Isolation Transformer - Use One!

[stj]

Re: Isolation Transformer - Use One!

get a pair of gloves!

[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...

[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).

[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.

[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?

[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.