UPS as an isolation transformer?

[-DOM-]

Hi Guys,

I have done a bit of research although thought i would check other peoples opinions on using a UPS as an isolation transformer. I realise that not  all use an isolation transformer in the design however if i found one that did would i be ok to use? I have studied power supplies so am not a complete noob but only just starting to do some repairs on them at home.

Any help would be great thanks Damien

[Rerouter]

a ups is not an isolation transfomer unless you unplug it from the wall, even the active types that charge up the battery and then directly invert off the battery tend to share a common ground,

[-DOM-]

Cheers for that, i didnt realise they shared the ground.

Rules out that idea. May look into DIY

[ConKbot]

Cheers for that, i didnt realise they shared the ground.

Rules out that idea. May look into DIY

Its pretty much a cardinal sin for any device with an outlet on it to not have the ground connected though.  I cant think of any consumer application where it would be safe/appropriate enough for the device to be sold readily. 


However, depending on the UPS type, you may find a useable transformer.   I remember a 300VA ups I had used a center tapped winding to step the 12v up to 120v, right at 60hz.  throw 2 of them back to back and you'd have your isolation transformer.

However some use a high frequency SMPS to step 12/24v upto rectified mains voltage, and then a class D amplifier to output the 60hz from the high voltage rail.  Not so useable in your application.

[ptricks]

You could operate a UPS unplugged from the AC source for isolation but you would need a UPS that could run a long time off battery which would cost more than an isolation transformer.

Depending on what current output you need it is better to look for old stereo amplifiers that contain transformers, easy to tell by the weight. Old amplifier transformers are good for several amps.

[ZOMGVTEK]

An online UPS could be used if you get a big DC supply to keep the batteries charged up...

It's going to depend on what you're trying to do, but it can be a bit expensive.

[edpalmer42]

I'm a little confused here.

My definition of an isolation transformer is one that ensures that neither of the power-carrying leads on the output has any connection to ground.  The presence or absence of the third wire safety ground lead is not part of the function of the transformer but is obviously a really good idea.  That ground lead is never switched or disconnected as part of an isolation transformer.  Is that what's being discussed here?

Based on that definition, most inexpensive UPSs do NOT provide any isolation since they just switch the AC through and then switch to the battery when the AC fails.  But a more expensive online UPS is continually running off of the inverter, so it very likely provides isolation between the input and output.  However, the output could be floating (isolated) or could have one side grounded.  This might be done so that the UPS looks more like the power line.  I'm thinking of North American power.  I don't know what would happen in 220V areas.

The only way to find out is to test the UPS or ask the manufacturer.

Ed

[amspire]

An online UPS could be used if you get a big DC supply to keep the batteries charged up...

It's going to depend on what you're trying to do, but it can be a bit expensive.

Online UPS's are designed with bypass relays or contactors to the mains in case of overload or large load spikes.  So you would have to disable the bypass, and this is a problem. If the UPS is designed to switch to bypass to protect itself and you disable the bypass, you may end up with the UPS unable to protect itself. Basically you have to know the UPS design very well before changing the operation.

But at the end of the day, the UPS was never designed for a floating output that is safe for humans to touch and any USP manufacturer would be totally horrified to hear of someone trusting their life to the isolation of their UPS output. If it is a switching UPS, the output will have pretty bad switching noise on it making it a poor choice as a source for test bench work.

I would dump the idea. Use a proper isolation transformer. Safe and noise free.

[ZOMGVTEK]

It really depends on the application.

I have a few Eaton 9120 3kVA models. They have bypass capability, but it is disabled by default. Typically "Online" ups models are sine wave topology. No modifications would be required, frequency and voltage can be quite tightly controlled, independent of the mains/input.

But yes, a mains isolation transformer can be used for most applications and it's probably cheaper.