Mixing AC and DC power in a circuit

Posted on Feb 6, 2014

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 conne

Mixing AC and DC power in a circuit
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cting 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. 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 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 Passerby Apr 2 `13 at 4:19 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. QuadrupleA Apr 2 `13 at 4:29 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. HikeOnPast Apr 2 `13 at 5:20 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...

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