[Amps] Transformer secondary resistance

[Steve Thompson]

On Tuesday 07 May 2002 11:14, Phil (VA3UX) wrote:
> At 09:42 AM 5/7/2002 +0000, you wrote:
> >Long ago, I read something that warned against simply paralleling
> > transformer secondary windings because of the risk of odd effects if the
> > voltages were not equal. Since then, at high power (whatever the
> > volts/amps), I've always used separate rectifiers on each secondary. Is
> > this over cautious?
>
> Yea Steve, the issue was with the potential for unequal load sharing
> between the two windings if they are not nearly identical in terms of
> impedance. This issue is raised primarily where two transformers are being
> considered for parallel operation but it also applies to the
> paralleling  of windings . In this case where the two windings are separate
> "donuts" wound in the same shop and intended to be 'identical, it's
> virtually a sure thing that the windings are close enough in impedance that
> they can be run in parallel with no concern at all.

That side of it was part of the piece, but I think the writer was also 
concerned about the possibility of 'backwards' current in the lower voltage 
winding - I can imagine it being minor where windings are nominally 
identical. I guess you'd need to measure the reactance of the windings to 
figure out if it might be significant where they were different - that's 
aside from figuring out what the effect might be other than reducing the 
primary current.

Steve