[GreenKeys] Another idea for a loop supply transformer

[Peter Gottlieb]

Regarding isolation when using a dual primary transformer, not all 
transformers are a problem.  I just dissected a transformer for another 
project which consisted of a tape wound dual bobbin design with each 
primary on a different bobbin.  Isolation would be phenomenal.

For my loop supply I used a simple isolation transformer.  I accepted 
the 170 volts for the loop and built a constant current supply with 
integral switching and I must say the selector magnet operation is very 
crisp and clean.  The current waveform is very nice.  I could have made 
a little regulator to drop to 120 volts but since everything worked so 
well I didn't bother.  I keep my fingers out of the gears and off the 
selector magnet wiring ;-)

Peter


Randy or Sherry Guttery wrote:
>
> 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...