William Chops Westfield wrote:
>
>     A cap reactive supply can be good. A 1uF poly cap
>     is about 3200ohms at 50Hz and 2700ohms at 60Hz if
>     I remember properly. Xc = 1/ 2pi f c
>
> And it has to dissipate as much power as a 2700ohm resistor, right?
> A bit of a problem, since caps are rarely rated for power dissipation.

Not so, no current flows through a cap, only charges built
up on it's plates. A good poly cap will run more efficient
and cooler than one of the micro-transformers you mentioned.


>     Capacitor quality is everything, I prefer the 630v
>     polyester caps, these are great especially if you can
>     get the blue "milspec" ones.
>
> Isn't a 630V 1uF "milspec" poly cap somewhat ... large?  Say, close to
> transformer sized?

Nope. I have buckets of them that the apprentices strip
from old TVs and such. You can get some smaller than others,
1uF about 30 x 16 x 7mm are the ones I use. You can get 0.33uF
in the same size package, just depends on the manufacturer.



>
> Actually, a shaver I took apart recently looked like it had a particularly
> interesting circuit involving a resistor in series with the primary of a
> very small transformer.  (It didn't actually have this, but it sorta looked
> like that originally.)  Can you do this in reality - limit the primary
> current to allow a smaller transformer?

I had one of those shavers too, a rechargable one with its
own charger built in. I pulled it apart too! I think it
is some type of saturation transformer system, so the NiCd
being charged acts like a short circuit and overall current
is limited by the resistor and special cored transformer,
dumping max and constant 100ma or so into the NiCd.
Fairly unconventional, and real messy when the NiCd
eventually fails.
-Roman

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