[GreenKeys] Another idea for a loop supply transformer

[Randy or Sherry Guttery]

gil smith wrote:
> 
> I use an isolation transformer with 115 output, and dual primaries
> for 115 or 230 input.  Wired with primaries in parallel you get
> about 160VDC supply (after rectifier/filter).

To which Douglas W. Jones responded:

> Eek!  Don't do this!  Wiring primaries or secondaries in parallel
> when they're wound for different voltages is not, in general, a good
> idea.

The type of transformer Gil is talking about - this is fine. Many
transformers have two PRIMARY windings - both for 115V input. If you put
both windings in series (phased adding) you can then apply 230VAC across
the pair - and the transformer will provide the correct voltage(s) out.
If you put the two primaries in parallel - you can apply 115V and still
get the same output(s) from the secondary(s).  In Gil's example - he's
talking about a transformer with dual 115V primaries and a single 115V
secondary. By wiring the two primaries in series - but instead of
applying 230V as usual - apply only 115V - the 115V (rated) secondary
will only provide half the voltage - or 57.5V - since only half the
"expected" primary voltage is applied.  As he noted "further on" in his
post - since the secondary is now only supplying "half" - after
rectifying and filtering - you can expect around 80VDC for your loop
supply. This is a good solution if you don't have a "well stocked" junk
box.

For those of us with the spare (free) parts - the "back to back"
filament supply is quite attractive.

The circuit for a battery eliminator for a "farm sets" is on this page:

http://www.comcents.com/mhrs/theprsup.html

While this supply is way more "complicated" than needed for a TTY loop
supply - it shows "back to back" transformers in circuit - and can be
quite useful for other similar projects.

best regards...
-- 
randy guttery

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