I have a system where there is a "master dimmer" (an 800W dimmer designed for household lighting). From that master dimmer, I have a few SSRs switching 40W incandescent light bulbs.

The idea being you can control the brightness of all of the lights with one dimmer.

However, the SSRs are only switching reliably when the dimmer is on full brightness, or set to very dim. Dimming mid way results in some lights just 'sticking' on.

Could the dimmer be messing with the zero crossing detection?

Any help would be much appreciated

Edit: this is the SSR being used

  • \$\begingroup\$ What kind of dimmer is it and how does its current curve look like? When you have that curve, think about how the ssr behaves with it. \$\endgroup\$
    – PlasmaHH
    Commented Jan 15, 2016 at 9:09
  • \$\begingroup\$ This is the dimmer (the 800W model). I can't find much data for it, unfortunately. \$\endgroup\$ Commented Jan 15, 2016 at 9:18
  • \$\begingroup\$ I'd use relays instead of SSR's to control the individual loads. SSR's typically have a minimum voltage rating for one thing, but more importantly rely on triggering from the incoming waveform in addition to the logic command. \$\endgroup\$
    – R Drast
    Commented Jan 15, 2016 at 11:17
  • \$\begingroup\$ My thinking is that a dimmed AC signal still crosses zero, which means the SSR should switch fine. Regarding the minimum voltage, even when the lights are on about 75% brightness, they still latch on. I'm on 220V, and the minimum for the SSR is 100V. \$\endgroup\$ Commented Jan 15, 2016 at 11:30
  • \$\begingroup\$ Your 'this' SSR link narrows us down to eight possible parts some of which are zero-crossing types. Which one is it? \$\endgroup\$
    – Transistor
    Commented Jan 15, 2016 at 14:02

1 Answer 1


The G3MB-202P 5V input (Zero crossing and built in snubber) is probably the problem. The data sheet says the operate time is '1/2 of load power source cycle + 1 ms max.'.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Simple SSR has zero-cross on/off switching - EDN.

This circuit demonstrates the action of a zero-cross detection circuit.

  • If \$V_{tri1}\$ is low (above but close to zero) and Q1 is turned on by photo-action from D1 then SCR1 will be triggered. This in turn will pass enough current through R6 to bring TRI1 gate voltage high enough to trigger.
  • When voltage exceeds a certain level Q2 will be biased on. The collector voltage will fall and there won't be enough to turn on SCR1.

The effect is that TRI1 can't turn on unless it is triggered close to the zero cross.

Triac waveform

Figure 2. Triac waveform.

When your master dimmer does this to the mains waveform the downstream SSRs go from zero (where there is no voltage available to trigger the triac) to 'on' where the voltage is high enough to disable triggering.

Your best option in this case is to replace with the G3MB-202PL.

  • \$\begingroup\$ Thanks for the great response! Although I'm concerned about using a non zero crossing SSR, since this circuit is near audio equipment. I'm also worried about inrush currents if the SSR is switched on at a peak in the AC wave. Will the non-zero crossing SSRs still be suitable? \$\endgroup\$ Commented Jan 15, 2016 at 17:56
  • \$\begingroup\$ Interference from an SSR will be less than that from a regular light switch as the SSR won't suffer contact bounce. Inrush current won't be any worse either - there is no zero-cross detection on a light switch. Inrush current for a filament lamp is determined by the 'cold' resistance of the bulb and most bulbs fail on switch-on rather than when warm. It probably takes a few cycles of the mains for the filament to heat up and reduce the current (to about half). Anyway, your dimmer switch is doing this every cycle! If there's no interference now there won't be with non-zero-cross SSRs. \$\endgroup\$
    – Transistor
    Commented Jan 15, 2016 at 18:17

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