Electronic – What type of transformer should I use to convert 24VAC to 12VAC (or vice versa)

power supplytransformer

I'm working on a project that combines a board from an electric organ with one of those cheap Chinese tube amps. The board requires +12V/-15V DC and the amp requires 12VAC.

I don't want to power the final product with two wall warts, because that would be fiddly and annoying.

The amp converts its 12VAC to ±30VAC internally, and my first thought was to power the amp off a 12VAC wall wart, then tap off the ±30 on the amp's board and power a 7812 and 7915 off of this (rectified first of course). But connecting them up causes a huge voltage drop, so it seems that this isn't going to work.

The current idea I'm working with is to power the whole project off a 24VAC wall wart, which will run the 7812/7915 combo fine, and also can step down at 2:1 to 12VAC for the amp.

My problem is that I can't find a 2:1 transformer suitable, but it seems impossible that such a thing doesn't exist. (The only 2:1 transformers I can find are huge, built to take 240V to 120V. This would work in principle but it would bring up the size/weight of the project by at least an order of magnitude which isn't ideal, to say the least.)

I bought some of these 78604-1C but they fry on 12VAC+ input. (The datasheet I have isn't forthcoming about a maximum voltage, which I am also confused about)

  • Is there a specific thing I should be searching for that will turn my 24VAC to 12VAC in a small package?
  • Or, is there some other way of solving this problem that explains why this type of transformer is so hard to find?

Best Answer

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) 12 to 24 transformation without isolation. (b) Fully isolated 12 to 24 V transformation.

Transformers are very flexible and versatile.

My problem is that I can't find a 2:1 transformer suitable, but it seems impossible that such a thing doesn't exist.

You should be able to find a dual 120V:12V transformer quite easily and use this with the mains side disconnected. If you feed 12 V in on the secondary you will get mains voltage on the primary but you just need to make sure that it is properly isolated. i.e., You don't have to use it.

In Figure 1b I have sketched just one of many arrangements that are possible depending on the transformers available. Here we are paralleling the 12 V "primaries" and series connecting the "secondaries".

Watch your VA rating overall. Also watch your winding current and make sure you don't exceed the design value.