Isolation Transformer - Use One!

[Longbow]

I'm sure it has been mentioned elsewhere, but it never hurts to remind techs to invest in an isolation transformer before they begin work on mains connected circuitry. Some variacs are isolated, some are not. But a nice isolated variac costs plenty. So does a differential scope probe. It is worth the money to invest in a 1:1 isolation transformer to prevent ugly electrical shorts between the mains and your scope, your soldering station, and other grounded test equipment.

Technically the test equipment should remain grounded through a 3 conductor plug. We've all cut that ground plug at some point for lack of a transformer, but you give up an important safety factor by doing so.

It is true that an electrical ground on all test gear can show up already existing sources of common mode noise, cause ground loops, etc. But in the end, these problems can be solved or improved without cutting the grounds. Working on the primary of a switching power supply means connecting your scope ground to one wire of the mains. Lots of computer supplies have "hot" heat sinks. Give it some thought - an isolation transformer is the right way to go.

[Joe Black]

Re: Isolation Transformer - Use One!

I have one (not sure if we mean the same thing though).
Transformer (EU) 220V-220V? I've been told it can't kill.
Also, you recommend plugging equipment after transformer, right?

[Th3_uN1Qu3]

Re: Isolation Transformer - Use One!

A big isolation transformer is too expensive for me - whenever i need to probe the primary of something, i use an inverter and battery as i already had those lying around.

[japlytic]

Re: Isolation Transformer - Use One!

I've read about a dangerous practice mentioned in older Pace satellite receiver manuals, which is floating the oscilloscope ground while the probe is attached to the primary side (no mention of an isolation transformer!) and not touching anything on the oscilloscope.
Many test equipment manuals have notices about grounding, such as "intentional interruption is prohibited".
Even with an isolation transformer, I still prefer to use a differential oscilloscope probe on the primary side, since I remember when taking waveforms on the primary side of an online UPS (even when isolated from the mains with an isolation transformer), the connected probe was inducing noise (most likely common mode noise), which fortunately was not enough to cause damage).

[eccerr0r]

Re: Isolation Transformer - Use One!

It would be nice if scopes didn't ground the shield... alas it hasn't been a problem for me as I've never had to scope out line voltages.

I think the only time I wanted to see line level voltages is on a generator. But in this case I stuck in a (FILAMENT!) transformer on the generator side and looked at the waveform that way. I suspect I'll need to do the same to look at UPS outputs... but I haven't needed to look at that yet.

I use my isolation transformer mostly for paranoia, just in case I accidentally touch those heatsinks...

[stj]

Re: Isolation Transformer - Use One!

get a pair of gloves!

[Th3_uN1Qu3]

Re: Isolation Transformer - Use One!

Speaking of UPS units - you can also use an UPS running off batteries in place of an isolation transformer - just remember to unplug it from the mains.

[fzabkar]

Re: Isolation Transformer - Use One!

I have 5kVA transformers which I've salvaged from some very, very old mainframes. They're heavy enough to anchor a battleship.

[Joe Black]

Re: Isolation Transformer - Use One!

Here is mine.
Obviously not using it

[SteveNielsen]

Re: Isolation Transformer - Use One!

I have this one, unused, bought new for $20 back in 2000. A dog chewed off the output cord before I ever got the chance to use it.

[budm]

Re: Isolation Transformer - Use One!

I have this one, unused, bought new for $20 back in 2000. A dog chewed off the output cord before I ever got the chance to use it.

That is a good isolation transformer.
https://cdn.badcaps-static.com/pdfs/...2f49c61139.pdf

[SteveNielsen]

Re: Isolation Transformer - Use One!

Thanks for the pdf Bud. Yeah, I knew it to be a good quality unit when I bought it and when was fiddling with tube amps. I've got to get it wired up to an outlet or power strip. I don't like molded in-line ac jacks that much so the dog picked the right wire to chew off
It seems heavier than 16# though.

[eccerr0r]

Re: Isolation Transformer - Use One!

Yeah these things are heavy...
Mine is a B&K Precision TR-110, it can only deal with 350VA continuous.
Nice thing about it is that if I need to muck with the voltage slightly, I don't need my variac... though my variac can handle more current...

[R_J]

Re: Isolation Transformer - Use One!

I have a Hammond E 77033 transformer rated at 1500va, everything on the bench is connected through it. that way anything I work on is isolated.

[SteveNielsen]

Re: Isolation Transformer - Use One!

I've got an older Trippe-Lite LC-1200b line-conditioner with a big fat transformer in it. I don't know if it is isolating or not. Can't find enough tech info about it. It's switching with relays between different transformer taps.

There's a 1K trimpot on the board but no indication of what it adjusts. Last time it was used the output was 110vac when connected to 120vac.

[SteveNielsen]

Re: Isolation Transformer - Use One!

Nope, not an isolation transformer but I adjusted the trimpot to switch to the 120v tap with 120v input.

[budm]

Re: Isolation Transformer - Use One!

It is AC Voltage regulator (switching taps on the transformer using relays to maintain AC output) and Line conditioner.
http://www.tripplite.com/1200w-120v-...ection~LC1200/

[SteveNielsen]

Re: Isolation Transformer - Use One!

I do know what it is, just not sure of exactly how it is doing it.

Measuring the transformer disconnected there is isolation between one winding and the other but I don't know which is pri or sec. I know in operation there is continuity between input neutral and output neutral and that is not an isolation. I can't find a schematic for the board so I guess I'll just have to get the board out and trace it out to see how it's being used if I really want to know. If I can. If I care enough to.

[budm]

Re: Isolation Transformer - Use One!

I used to have one, same principle as used the UPS with AVR. The relay clicking is very annoying when input live Voltage varies a lot.
Example of more complicated one (not using auto-transformer) than the one in yours, but same idea.

Lots of newer regulators us TRAIAC instead of relays.

[Topcat]

Re: Isolation Transformer - Use One!

Sencore PR570 variac & isolation transformer. Bought it new back in the early 90's, never had a single issue with it. Probably not too expensive on ebay.