[sdiy] Unmarked transformers

[TIm Daugard]

THANKS Harry

My comments discuss some of yours below. I appreciate your comments. Please
everyone remember, in my previous post I said, DON'T use my method unless you
know what you are doing. It is not a great method, but it does work enough for
me to use it . . . with extreme caution.

> It sounds like you are describing a phase control (triac) light dimmer.
> These are OK for resistive loads like a (er...) light bulb but could be
> very dangerous on a transformer

Yes it can - it puts out spikes of current at 60 Hz. The theory is that the
spikes have higher frequency harmonics the the orginal power frequency. Higher
frequencies are less dangerous for the transformer than lower frequencies -
higher frequencies require less metal in a transformer. Thats why the military
use 400 Hz for aircraft.

Inductance = 2*PI*F*L

> perhaps killing the transformer...

Occasionally, however this method is only used on transformers that are
completely unidentified. It's either going to kill the transformer and I have a
core that I can do my own transformer winding experiments on, or I will identify
a useable transformer and I can label it for use latter.

> A Variac is the correct way to do this test.

I agree completely. Variacs are at least 10 times the cost of a cheap dimmer. I
developed this method as a poor person who could barely afford the cost of a bag
of radio shack transistors to play with. At the time, any project that would
cost more than $10 would, require two to three monts to accumlate the money and
parts. I had a wife that didn't mind me spending time on electronics, but there
was no money.

> It would also be wise to monitor transformer
> primary current to have a clue as to saturation (current would suddenly and
quickly
> increase).

How? If you are using a meter it's looking for a sine wave current profile. I
don't think that monitoring the output current of a dimmer would give any kind
of accuracy. I monitor the transformer voltage on the leads that I hope are a
secondary. I generally lightly load the secondary with a 100K resistor. (Forgot
to list that, sorry.)

> Larry Hendry correctly warned that very small transformers often have poorly
> contained
> magnetic flux, so saturation might just cause extreme heating rather than
blowing a
> fuse..

Agree, the smallest transformers are likelyest to blow. Fuses are expensive.
Every power supply I build is properly fused. It's neccesary to protect the
equipment and the power system.

I don't fuse when I'm testing the transformer. If I thought it was a power
transformer and it's not, it's either going to be disassembled or thrown in the
trash. I rely on - circuit breaker in my workshop, very little current through
the dimmer to start (doesn't always work - some time the breaker trips), and a
quick hand on the dimmer located at least 4 to 5 feet from the transformer under
test (don't forget the clear plastic panel to deflect any blast damage.)

> which is also essential (yay Larry !!!)

Again anytime any one builds a powersupply use a fuse.

> If you INSIST on doing the light dimmer trick...a large resistive load in
parallel
> transformer will help to assure balanced firing. Try a couple hundred watts of
> lighbulbs. If you do misfire the load will quickly drain the excess flux.

I like this idea, I will now incorporate a second load on the dimmer with a
light bulb. I was using a split wall socket setup in a box that I could set on
the work bench. It was orginally set up to provide tempertaure control for a
cheap soldering Iron. I can replace the sockets with a normal two spcket outlet
and plug a light in.

> I really KNOW about this :^P   Welding controls use transformers fired on
giant
> 'light dimmers'
> and when one goes bad we get really, really unhappy customers.

Again in my message, I said Don't do this unless you know what you are doing. I
have been doing electronics for (whispering . . . over 40 years) along time. I
burnt up a lot of things when I started and still do occasionaly. But whats the
fun of a job or a hobby if you can't occasional generate strange acrid smells
and puffs of smoke. My son's favorite event is still a bright flash of blue
white light from a tube amplifier followed by a bang and a puff of conffeti and
smoke pouring out of the back of a tube amplifier. I still think the problem on
that one was a very old and slighty abused tube failed by saging and shorting
two elements.

Hurricane Ivan continues:
The insurance adjuster found losts of extra things to write up, that I didn't
spot, so that maybe we'll have enough money to put a new roof on instead of
patching the roof. It would be nice to have a roof that doesn't leak and no
parts blow off in the next storm.

The adjuster also said that if I do the work to cut my trees down, the insurance
company will pay me for the labor time I spend. I planed on cutting the trees
myself. One lady got charged $3000 for less work than I paid $600 for last
spring.

Tim Daugard
30.4078N 86.6227W
Who over slept and spent time on this, is going to start the week behind the
power curve.




> H^) harry
>