This is likely a noob question, apologies if the answer is obvious.

I'm trying to control a furnace using a 3.3V microcontroller. The furnace operates by connecting a common 21V AC "hot" line to one of two AC "neutral" output lines: Fan and Heat. When connected they have relatively low current flow, ~100mA.

I've had some success using a TRIAC to do the switching - but I'm directly mixing the power sources by connecting the DC battery negative to the common AC "hot" line. Battery negative/hot then becomes the reference at the TRIAC's MT1, and I can switch the furnace on and off by sending +3V or 0V to the TRIAC base.

A simplified schematic is below - SW1 would be in reality a microcontroller output pin with a current limiting resistor.


simulate this circuit – Schematic created using CircuitLab

Question is, is it kosher to mix power sources like this? Do I risk an exploding battery, excessive battery drain, weird circuit behavior or anything like that? If so, what would be the proper way to do this, to isolate the circuits?


  • \$\begingroup\$ I have no clue, but google shows some circuits setup in this manner. I'll let someone else give an answer to that. My question to you though, why no go for isolation and use a opto-isolated ssr or triac, and not bother mixing them at all? \$\endgroup\$ – Passerby Apr 2 '13 at 4:19
  • \$\begingroup\$ Don't know too much about the opto-isolated options out there - photo-triacs and SSRs look promising though, thanks for the tip. The few SSR's I had come across before were all high-voltage types. \$\endgroup\$ – QuadrupleA Apr 2 '13 at 4:29
  • \$\begingroup\$ Many household-grade thermostats use low holding current mechanical relays for this purpose. I've always been amazed that they can maintain several years of battery life with this approach, but they do. I'd expect a triac would be cheaper, so perhaps there are surge or mis-wiring risks inherent in such applications. \$\endgroup\$ – HikeOnPast Apr 2 '13 at 5:20
  • \$\begingroup\$ Yeah, had the same thought - my home thermostat uses mechanical relays but has somehow lasted 2+ years on a couple AA's. \$\endgroup\$ – QuadrupleA Apr 2 '13 at 6:28
  • \$\begingroup\$ Where do both power supply voltages come from? Are they floating, galvanic separated? \$\endgroup\$ – jippie Apr 2 '13 at 6:52

Some triacs, when controlled from a unipolar DC to the gate with respect to MT1, won't always conduct when the AC cycle goes negative i.e. they'll trigger during the positive cycle and reset as the voltage falls through 0V and won't retrigger again until the next positive cycle. The 2N6071 I have used before and this was OK - with unipolar DC control on gate the triac was basically equivalent to a closed contact.

Gate current needed to activate (say) the 2N6071A will need to be in excess of 10mA so this is a consideration to how you drive the gate from a MCU pin (quadrant IV is always the worst culprit for triggering with any triacs it seems). 2N6071B needs 5mA to guarantee conduction 360º.

If there is any doubt about the AC voltage possibly being at unsafe levels (i.e. connected to incoming main AC power) I'd use an opto triac circuit to isolate. BUT, with such a small working AC voltage (21Vac) trying to "manufacture" a gate drive circuit that "steals" power from the AC means volts dropped in driving the load.

So, given all this I'd probably go for a relay drive circuit unless you are totally happy that the AC you are switching is reasoably isolated from the incoming main AC

  • \$\begingroup\$ Interesting, thanks. So I guess any "opto" switching method is going to draw a little voltage from the AC circuit - not sure if that'd interfere with the furnace control or not, don't know much about it's internal wiring. I know there's a 3A fuse. Would the risk of a non-isolated circuit mainly be frying the MCU in the event of a surge? \$\endgroup\$ – QuadrupleA Apr 2 '13 at 10:48
  • \$\begingroup\$ Frying the MCU depends on whether your local AC low voltage supply can be tied to earth on the line you call HOT. Trying to avoid the MCU having to be floated up to some "strange" AC voltage that may somehow be linked to the incoming AC supply to the furnace \$\endgroup\$ – Andy aka Apr 2 '13 at 11:18

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