I want to actuate a light simultaneously whenever a triac based fan controller switch is on at any setting.

One idea I had was to use a 120VAC coil relay connected to the fan controller output. I only scrath my head cause I don't know if the modified waveform from the triac controller will keep the coil energized no matter the setting? If I had to guess I'd say yes because the max voltage remains 120v at 50/60hz, even if the rms voltage changes.

I suppose I could just buy a relay and give it try but thought I would ask here b4 I go to the effort.

As a clear example I would use this controller with this relay. https://www.amazon.com/Broan-72W-Electronic-Variable-Control/dp/B000UVWWGO https://www.amazon.com/American-Zettler-Power-120VAC-AZ2280-1C-120A/dp/B07587NZTC

the relay contacts at 30amps is overkill for a led light power supply but couldn't find a cheaper one with lower rated contacts (probably only need an amp at most)

alternatively maybe I can just hook up the controller output right to the led light power supply. Maybe the modified wave form won't affect the DC output voltage (12VDC I think) of that power supply.

  • 1
    \$\begingroup\$ The max voltage of a trial fan speed controller does not stay constant and a relay probably won't work. A power supply is not very likely to work either. I don't have a good suggestion, but there is a good chance that someone else will. You man need to wait 24 hours or longer for someone to provide a suggestion. \$\endgroup\$
    – user80875
    Aug 18, 2020 at 23:28

1 Answer 1


This sounds like an interesting project - I'd start by looking at the waveform of the triac to see exactly what you're working with, then identify nearby power sources (or if you're willing to go with battery power, wall wart, etc.).

A triac controller essentially cuts out pieces of an AC waveform. At very low settings, the signal basically is just 'burping' a few volts (depending on the controller I think this can be a pretty low voltage - I'm certainly no expert on all triacs), then being off for the rest of the cycle. So what you need is a way to keep the coil energized - as you said.

If you have access to the full AC waveform (ie before the triac cuts it up), then you can use a transformer to reduce the voltage, rectify to DC & use a relatively simple circuit to watch for those pulses -- depending on how you like to build things, this could be a microcontroller project or an analog circuit build. Since I'm not sure if you have this option, I won't take the time to go into that in detail.

One way to do this if you don't have that access would be to add a rectifier circuit (series diode + capacitor to neutral), rectifying the output of the triac to DC and providing a temporary power reservoir for the coil to draw from. The down side of this is that the higher the setting of the triac, the higher the voltage on the capacitor.

Depending on your relay, this might work, but I would think that it would either (a) only work at higher settings (more voltage), or (b) blow up at higher settings (too much current). To limit power to the relay coil, you can add a constant current circuit or just a resistor.

  • \$\begingroup\$ so what you are suggesting is rectify the output and use the capacitor to "fill in" and then use a DC coil relay instead?. I have a bunch of 5VDC coil relays and the coil only needs ~100mA. If the input DC voltage may vary (depending on the setting) is there a voltage regular IC that will always output 5vdc and handle 100mA. \$\endgroup\$
    – DKebler
    Aug 19, 2020 at 15:45
  • \$\begingroup\$ Yes, that's what I was thinking.If the coil draws 100mA it would be quite difficult to use a capacitor in this manner (it would have to be massive). Perhaps it was a bad idea.... \$\endgroup\$ Aug 19, 2020 at 22:11

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