Isolation Transformer

[oakkar7]

Dear All,

It is not a modern tech but still mystery for beginners. After I bought my test gears such oscilloscopes, I noticed that I should have an isolation transformer in my bench. Then, I forgot again with the other projects. One day, after seeing Dave and Todd videos blog about isolation transformer, I started the construction of DIY isolation transformer.

For those interests, here is Dave and Todd explained videos. Thanks both.
http://www.eevblog.com/2012/05/18/eevblog-279-how-not-to-blow-up-your-oscilloscope/
http://www.toddfun.com/2011/04/30/isolation_transformers/

And later, I recently found a post at Keith’s Electronics Blog. There is also original schematic from manufacturer. It isolates even ground pins of each output. Yes, system ground (Chassis ground) is purposely isolated.
http://www.neufeld.newton.ks.us/electronics/?p=500
and schematic,


And here is an AN from APC concerning about isolation in UPS system but worth to read.
http://www.apcmedia.com/salestools/NRAN-7NB2FG_R0_EN.pdf

I agree with Todd suggestion. This is the schematic from commercial isolation transformer. The output winding NEUTRAL is CONNECTED to main GROUND!!!

http://www.carebase.com/Isolation-trans ... -1000B.htm



This is the edited diagram as per suggestion. Disconnected secondary winding (NEUTRAL) from main GROUND.



All blogs and AN have their point of view in isolation but questions still exit.

My Notes:
- DUT (device under test) should be isolated with transformer rather than oscilloscope.

- BUT, Dave scenario-3… if DUT power supply is grounded to chassis (main ground), the problem is still existed. Disconnect DUT ground ?? or use with floating power supply?! or with battery?

- Test device such as oscilloscope should not be floated (must be grounded) without special reason.

- Most forum and articles talked about isolating oscilloscope not DUT. If someone know the best practice and pros, cons, let me know.

- Differential probe, usb isolator are also good but I cannot still effort.

- I need GFCI outlet at my bench.

- My thought is that isolating system ground has some pro and cons. Sometime we need to isolate for full safety or purposely. In most case, test equipment should be grounded for safety.

My construction is here.
http://okelectronic.wordpress.com/2012/12/04/diy-isolation-transformer/

Pls share your thought, experience and best practices.

[MickM]

Hi;
I built one to use when repairing old radios and TV sets.
These do not have a transformer, one side of the line is connected to the chassis for ground.

I used the transformers from two old UPS boxes.
The secondaries were connected together.
So 110v in, 24v out -> 24 in , 110 out.
The metal box is grounded.
The ground wire was left open on the output.

All test equipment is grounded, the radio/tv is not.
When the scope is hooked up, the radio/tv gets the same ground through the BNC.

Mick M

[Sleepwalker3]

If you buy a professional *purpose built Isolation transformer* (that complies with all the regulations), like I have on my bench, it will have the secondary not connected to anything - i.e. Floating. The Earth will still connect to Earth, just as it should. You should *NEVER* disconnect the Earth on your CRO, nor on your Isolation transformer. If you happen to contact one of the live secondary wires, you will get a minimal shock, even with the isolation transformer, though it will be minimal and likely only barely a tickle. You should note that Earth Leakage circuits will *NOT* trip, unless you have specialised types.

You should also note that having filter units or SMPS's connected to the isolated supply may (and likely will) affect the isolation of that supply, as many have filter caps to Earth, thereby causing a path to Earth if you contact the opposite leg - In other words, you would get more of a belt than if there were no caps on the opposite leg. Do not think that leaving the Earth disconnected on these is a good idea, it's not.

If you really need to measure floating voltage (e.g. you work with high voltage H bridges, High side circuits, EHT floating supplies, etc.), then you should either buy a purpose made floating differential probe, buy a floating isolated channel CRO (like some of -the great Tektronix ones), or use the differential functions on your CRO, providing the levels are within the range of your gear (obviously CRO being a generic term here).

When you work with an Isolation transformer on your bench, you need to be mindful of what you're plugging into the isolated supply (as mentioned above), otherwise it may not be as 'isolated' as you think. Some of my test bench supplies are double isolated (two completely separate galvanic isolation layers) , though the above comments still apply.

If you're working on this sort of equipment for a living (or if you just would like to be around for a while longer), you need proper high quality isolation gear that complies with the regulations, not homemade "I think this will work" stuff.

[oakkar7]

....

When you work with an Isolation transformer on your bench, you need to be mindful of what you're plugging into the isolated supply (as mentioned above), otherwise it may not be as 'isolated' as you think. Some of my test bench supplies are double isolated (two completely separate galvanic isolation layers) , though the above comments still apply.

yes, i also noted this (about to mind what DUT is plugged in). What you means double isolated? there is three windings? Can you explain with a diagram?

thanks all,

[logicnibble]

Hi!
I want to work on an offline power supply (flyback) 50W (100W maximum) prototype from 230V 50Hz and I want to be able to check signals with a Rigol DSO, and obviously, work safely.
What would be the professional approach?
I'd like to buy an isolation transformer but I don't know which one would satisfy my needs, nor where to buy one from Europe.