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I have an application that requires a transformer with 120VAC 60Hz on the primary and 7VAC/11VAC (200mA) on the secondary. Unfortunately, while this part is available in some countries I have not been able to get one shipped to the US (so far).

This ebay listing shows a transformer which converts 115VAC to 14VAC and 7VAC (250mA), which is very close (and the current rating is appropriate), so in theory I just need an additional step-down to get from 14 to 11VAC. I don't know the input tolerance of the board that uses the 11VAC otherwise I would try this directly since I assume it's just regulated to some DC value.

Unfortunately, the step-down turn ratio (~1.27:1) is not so easy to find, and the closest I've located so far is this digi-key component, but it's intended for DC-DC SMPS.

I'm guessing that this would not have the right properties to work for my AC application, so I was hoping someone would please comment to confirm, and if there is a good alternative for what I'm trying to do.

Edit: Sorry I wasn't clear before, but this is a one-off repair, so I was looking for a close replacement for the part I couldn't source. Both the 11VAC and 7VAC outputs are needed on the secondary, or an equivalent.

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  • \$\begingroup\$ find a 120 Vac to 12 Vac linear wall wart (power adapter) ... it may work ... duckduckgo.com/… \$\endgroup\$
    – jsotola
    Commented Jun 13, 2022 at 2:38
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    \$\begingroup\$ Perhaps separate 7V and 11V transformers? Or a 7V and a 4V in series? Though I'd be tempted to run the whole works off of a stereo audio amplifier with such low current requirements. \$\endgroup\$
    – Bryan
    Commented Jun 13, 2022 at 2:54
  • \$\begingroup\$ What volume do you need? Custom transformers are very common unless the volume is very low and an off the shelf transformer fits the bill. \$\endgroup\$
    – winny
    Commented Jun 13, 2022 at 9:09

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What you want is two transformers connected in a buck or boost circuit on the secondary. Take a 14V output and subtract the 3V output from another transformer, by connecting the windings anti-phase in series. That’s a buck circuit. Onto that output add another anti-phase 3V winding in series and you get 7V.

If you got a 14V output, then two small Nokia cellphone wallwart transformers like ACP-7U producing about 3.7V rectified would do it I think. You’d step down from 14V twice. These transformers can be tweaked for lower output voltage by unwinding a couple turns from the accessible secondary if needed, and the rectifier can be bypassed as well.

ACP-7U are Handy little things can put out 350mA and nicely power 3.3V MCU projects through an LDO (an aside).

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  • \$\begingroup\$ Thank you for the suggestion. If I'm understanding this right, then the primaries are all connected to 120VAC and by chaining the secondaries and choosing the right sets of wires you get the required boost/buck voltages? That makes a lot of sense! I was thinking everything had to be chained from input to output to input to output... \$\endgroup\$
    – Mandias
    Commented Jun 13, 2022 at 16:27
  • \$\begingroup\$ Yes, primaries are all powered from mains and you just stack output voltages like if you were connecting batteries together - except AC. Still works the same: connect a 2nd “battery” (winding) backwards and it will subtract the voltage and so on. If you connect several “batteries” in series, it will provide you with voltage taps as expected, but in AC. \$\endgroup\$
    – Kuba hasn't forgotten Monica
    Commented Jun 13, 2022 at 16:51

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