Variac?

ow.

Nope, two different things. There ARE isolating variacs but those specifica lly say so.

A variac is a toroidal coil with a wiper that varies the AC output like a p otentiometer, but it is better than a pot because it acts as an autotransfo rmer, just with a variable tap.

An isolation transformer is a one to one transformer that makes whatever is plugged into it floating with respect to the power lines which reduces sho ck hazard when working on the equipment. You can still get a shock, but it is less likely to get one from it to something external that is grounded or somehow tied to the power lines.

It also isolates the hot side of a power supply so you can use a scope on i t. Otherwise you have to "float the scope" which the manufacturers like to void the warranty if they find out. They are not designed to be floated. So you use the isolation transformer to "float" the unit being tested. Not on ly safer but also prevents a short circuit that could fry the thing (and th e scope) out in a fairly nasty manner. Like smoke.

Not usually. Most seem to be just a single coil where one side is common to the line and load. The other line wire is connected about 90 % up the coil. The load wire is then moved up and down this coil. As it gets up to the top you get slightly more voltage than the line .

Thank you for the advice. Just thought I could save money by buying Variac only. Will go ahead and purchase an isolation transformer. Ivan Vegvary

But if you ever need a variac, you might spend a little more now and get both.

Mikek

Just to add a mite: a Variac is a variable autotransformer. I'd say "nothing more", but that'd be showering a lot of disrespect on the mechanical guys and the heavy-iron magnetics guys that make them work reliably for long periods of time.

--

Tim Wescott 
Wescott Design Services 
http://www.wescottdesign.com

Uh oh. Years ago I smoked a scope* because of voltage conflict between it and the DUT. Since then I've isolated the scope.

So, why is that a bad idea?

Bob

• - it was one that I found at the dump and worth every penny

First and foremost, it makes the scope unsafe to use. The ground pin on the scope power cord plug is connected to the scope's metal cabinet and all exposed metal on the scope. If for any reason (such as mis-connecting the ground clip of the probe to the hot side of the 120 vac in the device under test) that cabinet gets gets connected to the ac hot side, the mains circuit breaker will trip, protecting you. If the scope is "floating" then the cabinet is not grounded. You could easily touch a "live" cabinet and get shocked, maybe killed.

There could be other issues such as noise pickup, but they don't matter if you're dead.

Ed

** Cos the chassis and BNC connectors etc on the scope can then acquire a hazardous voltage - and I mean *electrocution* hazard.

Leave the scope alone and use an iso tranny to float the AC power to the item you are working on. Then you can ground any part of it you need to make scope measurements.

It is also a bad idea to use the iso all the time, it creates a false sense of security when working with dangerous voltages and will hide the existence of leakage to chassis in the equipment being tested.

... Phil

Because the scope transformer is not "hi-potted". Would you rather risk the transformer in your scope arcing internally or the unit you are fixing (which is obviously already broken) ?

a hazardous voltage - and I mean *electrocution* hazard."

You may or may not remember the line of Tek scopes that were made to do thi s. They may have been part of the Sony/Tek line. They had batteries and tou ted that they were fully insulated, just like a modern DVM I guess. They sa id no problem with grounds. They had batteries.

But then there are the test probes, I bet only certain ones were approved f or fully floating action. I would think they would have to have insulation on the outer part of the BNC connector but then, that ground clip has to go somewhere. So there is no way to make it 100 % safe. Sorry liberals.

And even with a differential input, plugin or otherwise, I would not want t o subject that to a serious ground fault. If I am fixing your instrument, m y instrument is more important than your instrument.

So, they DID make a series of scopes that can do that, and the cases were 1

00 % insulated. I don't remember the models numbers but I do remember it. Y ou could float it and just use it like a DVM. That's what they said as far as I could tell.

Yeah, then you can run them in series. Just plug your IT into the wall socket, then the variac into the IT's output. Then just leave the variac at 100% except for those times you specifically need its lower Vout capabilities. And always power the faulty equipment from this setup, never the test equipment you're using to troubleshoot it.

On Fri, 29 Jan 2016 23:23:42 -0800 (PST) snipped-for-privacy@gmail.com wrote in Message id: :

Tek made a series of scopes that were completely isolated. The TPS2000 series.

Yuck. Check out that new logo!

I have two "Variacs", neither one is actually branded Variac. One is not isolated and the other one is. So maybe you can look for one that is isolated and avoid the isolation xmfr. Eric

** Variacs with isolated output are scarcer than hen's teeth.

... Phil

Ahhh, you know I never understood about iso-transfomers. I figured they where to used the float the 'scope and not the piece of kit. (Which didn't seem to do anything for me.)

I still have times where I need the piece of kit grounded to test something...(usually some ground loop AC pickup or something) and I have an AC power cord with the ground pin snipped.. I put a piece of green tape wrapped around the universal three prong input that goes into the back of the 'scope. Most of the time that lives in a drawer... doesn't get out much.

George H.