Ok, this feels like a stupid question, but my google-fu is not with me today.

I have an application in which I have a need for both 12V DC (or thereabout) and 24V AC.

I have a transformer that has two windings of 12V each. In the schematic below I have one winding connected to pin 1 and 2 and one winding to pin 3 and 4 of J3.

The first winding (1,2) is connected to the bridge rectifier and provides the DC voltage.

Connector J7 is connected to J3 pin 1 and 4 and provides the 24V to some other part of the device.

The thing is, most examples on the internet I can find about dual winding transformers make the center tap a ground and use it to provide both positive and negative DC thru the rectifier, but I do not need a negative voltage rail. To mee it seems this should work to. But because my electronics knowledge is rusty and I cannot find an example of this, I'm doubting. Am I missing a ground connection or something other reason why this won't work?

I could of course just build it, but I am away from home and do not have acces to my parts bin. So I someone could just say yes or no (in case of no please do correct me.)



1 Answer 1


This is more-or-less correct. If you put a capacitor on the output of the bridge it will charge to about \$12V\cdot\sqrt{2} - 1.4V\$ or about 15 or 16VDC. Any ripple will subtract from that.

Note that there is no common ground between the 24VAC and 12-16VDC.

Also, you are loading one side of the transformer more than the other so you won't get the full available VA out of it.

  • \$\begingroup\$ Thanks, but more questions: You say, "more or less correct", Is there a better way to connect things? I use the 12v to drive some dc-dc step down circuits and that is easier with a lower voltage. Also no common ground between 24VAC and (i think you meant) 12VDC. Is that a problem? The 24VAC is going to an completely isolated circuit. \$\endgroup\$
    – zu1b
    Apr 6, 2021 at 9:00
  • \$\begingroup\$ You could also ground the center of the two windings and use two diodes (or use half of the bridge), which would give a bit higher output voltage (by one diode drop). Probably similarly inefficient use of the transformer core. The lack of a common ground is not a problem if it is not required, just pointing out that it might make things more difficult in some circumstances- for example if the DC circuit was called upon to switch the AC side with a triac rather than a relay. \$\endgroup\$ Apr 6, 2021 at 11:58
  • \$\begingroup\$ Thank you for the clarification. So from your comments this is about the "best" possible option given the specified need for 24VAC and 12VDC on a dual 12V winding transformer. The better option (more efficient) would be having a transformer with both a 24VAC and 12VAC winding. The voltage drop of the bridge is a non issue as the DC voltage will be further converted down to 5 and 3.3VDC. The board is a replacement and I am stuck with this transformer. \$\endgroup\$
    – zu1b
    Apr 6, 2021 at 14:09
  • \$\begingroup\$ Yes, about the best. Suggest you ensure that the transformer is adequate though before settling on this approach. Also DC supply efficiency will be very bad (20-30%) unless you use a switching regulator \$\endgroup\$ Apr 6, 2021 at 14:15
  • \$\begingroup\$ I am using switching regulators, adafruit has several open designs I can use as examples like those with mpm3610. The IC's require a minimum of external components and seem efficient enough. \$\endgroup\$
    – zu1b
    Apr 6, 2021 at 14:28

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