Hi Everyone!

I have just obtained my first oscilloscope (BK 2120) Whenever I try to look at the wave form of some power supply unit, my garage G.F.I. trips!!
Can someone tell me why this is happening, and how can I avoid it??

Thanks a lot!!

JM

The scope has a three wire power cord and so the scope probe ground clip is at ground. If you try to connect it in the wrong place in the power supply the GFI will trip.

Thank you, David. But where is "the right place". If I want to look at ripple in a radio power supply, the ground clip goes to the chassis and the probe point goes to B plus, right? But this trips the GFI!

Before you do anything. Consider that tripping of GFCI is informing you of ground leakage in
the B&K scope. Deal with that before you use it.

Suggest that you get the schematic and find out if the leakage path is normal or caused by a fault.

If you have an old radio with a two wire power cord, the chassis maybe at several volts above ground and the GFI will trip when you connect the scope.

It would be better to connect the radio via an isolation transfomer first, before attempting to connect the scope

If the breaker does not trip until the scope is connected to something, then there is nothing wrong with the scope.......

If the device under test is an AC/DC radio (or anything else without a transformer), then an isolation transformer is the best solution.

Tell us what the radio is that you are tesitng.

I’m working on an “All American Five” AC/DC set plugged into what I think is an isolation transformer.
Although it doesn’t say “isolation”on its label -- it came from a flea market. The minute I touch the scope’s allegator clip to the chassis, the GFI opens. Would a new, properly functioning isolation transformer be the answer? I have hooked my scope up to a 6 volt filament transformer with no problem!

You just said the magic word "AC/DC". That means the chassis is HOT and you must use an Isolation Transformer. One from a flea market may not be a true Isolation one.

Your VOM or DMM will tell you if you have an isolation transformer----simply measure the resistance between primary and secondary.

Thank You, Pixellany!
I think possibly my transformer is NOT an isolation. What should the resistance be between primary and secondary be --I have no idea. Infinity?

Yes, infinity.

Unplug the thing first. Measure resistance from input plug to output socket. A reading lower than full scale (open) means that it is NOT isolated effectively. A reading of a few Ohms means that it is either an autotransformer or it has a winding/winding short. Either way, it will trip the GFI if the reading isn't wide open.
I think that the use of a GFI can be a lifesaver. It does involve working within its constraints but, to me, it is a good practice.

Moderate exaggeration---for effect. With typical meters, absolutely true---as in there appears to be no leakage when measured on the highest scale.

I think a GFI trips with a fault current around 4mA---that's roughly equivalent to a leakage resistance of 120/4E-3 = 30 Kohms. A good iso trans. should have leakage resistance maybe 100 to 1000 times that---i.e. many megohms.

From a Triad spec:

10 uamps corresponds to 120/10E-6 = 12 megohms

My Fluke goes to 40 meg and both iso's I have won't even cause a blip. Even with the Heathkit IT-28 I can't get anything at 600vdc from pri to sec, not a microamp.

Thank you, Everyone

I get the picture. Now, can someone recommend an isolation transformer for me? I basically work on old tube radios. What do I look for in a transformer?? (Specs.)

Thanks again!

Get an isolation transformer that you know is purpose-built for radio and TV service, that way you won’t have any doubt that it’s completely isolated and is the right tool for the job. There are a lot of good choices, a few brand names to look for are B&K Precision, Sencore, and RCA plus a few others who make professional electronic test equipment. This is a good forum to do show-and-tell, as in asking others before you decide to buy something you find. There are WAY too many of these things out there labeled "isolation transformers" that are merely AC line noise isolation transformers that are not intended to do what you want one to do.

-Mark-

Put the scope on the same or another isolation transformer .
Or you can use one of those 2-prong to 3 prong plugs with the green pig tail... but do NOT use the green pigtail to ground. Let the scope "float".
This way you'll only be using the two prongs.