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Registered Member #3026
Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54
Hi Everyone,
I just got my first oscilloscope. Whenever I use it to look for power supply ripple in an old radio I'm working on, it trips my shop GFI! Why does this happen and how can I avoid it??
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Many old valve sets had a 'live' chassis (for economy) due to the direct rectification of line voltage.
Best = power the radio via an isolation transformer and earth the chassis
Good= use two probes as a differential probe .............. invert channel B, add A+B channels, clip one probe tip to chassis, use the other tip to probe...do not connect the probe earth leads anywhere (un-clip them if possible). The chassis will still be 'live' so take care.
Worst= disconnect the 'scope earth wire from the mains ..very dangerous as now even the 'scope chassis will be live.
Registered Member #3026
Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54
Thank you, Sulaiman, I am using a flea market transformer I hope is an isolation transformer -- but it doesn't say "isolation" on it. Can I determine this by measuring the resistance between primary and secondary windings? And what should the reading be? Thanks again!
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
An isolation transformer will output the same voltage on the primary and secondary ends... so plug it in and see how much it outputs. If it's 120 vAc, then you're good.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I guess Inducktion is technically correct, since a 1:1 autotransformer is no transformer at all.
But it still strikes me as a triumph of logic over safety. If I bought a flea market transformer, I'd rather verify that the primary and secondary were actually isolated from each other and from the core, using a hi-pot tester, or at least an ordinary multimeter on the resistance range.
Whatever the reason, the GFI is probably trying to tell you that your radio isn't properly isolated from the line.
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oscilloscope and GFI
