[sdiy] Unmarked transformers

[harrybissell]

Crow is of course correct. We actually use inverse-parallel SCRs.  Light
dimmers
use triacs, which are worse.  And we DO control transformers by phase
control... but
you would not like to see the control circuits we use. Or maybe you WOULD, but
then
we would have to kill you.

The reactive load has a power factor (other than 1 = resistive)...so the SCR
does not
shut off until current passes through zero, and now that is phase shifted from
the actual
zero cross of voltage.  You cannot gate the opposite SCR until the first has
turned off.
Usual methods include KNOWING the power factor (or measuring it) and / or
making a
spray of gate pulses so that sonner, or later... you hit it at the right time

A Variac is the best test method for small transformers. Small is 'anything
you would EVER
power a synth with' :^P

H^) harry

The Old Crow wrote:

>   My transformer tester at my old workplace used inverse-parallel SCRs.
> MUCH better for reactive load driving.  My boss's used saturable-core
> reactors...  Triac control in many industrial situations is crappy in
> general.
>
>   Industrial controls use a lot of oddly-familar circuitry; VCAs in firing
> packages, tri-square LFOs in quartz heater controls, pulse width controls
> in about every 1ph and 3ph firing circuit ever made...
>
> Crow
> /**/
>
> On Sun, 3 Oct 2004, harrybissell wrote:
>
> > 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