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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?
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    \$\begingroup\$ You mention "a huge voltage drop" - have you checked the current capability of the relevant components? \$\endgroup\$ – Andrew Morton Sep 23 '17 at 19:29
  • \$\begingroup\$ @AndrewMorton I haven't done the exact calculations, but I do assume that the drop means I'm exceeding the current capability of the amp's circuitry; hence wanting to change tack and power the amp and board separately from 24VAC, rather than power the amp from 12VAC and then power the board from the amp. \$\endgroup\$ – buildsucceeded Sep 23 '17 at 19:32
  • \$\begingroup\$ You can't use 7812 with AC \$\endgroup\$ – HandyHowie Sep 23 '17 at 19:32
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    \$\begingroup\$ Your pulse transformers may be happy on 12VAC ... as long as it's at about 100kHz, not 50/60Hz. Their primary inductance is stated, and way too low to protect them at low frequencies - no wonder they fried. \$\endgroup\$ – Brian Drummond Sep 23 '17 at 20:16
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    \$\begingroup\$ Have you add a smoothing capacitor after the bridge? That should bring the (average) DC voltage to around 18V. Or gain (slightly) more voltage with a full-wave rectifier, center-tapped design - although that needs a more expensive tranformer. \$\endgroup\$ – F. Bloggs Sep 24 '17 at 15:24
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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.

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  • \$\begingroup\$ Something like uk.rs-online.com/web/p/pcb-transformers/1213841? \$\endgroup\$ – buildsucceeded Sep 23 '17 at 20:06
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    \$\begingroup\$ That's the idea. Note that on that one each secondary is rated at only 3 VA. You have to do the maths. See also their good old-fashioned versions. \$\endgroup\$ – Transistor Sep 23 '17 at 20:12
  • \$\begingroup\$ There used to be some transformer kits - typically with a 120 / 240 primary and the secondary bobbin left empty, laminations, frame and bolts. RS used to sell them but don't seem to anymore. If you could find one of these you could cut the primary windings out and make your own 12 / 24 transformer. \$\endgroup\$ – Transistor Sep 23 '17 at 20:23
  • \$\begingroup\$ I am not an expert, but isn't this going to get you the exact same result as simply removing XFMR1 from the schematic, and just connecting the mains to XFMR2? It'll require XFMR2 to have a higher VA rating, but it's (A) simpler, and (B) doesn't have a couple of unconnected wires with mains voltage on them. Point (B) is less of an issue in the US, but having grown up in the UK, and having had exactly one 240 VAC shock from the mains in my lifetime, I have a very healthy respect for mains voltages, \$\endgroup\$ – dgnuff Jul 8 '18 at 8:33
  • \$\begingroup\$ Thanks for making me look at my answer again. There was an error in Figure 1b which I think I've fixed. It's most of a year since I answered this so I don't remember my thought process. I did point out that the mains terminals have to be properly isolated. \$\endgroup\$ – Transistor Jul 8 '18 at 9:03
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If you find a transformer for power grid voltage with two secondary winding for 24 and 12 V AC, you may use it as a 24 to 12 V transformer. Just leave the higher voltage winding unconnected and use the two former secondary windings as new primary and secondary.

A 240 to 120 V transformer could be used as a 24 to 12 V transformer only for very low currents. It has too many windings (10 times too much) and the wire diameter is much to small.

The best solution is to use one single transformer only with secondary windings for both 24 and 12 V AC. You get both needed voltages from that single transformer.

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