Transformer Isolation Question

[LyleCaldwell]

I am trying to get 9vdc to a tuner on my pedalboard using the 9vac supply going to my Axess FX1.  I have the means to convert to 9vdc, but I need to isolate the ground between the tuner and the FX1, and I'm not sure A) whether a transformer would go before or after the schematic below (I think before) and B) what transformer to use and how to wire it up.  I did read RG's excellent article on transformers, but it raised questions I didn't even know existed before reading it.  But that's part of the fun.

Anyway, here's my little voltage doubler/regulator circuit:

[R.G.]

Clever application, very much like one of my background projects.

A) whether a transformer would go before or after the schematic below (I think before)

It goes before. You have AC at the input, but only DC at the output, so the transformer has to go where there is AC; transformers work on AC and die on DC.

B) what transformer to use and how to wire it up.

In the words of the bard - aye, there's the rub. You need a 60Hz 9Vac to 9Vac transformer with some modest power ability.

To the best of my knowledge, there is no transformer made commercially that does that. They're all intended either for 120V ac power use, output transformer use, or low power audio use.

I've been messing with the little Xicon series trying to come up with a 9Vdc to 9Vdc isolator, chopping the 9Vdc into AC and running it through a transformer to make 9Vdc after rectifiers/filters/regulators. I've been trying to creatively misuse several transformers to do this, with some limited success. The most promising seems to be audio distribution transformers - the so-called constant voltage audio distribution devices. But there is nothing directly intended for this.

The right transformer for it is trivially easy to wind on your own, but you can't buy them.

You know, it struck me that you can buy them just as I started to hit the "post" key.

Get a small power transformer with at least two 10Vac secondaries. The little flat pack things would work. Insulate the nominal 120Vac primaries well - they'll still be at over 100Vac. The power one secondary from your 9Vac, connect the other secondary to the input of your little regulator. It's big, expensive, and heavy, but it should work.

[LyleCaldwell]

Thanks.  But the circuit needs two AC inputs, doesn't it?

[R.G.]

Yes - and a transformer gives it to the circuit. The two AC inputs are the two ends of the transformer secondary.

[LyleCaldwell]

I thought you meant connect one secondary to the circuit VIN and the other secondary to the 9vac supply.

So I'm completely unsure how you mean for me to connect the transformer.  I don't mean to be obtuse.

[R.G.]

Sorry. I'm not being clear.

I thought you meant connect one secondary to the circuit VIN and the other secondary to the 9vac supply.

Yes, I did. I proposed getting a commercial transformer with three or four windings, 120:120:9:9 Volts. Then ignoring the two 120V windings, and using only the 9Vac windings. The nominal primary of a transformer is always where the power goes in. The secondary of a transformer is always where the power comes out. By driving one 9Vac winding with 9Vac, that winding that used to be a "secondary" is now a low voltage "primary".

I think I got confused at:

But the circuit needs two AC inputs, doesn't it?

You have 9Vac available from an external source, your Axxess FX1. I took it that you wanted to make that 9Vac into an isolated 9Vdc.

To do that, you need to transformer isolate the circuit you have shown in the drawing. My proposal was to use a 9Vac to 9Vac transformer, and to get that by abusing a 120:120:9:9 vac  commercial transformer, using only the two 9Vac windings and ignoring the 120Vac ones.

From there, you are correct. One 9vac winding goes to the circuit Vin, the other goes to the 9vac from the Axxess.

[LyleCaldwell]

Gotcha.  I got confused by "Get a small power transformer with at least two 10Vac secondaries. The little flat pack things would work" and then "It's big, expensive, and heavy, but it should work."

Time to hit the data sheets again.