Re: hot scope leads?

On Sat, 22 Jul 2006 11:36:25 +1000, "Phil Allison" <philallison@xxxxxxxxxx>
wrote:


<BobFlintsTone@xxxxxxxxxxxxx

Normally, you DO NOT use a scope to measure AC mains leads, unless you
have a special attachment to allow it, usually a signal transformer. At
work, we
rent a power analyzer when we want to see the AC.


** That seems over cautious to me - a standard 10:1 probe is well able to
safely cope with 120 / 240 volts AC.

I'm just going with the idea that you don't need a scope to check your AC,
you
can use a meter.


** Nonsense - you *need* a scope to VIEW the AC wave .


I use a meter to check the AC, and very rarely see the need to scope it... most
power supplies don't care a lot about the AC except for amplitude. Newer stuff
works from 100 to 250v at from 50 to 60hz. and that covers it all!

Anyway, you can't tell much by looking at a wave, compared to actual harmonic
content analysis. Hard to guess distortion by eyeballing a trace. If the AC
trace is poor enough to see, it's really bad! Sometimes the AC here gets it's
peak clipped, due to the industry in the area.


( I also own a 100:1 probe rated at 1500 volts max - guess what THAT
is
for, Bob.)

Gee, Phil, it might come in handy on vacuous equipment...


** Yep - the plate waveforms on tube guitar amps.


I guess an Ampeg V22 can max out your probe!


Also, when working with triac and SCR circuits that "chop" the AC supply,
one cannot use a transformer to safety isolate the waveform from the scope
as it may well have a large DC component.

Yes, a transformer isn't a good idea with a switcher anyway,


** Wrong topic - Bob.

The subject is light dimmers and the like.

Which we call switchers here...



I've seen lots of gear that had a cap in series with the transformer.


** AC coupling is NO GOOD.

You need to see that DC component on the scope.



Yes, DC offset can be important and is readily visible on a scope.



......... Phil






.

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