker unacceptably

  • \$\begingroup\$ Thaks for your answer. What is the problem if we turn on heater or motor at the middle of the cycle?please explain. \$\endgroup\$
    – user_fs10
    May 10 '17 at 10:02
  • \$\begingroup\$ if you turn on after ZCS ok , but then ZCS is useless, if turn on before ZCS nothing happens until zero crossing \$\endgroup\$ May 10 '17 at 14:49

To make a light dimmer you have to use a non-zero crossing triac driver. For example, the MOC3051/52. These are sometimes called "Random Phase Triac Drivers".

You typically also need a zero crossing detector. To control the brightness you vary the time delay between each zero crossing and triggering the triac, from close to zero (for close to full power) to almost 1/2 cycle (for almost no power). This is called phase control, and it causes continuous EMI from the switching (which should be filtered) and sometimes the filaments 'sing' audibly with the current pulses.


You cannot realistically smoothly control the brightness of an incandescent bulb with a zero crossing triac driver, though you might be able to get a few discrete steps, especially if you don't care about putting harmonics on the power line- for example you could turn the lamp on or off during 3 consecutive 1/2 cycles (then repeat) giving you 0/3, 1/3, 2/3 and 3/3. And get 1/2 by turning on and off for 1/2 cycles like a diode. You might still see undesirable flicker, especially at 2/3.

PWM as you suggest can be used to control something like a slow heater or other device that has a long time constant relative to the PWM timebase, which in turn should be many AC cycles. For heaters we often use a timebase in the 2-20 second range. Obviously that would cause undesirable flicker on an ordinary light bulb.


ZCS and PWM cannot be used together. You can soft start with ZCS or soft start with increasing PWM duty cycle then adjust final duty cycle to get average voltage.

The reason for ZCS control is that light bulbs with tungsten have a cold resistance about 10% of hot and thus draw 10x the current if started at mid phase or peak voltage.

Dimmer triacs have a limited gain and the cold load resistance often has a higher dimmer threshold to get started, then you can reduce the level. Other types with higher current pulses are better for low dimmer starting.

But you cannot use a ZCS with a PWM.

I suggest a small pulse transformer and a Triac instead if you are driving a tungsten bulb and you want a smooth start. A simple bridge from AC to R divider to CMOS inverter can make an effective ZCS then use a variable one shot to adjust phase (555) then drive a small pulse transformer. This prevents heat in Triac from excessive gate current using a pulse for PWM at 100Hz or variable phase.

or start to use LEDs that are designed to be dimmable...

  • \$\begingroup\$ Thanks. What is the meaning of cold resistance? \$\endgroup\$
    – user_fs10
    May 10 '17 at 10:08
  • \$\begingroup\$ Tungsten R vs T changes from 25'C cold to 3000'K hot by R/10 to R for R=W/V^2 or V/I rms. we call this positive thermal resistance temp coefficient or PTC \$\endgroup\$ May 10 '17 at 14:46
  • \$\begingroup\$ cold = room temp for a Bulb \$\endgroup\$ May 10 '17 at 14:51

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