What's a good isolation transformer?

[Spork Schivago]

Hello,

I want to buy an isolation transformer. I had some questions. I understand some people hook their scopes up to the isolation transformer, but generally, it's wiser to hook the DUT up to the isolation transformer.

What's your take on that?

I wanted to use it for testing some PC power supplies. Because these PSUs can provide a lot of current, I figured something that could handle 700 watts would be good.

Even though this is a UPS, it says full isolation: https://www.mouser.com/ProductDetail...alOmbPy0Kls%3d

Would something like that work?

I plan on using my oscilloscope a lot and never really had a need for it until now, so I figured while I played around a bit, it'd be best to take some safety precautions. I think an isolation transformer, along with some other safety precautions is needed. I don't have differential probes but might purchase some eventually.

Thanks.

[budm]

Re: What's a good isolation transformer?

https://www.badcaps.net/forum/showpo...&postcount=201
You will want the transformer with secondary winding that has no connection to safety GND.
That UPS spec sheet does not give full details how it is isolated from primary side.

[eccerr0r]

Re: What's a good isolation transformer?

I only have a ~300W iso transformer. It's an old BK Precision. Since PSUs are either dead or alive, I am not worried about testing a 700W PSU on the 300W iso transformer - I use the iso transformer to make sure the 700W PSU works at 40W, and then just run the PSU without the isolation to test the rest of the way to 700W. In fact I was debugging and testing that Coolmax RM-850B PSU on the iso transformer...

[R_J]

Re: What's a good isolation transformer?

something like this Stancor GIS-500 (va) or GIS-1000 (va) would work.
they are simple 1:1 isolation transformers
or by hammond http://www.hammondmfg.com/171.htm

[Spork Schivago]

Re: What's a good isolation transformer?

https://www.badcaps.net/forum/showpo...&postcount=201
You will want the transformer with secondary winding that has no connection to safety GND.
That UPS spec sheet does not give full details how it is isolated from primary side.

Budm! Haven't seen ya post to one of my questions in a long time! I hope everything has been fine with you.

Thanks for the link. Yes, I am aware that I want the transformer with the secondary winding that has no connection to safety GND. But if I purchase, let's say that UPS, and it has the connection, can I not just simply severe the connection to GND? Or would that cause issues?

I see they make isolation transformers specific for this task, but I can't seem to find any that can handle something like a 700 watt power supply. I'd hate to waste the money on the UPS, because I don't need battery backup, just the isolation transformer. I'm just having trouble finding one large enough.

[Spork Schivago]

Re: What's a good isolation transformer?

I only have a ~300W iso transformer. It's an old BK Precision. Since PSUs are either dead or alive, I am not worried about testing a 700W PSU on the 300W iso transformer - I use the iso transformer to make sure the 700W PSU works at 40W, and then just run the PSU without the isolation to test the rest of the way to 700W. In fact I was debugging and testing that Coolmax RM-850B PSU on the iso transformer...

Yes, but you're also probably very good at using an oscilloscope, right? Someone, somewhat knowledgeable with scopes once said to have fun exploring. It wasn't until I attempted to explore the high side of a transformer that I realized this could be dangerous! That kind of scared me and kept me away from the scope for a long time.

Now I want to get back into it, assume the scope isn't damaged. So the purpose of wanting such a large isolation transformer is so I can explore a bit more safely, although I realize an isolation transformer by itself doesn't necessarily provide all the safety I need and other precautions will need to be adhered.

Just so I'm clear, I want one not just to diagnose DUTs, but to learn how to use the scope. I have a PSU that I will be using to learn how to safely use the scope.

[Spork Schivago]

Re: What's a good isolation transformer?

something like this Stancor GIS-500 (va) or GIS-1000 (va) would work.
they are simple 1:1 isolation transformers
or by hammond http://www.hammondmfg.com/171.htm

That Stancor GIS-1000 seems like what I'd be looking for, and it's cheaper than the UPS, which is good.

If I've done the math right, the Stancor GIS-1000 and the Hammond 171G would be equivalent with the amount of power they could provide, right? Around 1,000 watts. The Hammond has a built-in breaker though, which could be nice.

Even though it probably wouldn't do any good, for safety reasons, I had planned on always plugging the isolation transformer into a GFCI receptacle. Would this be pointless and could people suggest other safety measures I should take?

I plan on keeping an eye on my free hand and making sure it's not touching anything it shouldn't. I plan on trying to make sure I'm not grounded.

The reason I think the GFCI might be useless is although they will trip, I don't think they limit the amount of current going to a short. For example, if I mess up and trip the GFCI outlet, but have 10 amp going through the outlet, I don't think that outlet will prevent that 10 amp to getting to me, if I'm the reason for the short (ie, my finger being somewheres it shouldn't be!) What do you guys think?

[Sparkey55]

Re: What's a good isolation transformer?

Many years ago back when I was working on hot chassis large tv sets I took a 400+ watt power transformer from an old tube tv and stripped the HV and filament windings out and wound another winding to match the primary with Hi and Lo taps. Worked very well.

[Spork Schivago]

Re: What's a good isolation transformer?

I have a CyberPower 1350PFC LCD UPS and when I measure the continuity between Earth mains and neutral, I get around 13 megaOhms (for the battery + surge side). There's a side that says Surge, with a note that says:

Please Note
Surge - Only receptacles are always on.

With that side, the resistance between Earth mains and neutral is around 6 megaOhms. Would this be considered an isolation transformer?

I'm curious and wondering if I could tear it apart and physically separate it more so there's infinite resistance between neutral and Earth mains on both sides. I believe the battery pack(s) inside are no good anymore, so I'd either have to replace them to use the unit again or ditch it and purchase a nicer UPS (which I plan on doing).

Eventually, I want a UPS, I don't know what they're called, box that holds a bunch of smaller UPS units. And I want that to power my systems long enough until the whole home generator kicks on (which wouldn't take too long). Or maybe, if they make one, a rack-mount UPS that's very powerful and could power an entire house long enough for a whole home generator to kick on.

[eccerr0r]

Re: What's a good isolation transformer?

Honestly I have yet to need to 'scope the live side, it's the low side that I find all the problematic circuits. Sort of fix the problem a bit at a time.

The trouble being is if the PSU is failing at high load, then I'm screwed. But I can still collect observations from the isolated side of the PSU that can help figure out unisolated side problems.