What if you wire an isolation transformer like so?

[leadbelly]

Example: an AC isolation transformer, rated 500mA, with dual 120VAC primaries and dual 120VAC secondaries. One primary winding is wired to 120VAC. The other 3 windings (1 primary and 2 secondary) are wired in series for 360VAC. Could the transformer be expected to deliver 250mA? I realize this is not generally applicable, but I though it might be acceptable service for an isolation transformer where the windings are more similar to each other.

 

[poobah]

Yup, and you will get the current ratings of the individual windings (not combined). This is a mickey mouse though...

 

[AudioFreak]

And kiss the primary : secondary insulation / breakdown specs good bye.

 

[Eva]

Assuming that the transformer is ratet at 500mA for paralell winding connection, you will get 360V 250mA by doing what you propose.

However, note that insulation between primaries has not to meet any particular safety requirement, and the wires from both windings are usually in direct contact, so your 360V output is likely to be poorly insulated from the mains side and may become live if the wire enamel breaks down.

I would only recommend that arrangement if the load connected to the 360V side is well insulated and its ground or any of its nodes are never to be touched by the user. If you want to power a valve amplifier, you should look for another approach. If you have several transformers of the same model (ie: from some surplus sale), you may use two units instead to get 360V or 480V with proper isolation.

 

[KBK]

ouch

that reminds me. I've got to get my isolation transformer home from work, where it was delivered. But each time I look at it, I get discouraged and walk away. It weighs 600 lbs. Ouch.