Transformer question (only slightly OT)

[Roy J. Tellason]

On Saturday 02 September 2006 09:58 pm, Chuck Guzis wrote:
> On 9/2/2006 at 11:11 PM ard at p850ug1.demon.co.uk wrote:
> >A useful trick is to connect a 100W mains light bulb in series with the
> >input to the transformer when first applying power (and with the
> >secondaries of the transformer unconnected). Such a bulb will easily
> >handle the magnetising current of a small transformer, anf won't even
> >glow (or drop much voltage), so you can check the output voltages, etc.
>
> Hmmm, the way I'd do is to hook up one winding to the mains, then tie one
> lead of the second winding to either side of the mains supply and measure
> the voltage between the other lead and the other side of the mains supply.
> If correctly phased, the voltage difference will be closer to 0 than
> 2xmains.

This is correct.  But the light bulb as a current limiter is a very handy 
thing to have,  in any case.  If you have a transformer that's developed a 
short,  for example,  or some load is connected to it while you're testing,  
or a rectifier turns out  to be shorted,  or all sorts of other things.

I built myself a little box that uses a center-off DPDT switch to select 
whether that light bulb is inline with the outlet in the box or not.  And I 
also have a set of binding posts connected across that switch,  which is 
labeled "Bypass/Meter/Limit" for no limit,  measuring current with the meter, 
or using the light bulb to limit it.

This sort of thing is very handy when you're working on,  say,  solid-state 
audio amplifiers,  and you've just replaced a whole set of blown output 
devices and want to find out if anything else is bad without blowing them 
again.  Stuff like that.

-- 
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ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
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