[sdiy] Unmarked transformers

[harrybissell]

TIm Daugard wrote:

> > ahh yes.  I was thinking "current transformer" or "Current Probe" and
> oscilloscope.
>
> And we're back to high cost. A Variac is cheaper than a current probe that can
> give an accurate reading of a pulsed AC source

LOL did anyone say this is a cheap hobby ?   OTOH once you get something you can
use it for life.   I know at work we pay about $12 for an iron core transformer...
600:5
ratio (thats 600A in gives 5A out) Plus another maybe $2 for a resistor for the
secondary
(maybe some fairly low value - 0.2 ohms ?).   Now if you want to multiply you can
make
multiple turns through the core.  Calibrate it one time with your multimeter on AC
current
scale and you are there.

BUT you don't NEED to calibrate it. The shape of the current pulse should be sort of
a sinusoidal
pulse.  If it saturates the pulse will have a sharp upward turn.

> > Possible to use a series resistor (albelit a really high power one)
>
> A light bulb. At lunch, I thought about putting a light bulb in series with the
> transformer instead of in parallel. This would limit current for at least the
> intial test.

Light bulb is a variable resistor... maybe good for protection but not for measuring

saturation.

>
> I never connect my scope anywhere need line power - scopes are to hard to come
> by. I only use meters with line power.

I almost always use a scope. Meters LIE to you.  But I use an external differential
probe. It would be fairly easy to make one with a battery powered opamp (maybe
+/-9V)
and a 100:1 resistor ratio...   1M input and 10K feedback resistors.  Of course you
NEED
TO KNOW WHAT YOU ARE DOING if you attempt this or the scope is toast (and maybe so
are you!)

> Then we get into the phase angle problem. A series resistor will give us a
> voltage that is partially determined by the inductance.
>
> A cheaper way to check a transformer might be to measure the inductance at 60 Hz
> and then calculate power draw at line voltage level vs. core size. Charts for
> the power capability of cores are fairly easy to find. However we rapidly
> approach the point where the poor teenager starting out is totally lost.

Yes... considering that a PowerOne supply is maybe $20 surplus. And there are a lot
of
slightly-used ones out there. Why tempt fate ???

H^) harry