It's time to restore the 11/45.

tony duell ard at
Thu Feb 5 12:04:39 CST 2015

>    > 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!.

The official manual for the Philips PSU in my P854 computer gives all the waveforms
in the chopper circuit. It actually states that you should use an isolating transformer 
on the mains input of the PSU to look at these, but if one is not available to disconnect
the mains earth lead of the 'scope. Rather you than me.

Variacs, BTW, are variable autotransformers and do not provide isolation.
So the same problems exist if you are running something off a mains-powered
variac. Of course you can use a variac together with an isolating transformer.

>    > From: Mouse
>    > Perhaps fortunately, it appears that here, at least, incandescents
>    > aren't completely banned, just vanilla 100W ones.
> In the US, 'vanilla' 75W bulbs are now also banned (and I think 60W's are
> headed that way, too); 'specialty' bulbs (not sure of the exact definition -
> think coloured, etc) of 75W and 100W are still allowed. (As are, ironically,
> plain 150W and 200W!)

Over here bulbs above a certain power (is it 60W now?) can't be sold for
'domestic lighting', but can AFAIK, be sold for any other purpose. And 
there is no restriction on what you use the bulb for after buying it, although
I am told some insurance companies won't pay out if they discover a fire
was caused by such bulbs used for domestic lighting.

More usefully, you can still get normal incandescent bulbs for special
applications (e.g. lighting the inside of a cooking oven), 'rough service'
bulbs, and so on. All of which make perfectly good ballast resistors.

Am I the only person to have a cable with an IEC 'kettle socket'
on one end, a similar plug on the other (think of the plug used
to power the IBM5151 monitor) with a lampholder wired in series
with the live wire? I connect it between the power cable and 
input socket of something I suspect of having a mains short, and
put a suitable bulb in the holder.

For larger appliances (the iron somebody mentioned), I have read
that those 1kW halogen security lamps make quite good ballasts.


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