[sdiy] Unmarked transformers

[harrybissell]

TIm Daugard wrote:  <snip>

> 2) I use a remote switch box with a dimmer switch with a long cable to the
> transformer
> 5) I just barely crack the dimmer on to start with while standing

ooohh I'm a little worried about this tip....

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

perhaps killing the transformer...

The dimmers are not expecting a reactive load. Unless specially designed
to compensate for this, they may half-cycle or do other bad things. Half-cycling
essentially puts DC on the transformer primary... guaranteed SATURATION !

A Variac is the correct way to do this test. It would also be wise to monitor
transformer
primary current to have a clue as to saturation (current would suddenly and quickly
increase).
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..
which is also essential (yay Larry !!!)

If you INSIST on doing the light dimmer trick...a large resistive load in parallel
with the
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 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.

H^) harry