Lawrence Lile wrote:
>
> I was hoping someone would suggest the dropping capacitor.  Take apart any
> steam iron with AutoShutoff,  X-10 module, hair dryer with autoshutoff, etc.
> and you are likely to find this type of supply.
>
> You have to full-wave rectify after the capacitor, or it won't work right.
> Place a 22 ohm 1 watt resistor on the hot side, as well as a fuse if you can
> afford it.  We actually use the 22 ohm resistor as a fuse (cheaper).  I
> never regulate the output with anything other than a load.  Usually use a
> resistor-zener power supply to step down to 5V.  Everything between the
> rectifiers and the step-down resistor may see 24V, may see 50V, who knows?
> I usually am driving a relay in there, with a transistor rated at least 150V
> (2n5551 = US$0.05) and a capcitor rated 63V, 75V surge.    Once the relay
> turns on, I can rely on 28VDC +/- 5v (which is good enough for a 24v relay).
>
> Now there are many people who will begin flaming me, saying this type of
> power supply is unreliable, cheap, problem-prone, cheeesy, etc. etc.  They
> are right!  This is only a useful design in an under US$20 appliance with
> heavy cost pressure and a high allowable failure rate.  No isolation, no
> regulation, lots of ripple, and the whole thing costs about US$0.25 -
> everything a person could want in a power supply!


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

So at 120v 60Hz a 1uF cap will supply average
current of say 110v/2700= 41mA.
This is with full wave rect bridge and a 10v load.
With a half wave rect, as per the Microchip
"transformerless" appnote, you only get 20mA.

Some good design tips, always use a series resistor
between active and the cap, about 5% of the Xc ohms
value, so 120 ohms is good for 1uF cap. A 1.5 megohm
resistor across the cap is a good safety measure and
is required for UL listing also.

Then important stuff!!
Capacitor quality is everything, I prefer the 630v
polyester caps, these are great especially if you can
get the blue "milspec" ones. With a 120v mains you
might get away with 250v poly caps. Always use a zener
on the output or voltage can climb to full mains
volts.

The full wave bridge rect gives double the current
for the same size cap, but BEWARE all parts of your
load circuit will swing to full mains potential. Be
real careful with buttons, etc that someone might
touch of get finger sweat etc.

My preferred type is the two diode half-wave example,
with this type your load has it's neg lead at the
mains neutral potential, normally 0v. Much better
if you have buttons etc attached. I have a Ni-Cd
charger built like this that has given many years
good service, as a constant current supply with
a meter and two alligator clips.
-Roman

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