Alexandre Domingos F. Souza wrote:
>
>         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 120mA.


Hi Alexandre! This can be done if you REALLY want
to do it. The Mchip app note shows 45mA, and that
is with a half-wave rectifier (two diodes).

Just change to a proper rectifier bridge, this will
double the current to 90mA. Then using a 30% larger
value of the main Xc cap will give you 120mA.
This web page has Traffic.PDF sent by Richard Ottosen
which shows a larger current Xc supply with a
full-wave bridge:
http://centauri.ezy.net.au/~fastvid/tube4w.htm

IMPORTANT! using a full wave rectifier will give
lethal voltages at ALL points of your 24v circuit.
You no longer have a reference ground! I know you
said you were using an optocoupler but remember at
all times this circuit is dangerous...

Some handy points, you can get reliable relay
operation at half the hold current usng the old RC
trick, which gives it full current for pull-in
but only half current under hold. Costs one resistor
and one 470uF electro. Maybe this would get you
down to 60mA circuit current. I have used this trick
down to about 25% hold current.

Remember the Xc supply ALWAYS runs full current,
so if your 120mA relay is OFF, something else
(large zener?) must be drawing the 120mA. Have a
serious think about this. Xc supply is best for
circuits that always draw the same current.
:o)
-Roman

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