constant voltage vs isolation transformer

Thread Starter


Joined Mar 3, 2010
looking for any additional knowledge and tid bits on the subject of these transformers..

i am currently running an isolation transformer to power a pc.. and looking to solve other issues was thinking about constant voltage transformers.

can a constant voltage transformer substitute for an isolation?

new to the constant voltage transformer.. i know that it involves a little more circuitry? filtering?

any comments welcome


Joined Mar 12, 2010
The "constant voltage" transformers I have met were called, "ferroresonant". It seems that a capacitor connected to a third winding compensated for something. They weren't all that good at being constant, but better than nothing. As for isolation, all transformers isolate. It doesn't matter if they are constant something-or-other. As long as no conductor connects the primary winding to the secondary winding, it's isolated.


Joined Apr 20, 2004
The third winding soaks up spikes and can compensate for a couple of cycles of voltage sag by depositing/taking energy from the capacitor. Unless the application is critical, the added expense is not worth it.


Joined Jul 7, 2009
I'd like to add a tidbit more to Bychon's response. His statement that as long as a primary winding isn't connected to the secondary is quite important -- and yes, there are transformers out there that do make such a connection. A Variac is one common type and it's important for folks new to electronics to realize that a Variac is not an isolation transformer.

To find out whether your transformer's windings are isolated from each other, disconnect all connections to the transformer, then measure the resistance from each input lead to each output lead. If you get a non-OL reading on a DMM (say, on the order of 1 MΩ or lower), you'd start to wonder if it was isolated (or some insulation is damaged). If you get a resistance on the order of 1-10 Ω, you definitely know it's not isolated.

Also, the isolation drops as the frequency goes up because of the distributed capacitance between the windings. This normally isn't an issue for the usual 50-60 Hz line frequencies.

I'm also curious why you feel it's necessary to run your PC from an isolation transformer. The usual reason is to protect against leakage currents when testing hot chassis electronics, but I hope that isn't the case with your PC!


Joined Nov 6, 2005
Constant voltage transformers were fairly common before UPSs were available.

Nowadays, a UPS is far cheaper.

If you are worried about supply volltage fluctuations rather than just power failure, get an 'Online' type rather than 'Standby' type.

The standby type only switch in to circuit if the power fails.

The Online type permanently run through the inverter and actively regulate the supply to the PC or whatever.

They are more expensive new, but they do appear regularly on ebay - I got two 1500VA APC SmartUPS for 50- each rather than 400- each..
Even if you have to replace the batteries after a year or two, it's still waaay cheaper.


Joined Apr 19, 2016
I have a similar problem as above.
I have started repairing minor electronic items. I wish to use an isolation to be safe as everyone else mentions it.
I have an old constant voltage transformer- from sola electrical USA. I know it has a large capacitor in along with a one to one ratio winding have an input and output as mention below.
> Input 180-260V
> Output 220-240 v

> I tested the transformer using serial bulb connection and the lamps remain bright at all time.
> I am not sure if transformer is ok or not ,I am afraid to connect it to direct power main source. Please advise .

So my question is bright serial bulb a good thing or bad thing. i my opinion this must be the case contrary to using a serial bulb test in other test condition.?

Secondly can I use a constant voltage transformer as an isolation transformer?
Waiting for your prompt reply

> Ramputty