Which Isolating Transformer should I use

[onpiopah]

I want to do some testing with my scope on the primary side of a switch mode power supply (from a plasma) but i have been advised that i should use an Isolating Transformer. My reason for using one is to cut down on the risk of me getting an electric shock. I already have a RCD fitted to the socket.

Would one of those small yellow ones that you see workmen using be sufficient. Something like this BBH Saftey Isolating Transformer 240 Volt - 240 Volt.

A proper description is here

http://www.ebay.co.uk/itm/BBH-Saftey...ht_1782wt_1275


thanks for any help.

[tom66]

Re: Which Isolating Transformer should I use

That will be suitable, but you must make sure not to exceed 500W (if PSU has working PFC, VA = Watts for most purposes.) 500W will only be exceeded if you try full white or high contrast scenes, or analog snow, if it is an older panel. A "no signal" display will use a lot less.

BTW, you must isolate the TV, not the scope. At this point, the primary side will be floating, so in THEORY, if you were to touch ONE point on it, you would not be electrocuted, since there is no path for the current to flow from the output of the transformer to the earth, which you are indirectly connected to. HOWEVER, if you then put your finger on another portion at a significantly different potential you WILL BE SHOCKED. In addition, most isolating transformers connect output earth to mains earth in my experience, and you may find you have to disconnect this, because your scope probe is earthed, and if you try to probe something on the primary side you may get a nice BANG.

[japlytic]

Re: Which Isolating Transformer should I use

I also recommend the use of high voltage differential probes for the oscilloscope, even though the SMPS primary is isolated with an isolation transformer.
Also, attach the probes before connecting the TV and remove them after disconnecting the TV via the isolating transformer.

[onpiopah]

Re: Which Isolating Transformer should I use

Thanks to you both, i do plan on being very careful, however I just want to stack the odds in my favor. Once I fix the thing I'll make sure I post an a-z on my progress.

[mertsok]

Re: Which Isolating Transformer should I use

This isolating transformer will be suitable for your application. At all times, appropriate safety precautions and standard safety practices should be followed. Basic safety precautions should always be followed, as it is 240 volts.

[ben7]

Re: Which Isolating Transformer should I use

That will be suitable, but you must make sure not to exceed 500W

Btw, transformers are fairly tolerant of overloads. This one is rated at 500Va continuous, but that doesn't mean you can't draw 600Va from it at all ... just not continuously. The output voltage may go out of spec though.

[Kiriakos GR]

Re: Which Isolating Transformer should I use

My GDS-2102A scope comes with 400V input limit, that is much lower than 220V Mains, or about the 314VPP.

What is your scope model?
From the spesifications of it you will have to adjust your shopping list.


Edit:
Regarding the plasma cutter, the portable transformer sound as good idea.
If you calculate correctly the sum of needed K Watt, for the two X&Y motors plus the needed power for the electronics,
you will be good to go.

And it will cost you much less than a portable scope with isolated inputs = 3.000$

[capkid]

Re: Which Isolating Transformer should I use

If I remember correctly, I paid under $100 on eBay for my Stancor iso transformer. It's rated at 1300VA.

[Behemot]

Re: Which Isolating Transformer should I use

I got 2kVA for something similar. Just the transformer though, had to connect cables myself…and some NTC as it was kicking of brakers. Have to buy 8A/10A NTCs from DigiKey next time, now I am limited to 5 A i think, don't have bigger in here.

[budm]

Re: Which Isolating Transformer should I use

"that is much lower than 220V Mains, or about the 314VPP." 220V R.M.S. = 311VP (Volt Peak), or 620VPP (Volt Peak to Peak).

[tom66]

Re: Which Isolating Transformer should I use

My GDS-2102A scope comes with 400V input limit, that is much lower than 220V Mains, or about the 314VPP.

Most scopes I've seen nowadays are 5V/div or 10V/div limited, so your fantastic scope can only display up to 25 or 50V peak from 0V.

And it's a bad idea to probe without an isolating transformer because otherwise you are measuring live-earth potential instead of live-neutral which will introduce significant error. Aside from the danger to your instrument as well as it is only CAT-I rated.

Also, mains electricity can have transients exceeding 1kV daily which will blow the arse-end of your input preamplifier up.