Tony Roe said:
The most problem with identical filament transformers back-to-back is the
regulation, which can easily be 10-20% for each small transformers, so 20-40%
for the pair. So with 115VAC in, the output starts out with 110-114VAC at no
load, but might drop under 80VAC on load.
There's another probem waiting for you too Jim. Transformers have
copper losses, so your open circuit V_out for the first TF might be
say 10% or 20% high off load. This means the 2nd TF dosent have the
turns ratio one would think, and when driven at nominal V on its
secondary will never produce V_mains at its primary, it will always be
low.
The quick dirty best avoided way is to use TFs with primary taps -
here TFs had primaries with 200,220,240v taps for a long time:
adjusting the tap setting can correct the problem. But to do that one
must ensure the thing is properly fused, thermally cut-out-ed, and
fireproofly housed, since doing so can result in substantial
overcurrents, shorting metlting and fire. So for those that dont
understand how to engineer safely round the implications of that,
dont.
Using TFs rated at well over what youre drawing helps, since the
deregulation drops to only a percentage of the nominal regulation
figure.
A better way is to use a variac with your iso setup. A better way than
that is to add a few more turns onto your first TF's secondary to get
the right off load V_out on TF2. And an even better option is to add
more primary turns on TF2, long as you understand how to do it without
compromising insulation.
At best back to backing is a rough hack. Is that what you need?
Regards, NT
