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'[EE]: Custom transformer source anyone?'
2002\06\21@071925 by Roman Black

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Chris Loiacono wrote:
>
> Does anyone have a source for custom PC mount transformers that they can
> share?
> I need to convert a 7805 based supply from a 230VAC line to 277V and 230V
> 'standard' transformer primaries are only OK'd for use 10% above their rated
> voltage. I am hoping to be able to have a few specials wound, but I need a
> source.....anyone, please?


(crazy idea following) :o)
If your output current is relatively constant you
can use a non-polarised capacitor in SERIES with
your transformer primary. Use a 630v Poly cap,
probably start with about 0.47uF and see how that
goes. The idea is that the cap drops *about* 45vac
(like a resistor would) but without dissipating any
real amount of heat. That way you can run your
230vac transformer from 277vac, and use a switch
to short the cap (voltage selector switch).
You should also use a resistor about 470k across
the cap, and one in series dropping about 10% of
what the cap does. These are needed for UL cert to
I think. Yeah it's a bit crazy, but cheap and
*probably* a solution for you?
:o)
-Roman

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2002\06\21@121033 by Dwayne Reid

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At 09:18 PM 6/21/02 +1000, Roman Black wrote:

>(crazy idea following) :o)
>If your output current is relatively constant you
>can use a non-polarised capacitor in SERIES with
>your transformer primary. Use a 630v Poly cap,
>probably start with about 0.47uF and see how that
>goes. The idea is that the cap drops *about* 45vac
>(like a resistor would) but without dissipating any
>real amount of heat.

This can work but you have to be REAL careful about avoiding
resonance.  For example, one of my neat tricks is to take a fan rated for
use at 230 Vac and operate it from 120 Vac by putting a capacitor in series
with the fan motor.  With the right sized capacitor, the fan motor sees
about 180 - 190 Vac.

dwayne


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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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