I built a circuit (like the one below) to control an incandescent light bulb from an Arduino controller for the digital input.

enter image description here

I did not use the zero crossing part of the circuit and mine is running on 120V and a 220 ohm resistor for the triac. My program turns the triac on and off making it dim but also flickers which is an effect I am looking for cause this will be used to simulate a fire. Works great!

My problem happens when I have it plugged into a computer UPS. When I unplug it, and it switches to battery use, the light bulb switches on and stays on. (I swear I had tested this and it didn't do that before.) The circuit works perfectly if I put the UPS back online. The output of the UPS isn't the nice sine wave I expected it to have for a computer back up and, instead, looks like the output of an inverter I tried before; very spiky. I also measure the voltage dropping to around 90V with a meter.

The whole purpose of this is to make this circuit be portable inside a bucket with a UPS powering it and now I'm stuck and running out of time. Does someone have a fix and possibly a lesson in the process? (I've been out of electronics for a while and not a power electronics guy in the first place.)

EDIT: It is not necessary for me to use this circuit exactly. I once thought of using a FET and driving the bulbs directly from an inverter but my UPS idea solved the problem of charging the battery. But I have the same problem in that I have not done any circuit design in over 10 years.

  • \$\begingroup\$ Why oh why are you showing a circuit that you only partly used. Why not just show the circuit you used. Everyone is lazy now and then I suppose. What is the bucket thing about? Is it meant to work submerged? If so, what is the fluid? \$\endgroup\$
    – Andy aka
    Oct 19, 2013 at 23:07
  • \$\begingroup\$ @Andyaka Because that circuit was available to show and I don't have an editor to create one where I'm at. I AM using the bottom half of the circuit. As I said, it's to simulate fire in a bucket someone will be carrying on stage. I'm panicking a bit as I just found this problem and it's due Wednesday. \$\endgroup\$
    – Rob
    Oct 19, 2013 at 23:16
  • \$\begingroup\$ @Rob what is the power rating of the AC lamp and power rating of UPS,battery spec \$\endgroup\$
    – yogece
    Oct 20, 2013 at 5:30
  • \$\begingroup\$ @yogece I have two UPSes. One is 550VA and the other is 700VA. The light bulbs are two, 60W each, though I may go lower to 40W. \$\endgroup\$
    – Rob
    Oct 20, 2013 at 5:34
  • \$\begingroup\$ @Rob can you come to chat room chat.stackexchange.com/rooms/15/electrical-engineering \$\endgroup\$
    – yogece
    Oct 20, 2013 at 5:46

1 Answer 1


Your UPS likely has a square (non-sinewave) output. The square output likely exceeds the dv/dt rating of the triac, causing it to turn on. You might want to consider a snubbing network to reduce the dv/dt transients.


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