It's time to restore the 11/45.
js at cimmeri.com
js at cimmeri.com
Thu Feb 5 14:12:33 CST 2015
On 2/5/2015 1:04 PM, tony duell wrote:
>> > From: tony duell
>> > A plain Variac does not provide isolation from the mains so you are
>> > actually now working on mains-connected circuitry (read : dangerous and
>> > you can't clip a 'scope on it).
>> I'm clearly a little slow this morning: why can't you use a 'scope on it?
> With most 'scopes one side of the input is grounded -- you have a coaxial cable
> with the shield connected to the 'scope case and true ground. In most, if not all,
> countries, one side of the mains is connected to ground too. So if you have
> something directly connected to mains (the common example on this list
> would be the the chopper circuit of an SMPSU) then you can't just clip the
> 'scope ground lead on it. If the input is a bridge rectifier (as it is in most
> SMPSUs and in the DEC regulator bricks that started this), then clipping the
> 'scope ground to the obvious point -- the -ve output of that rectifier -- will
> effectively put a diode across the mains. This is a good way to let lots of magic
> smoke out.
> The correct way round this is to run the item under test off an isolating
> transformer. Then no point on the secondary side of that transformer is
> necessarily grounded (that is the whole point of it being an _isolating_
> transformer), so you can arbitrarily connect any single point in the circuit
> to ground. So you can clip your 'scope ground lead where you like.
> The dangerous way is to 'float the scope' -- that is to disconnect the mains
> earth from the 'scope (in the mains plug, for example. You can now connect
> the 'scope ground lead where you want, but since that point is not necessarily
> the same voltage as true ground, you can end up with the metal 'scope case
> at a high voltage wrt true ground. Very nasty!
I'm glad you said this as, at times,
I've put my scope on the isolation
transformer instead of the item itself.
More information about the cctech