Transformer - using only the primary coils as an isolation transformer

Thread Starter


Joined Feb 6, 2020
Suppose I have a transformer with 2x110V (isolated) on the primary side. Could I connect these two coils up as an isolation transformer (within the coils' voltage/current specifications)?

primary A -> AC source
primary B -> output
secondary -> not connected

In other words, am I right to assume that in a typical transformer the terms 'primary' and 'secondary' only reflect the intended use, and not so much the design? They're all just coils around the core, right?



Joined Feb 20, 2016
The only problems that may come up are that usually, to get the current rating, both primaries may be designed to run in parallel on 110V. This may limit the max power you can use without overheating.
Also, the insulation isolation voltage rating between the windings will not be as high as primary to secondary.


Joined Jul 18, 2013
Windings of equal value are often wound Bi-Filar, so if this is so the voltage would be at the highest when ran normally in the high voltage (series) mode, so there should be no problem in insulation, plus just about all transformers are wound with the modern high rated varnish now.
Although as mentioned, the Va rating may drop accordingly when one winding is used for 10vac.

schmitt trigger

Joined Jul 12, 2010
Also....because the primaries are bifilar wound, there is significant capacitance among them, which means leakage.

Not enough to do heavy duty damage, but it will still tingle you, and cause your scope waveforms to have lots of noise.

Ask me how I know.......


Joined Mar 14, 2008
both primaries may be designed to run in parallel on 110V. This may limit the max power you can use without overheating.
Yes, the current limit would be half of the 110V rating or equal to the 220V rating in series.

Another consideration is that the windings are not wound to compensate for the voltage drop due to winding resistance as secondaries are, so the output voltage will be reduced by the output current times the winding resistance of both windings in series.


Joined Feb 20, 2016
I would be loath to run a transformer for isolation like that. It may be true the varnish is fairly well rated insulation wise for the maximum 110V AC it would normally experience, but the insulation between the primary and secondary windings is in the KV range.
The extra layers of insulation added between windings is a lot better that just the varnish.
The whole point of an isolation transformer is for safety after all. Keep safe!


Joined Mar 14, 2008
As a safety backup, use a GFI plug, such as this, for the transformer.
(I salvaged one from a broken hair dryer).
That will protect you even if the transformer insulation failed.