# Using ballast as isolation transformer

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#### Diy1995

##### Member
Hello,
I need an isolation transformer for my tube amp project and I wonder if I can use or convert my ballast as one. I have seen tutorial on how to use ballast as isolation transformer http://www.instructables.com/id/Ind...sh/step4/Preparing-the-Isolation-Transformer/
But my ballast is quite different, its closed and it has only 4 connections IN: common, phase, OUT: 400w, 250w.
I cant measure resistance between common and any other contact -it seems that its higher than 200Mohm (highest settings on my DMM),
resistances between other contacts (phase, 400w, 250w) is 9mOhm

Thank you for anwsers

#### MikeMl

##### Well-Known Member
Ballasts have an air gap in their magnetic path that makes them unsuitable to use as a transformer, even if they had isolated windings, which most do not. They are usually wound as an "auto-transformer", meaning no isolation between primary and secondary.

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#### tcmtech

##### Banned
Unfortunately your ballast is the most basic inline inductor type and not a real transformer type of which most any of those that are not in potted and sealed containers are fairly easily modified to work as transformers of various configurations.

#### Diy1995

##### Member
Ok, its plan B then I guess. Using 2 220-24v transformers with secondaries connected together. Can I mount them like 1cm close one to another? Thanks
And if I rating of transformer is 24v 60VA, is VA rating the same on 220v then?

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#### MikeMl

##### Well-Known Member
Why do you need an isolation tranny ahead of a tube amp. Wouldn't the regular power transformer (B+, filaments) have isolated windings?

#### MikeMl

##### Well-Known Member
[QUOTE="Diy1995, post: 1286310, member: 264139"...
And if I rating of transformer is 24v 60VA, is VA rating the same on 220v then?[/QUOTE]

Not quite. I would derate the tranny that is stepping up 24Vac to 220Vac.

#### Nigel Goodwin

##### Super Moderator
Why do you need an isolation tranny ahead of a tube amp. Wouldn't the regular power transformer (B+, filaments) have isolated windings?
You must have missed his other threads?, he has a live chassis valve amplifier.

#### tcmtech

##### Banned
If you have access to better quality old HID lighting ballasts that are built on the standard E and I core transformer design they are pretty easy to rework into a very good isolation transformers. All you need is a pair of them so that you have two matching coil sets to make a 1:1 ratio unit.

The best part is being extremely mass produced they are made of sectional laminations rather than interleaved with simple easy to split welded seams so a few minutes with a hacksaw is all to takes to pop one apart without ruining it. After that all you do is slip the unwanted coil set and core shunts off and put your matching coil set from the other ballast and use a bit of epoxy to put it all back together.

On top of that unlike microwave oven transformers that are made to run for short duration at near core saturation limits lighting ballasts are built to run continuously with high overload capacity and do it while in extramly high temperature environments on top of that, class H or H+ (300 - 320+F rated) insulation, which for DIY repurposing makes them extramly robust and forgiving to heavy overloading.

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