[sdiy] Unmarked transformers

[Oren Leavitt]

Good ol' thrift stores and their "wall warts, cords, and misc." bins.
I've scored three Sola encapsulated +/-15V linear power supply units at 
a thrift store once.

Never stick your test equipment (other than an AC voltmeter) on the mains!

Series connected light bulbs and a variac have been my best friend for 
testing unknowns and powering up old equipment for the first time.

Oren
32.765N 97.065W

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
> 
> 
>>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.
> 
> 
>>and a TRUE
>>DIFFERENTIAL scope probe... none of this pseudo-differential crap.  I never
> 
> ever
> 
>>connect a scope ground to either side of the AC line.  Without one of these
> 
> you
> 
> I never connect my scope anywhere need line power - scopes are to hard to come
> by. I only use meters with line power. When I was active in the Air Force, I
> came back from lunch to the familiar smell of electronic smoke. A civilian
> technian with a shortage of skill decided to determine which power cable was
> carrying the 400 Hz 115 V power by using a frequency counter. PMEL (test
> equipment calibration people) weren't sure they could repair the input amplifier
> board.
> 
> 
>>cannot
>>estimate the saturation starting...
> 
> 
> 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.
> 
> BTW my favorite way to get a power supply is just go to the Goodwill Store and
> look through their box of wall wart type power supplies. I occasionally find a
> power supply with a cord attached that is marked +12V DC at some current rating.
> I have found 12 Volt 2 Amp supplies for $1.00. With something like that, who
> needs to worry about power supplies.
> 
> Of course, then the unmarked transformer, stripped out of equipment or found in
> a flea market box of junk, gets tossed in my transformer bin, until it's time to
> start this whole process over again.
> 
>  Tim Daugard
> 30.4078N 86.6227W
> 
> 
>