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I have a device that requires 9V AC and rated for 2.1 amperes. I thought I could modify the output of either of the following voltage supplies to meet those values:

  1. A power transformer that outputs 24V, 50VA

  2. A power supply wall-wart - output 15VAC, 1.5 amperes

Is there a simple way to do this - any schematics/component values possible?

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    \$\begingroup\$ A 24V to 9V transformer, or a 15V to 9V transformer. But they will be about the same price as a mains to 9V transformer and harder to find... \$\endgroup\$ – Brian Drummond Feb 11 at 18:58
  • \$\begingroup\$ Thanks, it is a hassle to find them at good cost. \$\endgroup\$ – Michael Van Wagner Feb 12 at 19:14
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The component you're looking for is.. another transformer.

Are you looking for "9v a.c" RMS? Peak-to-peak? What does the input to the device you're trying to power look like (electrically)?

It's not trivial to adjust the voltage of an AC signal after transformation; but it's generally pretty easy to simply get a new transformer.

You can try disassembling the power supplies and checking if they have an additional tap that happens to provide the voltage you're looking for (which is rare in low-cost parts; but you can often salvage multi-tap transformers out of other equipment!), or simply replace the transformer inside with one that has the correct turns ratio.

Alternatively; depending on how long you need such a solution; you could also simply power either existing power supply with an Autotransformer; (A device that simply allows you to "tap" the output anywhere along the coil, giving you a variable turn ratio transformer) to step down the voltage going into the power supply before it steps the voltage down again. (This is NOT efficient; but for testing and temporary use; would also allow you to convert the incoming power to the output you're looking for.)


If you're feeling very adventurous (I'd HIGHLY recommend against this unless you really know what you're doing!) you can remove some turns from the secondary side of the transformer to get closer to the voltage you expect (though you WILL de-rate the total power capacity quickly!) Depending on the rest of the power supply; any AC supply should run at a lower voltage without a problem.

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  • \$\begingroup\$ Autotransformers can be very efficient. Variacs are up to 98% \$\endgroup\$ – Phil G Feb 11 at 19:36
  • \$\begingroup\$ Sorry; the "This is NOT efficient" line was not intended to point out the Autotransformer as the SOURCE of loss; but the idea of chaining transformers to circumvent replacing a single one. The losses in the (likely) inexpensive power supply will only be compounded by decreasing the input voltage; as many of the components in the secondary power supply will have been designed with a fixed set of conditions in mind. \$\endgroup\$ – JustinDavidow Feb 11 at 20:30
  • \$\begingroup\$ thanks for the info! \$\endgroup\$ – Michael Van Wagner Feb 12 at 19:10
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Use the secondary output from the 15 volt AC supply in antiphase series with the secondary output from the 24 volt AC supply and, you should get the difference. That difference is 9 volts AC.

Well, it’s worth a try anyway. Make sure you test with a meter first and leave it to soak on load in a safe area for a while. If you measure 39 volts just reverse the secondary connections on one of the transformers.

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  • \$\begingroup\$ Thanks for the info! \$\endgroup\$ – Michael Van Wagner Feb 12 at 19:11
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If you have a split-bobbin transformer (primary and secondary on separate segments of the bobbin) you could consider removing the existing secondary and winding a new one. You would probably need something of the order of 35 turns, but you would want to measure the volts per turn and base the number on an actual measurement. Gauge would probably be something like AWG 18 or equivalent parallel strands of thinner wire. Ideally you would try to mostly fill the winding window but if you're going way down in VA rating from the original that's not essential.

If it can't be done without removing the E-I laminations, it's probably not worth the hassle.

Of course it's important to maintain proper clearance between primary and secondary for safety and it would best be lacquered afterwards to prevent hum, and taped for protection.

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  • \$\begingroup\$ Thanks for the info! \$\endgroup\$ – Michael Van Wagner Feb 12 at 19:13
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Best is to buy a new 9v transformer. If you are adventures and spoiling one of the available transformer is no problem then use the 24v transformer, because its wire gauge will give you the required current rating, and start removing few turns at a time of the secondary winding and measuring the voltage every time till you get the required 9v. Even if it is not split bobbin or side by side winding, because normally the secondary winding is always on the top.

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