>     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?

No.  Ideal capacitors are incapable of dissapating any power.  This is
because their current and voltage are 90 degrees out of phase.  Energy is
only temporarily stored in one part of the cycle, then released in another.
This why a capacitor coupled power supply can be so efficient.

Real capacitors do dissapate power because the dielectric material
dissappates energy in the process of storing and releasing the charge.
Fortunately this isn't much of an issue at 60Hz.

Back in college I once made a small radio transmitter at about 1MHz that fed
its signal into the AC line.  I had a push-pull class C output circuit
driving an LC tank.  Each half of the L had an adjustable slug and a .1uF
ceramic capacitor accross it.  I would tune it by adjusting the slugs for
maximum output on an oscilloscope.  I noticed that I had to keep re-tuning
it for the first few minutes after switching it on.  I figured it was
settling thermally, but the drive transistors stayed pretty cool the whole
time.  Then I accidentally touched one of the tank caps and almost burned
myslef!  I was amazed that the capacitors were that inefficient at 1MHz.
Then I calculated the current and realized I was dumping over 3A RMS thru
them.  It was a great lesson on real world versus theoretical electronics.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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