Working on a design where power supply (on the PCB) is 12VAC to full wave rectifier to 5VDC linear regulator.

Customer switch box is also power via same 12VAC source and will be switching that 12VAC back to my device (retro fit application, must switch the 12VAC). I am looking to use least amount of components. Therefore was thinking I’d half wave the switched 12VAC from customer supplied box and use inverter IC (we stock this so it’s ideal) to interface to my microcontroller.

I am looking for feedback regarding my half wave circuit interfacing to the full wave common (10uF and ULN2804). Have bread boarded this and all looks good. I’d just like verification that the only use of the 12VAC common (not including power supply) is to power the customer supplied device and should not be referenced to any of the input circuit. More or less just want opinion from others who may have more experience with this.

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Your scheme will probably work. You should be aware that the AC ground and the DC ground are at very different and alternating potentials.


simulate this circuit – Schematic created using CircuitLab

Figure 1. The same circuit drawn three times but (b) and (c) have the reverse-biased diodes removed for clarity.

As shown in (b) and (c) the DC ground will vary in voltage between +0.7 V and -12√2 + 0.7 V with respect to the AC ground. You need to be sure that this won't upset your requirements.

  • \$\begingroup\$ Just out of curiosity, what is the purpose of that 1uF capacitor before the bridge? \$\endgroup\$ – Maple May 14 '18 at 19:28
  • \$\begingroup\$ Don't ask me! It's strange. It's polarised and it's rated at 250 V on a 12 V AC section of the design. There's also a 35 V varistor. They must be expecting trouble. \$\endgroup\$ – Transistor May 14 '18 at 20:35
  • \$\begingroup\$ With that cap they are not "expecting" trouble, they asking for it :). Varistors at least I can understand, they are there to help blow the fuse, I think. \$\endgroup\$ – Maple May 14 '18 at 22:02
  • \$\begingroup\$ I'm in the early stages of the design and just put together (from other existing designs) some screen shots to formulate my question. It got the point across didn't it? \$\endgroup\$ – becjasl May 17 '18 at 17:07
  • \$\begingroup\$ NOTE the 1uF is a Metallized Polyester Film Capacitor. \$\endgroup\$ – becjasl May 17 '18 at 17:49

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