At 09:52 PM 7/25/01 -0300, you wrote:
>        Dear guys,
>
>        I think it's a basic one, but maybe fits the list
>
>        I designed a circuit with a PIC (12C508) that turns on and off a
relay, controlled by an external source. The power to this circuit (around
120mA) is a small transformer. I'd like to build a transformerless PSU into
this circuit. But EVERY scheme I found on the internet was unusable. I saw
something on the Embeeded Control Handbook from Microchip, but it only goes
to (if I'm not mistaken) 45 mA. Maybe someone can help me to develop (or at
least learn to) a simple transformerless PSU that can give me 12vcc / 120
mA???

I'd suggest not doing it. This kind of power supply has marginal
reliability in many cases, and
transformers are not that expensive in volume. If you must, then go to as
high a voltage DC relay
as possible to reduce the current. 24V at least. Keep in mind that without
isolation any input or
output is a potential source of DEATH to the user at any time in the
future, over the life of each
unit that you make. A typical 24VDC 360mW relay draws only 15mA. If you
don't know how to scale the
one shown in the MChip data then all the more reason to steer clear...
transformers not only are more
reliable but they are usually very effective at filtering noise and
transients out of your circuit.

A (bad) example of these is the crappy China-made IR light switches that
sell for about $7 US retail
on special (including die cast and molded housing, two CSA/UL light
sockets, PCB, relay and IR sensor,
supposedly weatherproofed, several pots and with a 4-color printed carton.
These things last about
one season around here.

Best regards,
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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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