Isolation Transformer

[requim]

So I've been reading about how I should use an isolation transformer if I'm going to work on electronics gear, especially high voltage while it is powered up.  I've been looking around and seen isolation transformers for audio gear, hospital grade, and standard stuff.  Tripp Lite makes regular and hospital grade isolation transformers.  Tripp Lite also makes huge step-down/isolation transformers.

My question is, if I were to buy a massive step-down/isolation transformer, would that be sufficient, or is it not really what I'm looking for?  Here's an example:

http://www.tripplite.com/en/products/model.cfm?txtModelID=2909

Also, can you explain what the difference what the difference is between the regular isolation transformers and the hospital grade isolation transformers?  For example: Tripp Lite IS1000 vs Tripp Lite IS1000HG

[IanB]

You need to make sure you know why you should use an isolation transformer. If you just blindly use one because you are told you are supposed to, without knowing the reasons, you could still electrocute yourself.

[ejeffrey]

An isolation transformer is useful if you are working with live mains voltage stuff.  If you use wall warts, batteries, bench power supplies, or enclosed power supply modules you don't really need it.  You probably don't need or want a step down transformer, just a standard 1:1 isolation transformer.  Generally I wouldn't expect you to need a huge transformer, just enough to power your device under test and maybe an oscilloscope.

The biggest difference with medical grade isolation transformers is that the leakage current limits are much smaller, and the insulation rating is much higher.  Capacitance between the primary and secondary windings, or secondary to ground allows some leakage current to flow.  Also, surges on the primary can be coupled to the secondary through capacitance or insulation breakdown.  Normal isolation transformers are intended to be safe in the case of incidental contact.  If you are powering an EEG probe that is attached with conductive gel to someones chest you need to be a bit more careful.

An isolation transformer does not protect against everything, and can easily be rendered useless.  Of course the standard problem is that if you touch both live and neutral lines at once, you complete the circuit in spite of the isolation transformer and still get shocked.  More subtly, if you connect a device under test to a grounded instrument, you provide a connection between the primary and secondary, and you are essentially no better off than before.  This can be a computer via a USB interface, an oscilloscope, an audio jack, or simply because the device is in a metal case resting on a metal bench.    You can start connecting everything in your lab to the isolation transformer, but now there are so many more ways you can be connected to the local 'isolated' ground that you increase the risk of the first problem.  Isolation transformers are great for measuring a single device that doesn't need to be connected to anything else to operate properly, but those are becoming less and less common.

A much cheaper option that provides many of the same benefits is a GFI.  This doesn't isolate the voltage, but instead senses a leakage current and shuts off the power.  The nice thing about GFIs are that if you provide a leakage path to ground, it shuts off immediately rather than silently degrading to a non-safe, non-isolated condition.

What you need in the end also depends on whether you are working with line powered low voltage electronics, non-isolated line voltage power, or true high voltage electronics.  Without more information, it is hard to judge, but I suspect that a 5 kVA medical grade isolation transformer is not what you need or want.

[Richard W.]

In a hospital-grade transformer one side of the secondary-winding is connected with protective earth.
The output is isolated from the mains, but NOT from the earth-potential.




Richard

[Uncle Vernon]

You need to make sure you know why you should use an isolation transformer. If you just blindly use one because you are told you are supposed to, without knowing the reasons, you could still electrocute yourself.

Too true. More importantly If you do manage to get across both sides of an isolated supply the safety devices in your mains wiring such a RCDs wont be able to sense or interupt the fault. This is impolite to friends and family who may not appreciate the smell of burning flesh for extended periods.

Precautions and a good knowledge of what is going on are essential before working with live mains level voltage from any source, including generators, inverters and safety isolation transformers.

[requim]

No worries.  I'm acutely aware of my lack of knowledge, but if I don't ask questions, I won't learn.  Having blown a power company transformer in my youth by arc welding a screwdriver to the main panel I would like to think I know how dangerous it can be.  That I didn't become a Darwin awards winner was only by shear luck.  Needless to say ever since I have been extremely cautious while working on anything electrical.

As for how much juice I need I have a high end mixer that needs repair along with a high end color laserjet printer so I figured I'd need something beefier, and since I can get a great deal on a 5000VA iso transformer (ie significantly cheaper than an 1000 va or 1800  va) I figured I should at least investigate it.  I'm thinking I'll also pick up a 250 VA iso transformer since that should cover me in the majority of cases.

[codeboy2k]

A much cheaper option that provides many of the same benefits is a GFI.  This doesn't isolate the voltage, but instead senses a leakage current and shuts off the power.  The nice thing about GFIs are that if you provide a leakage path to ground, it shuts off immediately rather than silently degrading to a non-safe, non-isolated condition.

I have to agree with this.  I think all benches (especially home benches) should have individal GFI outlets
(like it's required by code for wet areas and garages in some regions) or at least a GFI breaker powering the whole bench.

Everyone, go out now and replace all your bench-top outlets with GFI outlets.

This will save your life one day.

[Psi]

One thing that an isolating transformer is good for though, is allowing you to attach the oscilloscope ground clip to any part of a circuit.

With an isolating transformer on a device you can safely ground any part of the circuit and measure the voltage to any other part of the circuit.

You do have to check for any inputs/outputs though.
Say you put an isolating transformer on your TV. If the TV was connected to a VCR via some cable then the TV may still be grounded through that cable to the VCR's mains ground.

[alm]

This works as long as you connect the DUT to the isolation transformer, and not the scope (ask your favorite scope manufacturer about floating their scopes). Note that by grounding the DUT through the scope, you eliminate most of the added safety from the isolation transformer, touching one point in the circuit can result in a shock unless you're fully isolated from ground, just like without isolation transformer.

[Psi]

yeah, i should have said that.
Don't isolate the scope.