[sdiy] transformer question

[jhaible at t-online.de]

> Ok, torroids are nearly humless.

Even these can do nasty things (like producing noise) when
the core is saturated. Once I used a small pcb mount toroid,
and even though I did not exceed the rated power, it produced
an ugly sound. Why ? Because I used half wave rectifiers
(never a good choice, but sometimes necessary ...), and the
whole power was transmitted during a very small fraction of
each 20ms cycle, i.e. the momentary current was too high.

> They have low stray inductance. This means they represent 
> a  
> very stiff voltage source, e.g. if you short them
> you'll be sorry. They also tend to blow fuses when 
> turning on. 
> For rectifiers with caps this means huge
> peak currents, a lot of noise in the mains supply.
> At least I think so. Ok, we can handle that and use a 
> inductor coil to smooth that current. ANother component.

I think one (the ?) reason for the low impedance is the low
copper resistance of the windings. Don't ask me why this
is typically lower than on other transformers - maybe
it's just that there is more space available for thicker 
wires, or maybe you need less windings because of a different
core material ? Anyway, a simple measurement with the Ohm-meter
reveals a typically smaller DC resistance. You can add 
an external resistor in series with the secondary winding
to get a less "stiff" behaviour. And if fact this is what I 
normally do. However, this will *reduce* the secondary voltage
under load: A transformer which has a higher DC resistance
in its winding has its rated voltage *under nominal load*
(and it will goo higher without load). If you add an external
resistor, the voltage under load will be smaller than the rated
voltage of the transformer, of course.

So the solution is: Select a toroidal with slightly higher
secondary voltage, and use external resistors to fit your needs.
There are also special rectifiers with considerable resistance,
but I've never used them. I think Harry can tell us about these.

JH.