If the relay is a simple single pole on/off type, and not too many amperes are
going through it, you could replace it with a triac and a zero crossing triac
optocoupler interface (as low as 5ma current draw).  If the load is inductive,
you will need to add the proper snubber circuit too.

You could then use a VB409 (from ST microelectronics) which is rated at 70ma out
of its 5vdc 5% regulated pin (which should be more than enough to drive the
triac opto and the pic), but I think it can kick out a bit more out of the
12~15Vdc unregulated pin (something like 140ma for 10ms max).  Like the X cap
setup, this part does run your circuit at line potential.  Unlike the Xcap
setup, it don't care if your not drawing anything from it, but it may need a
heatsink if your drawing more than 40~45ma.


-----Original Message-----
From: Roman Black <fastvid@EZY.NET.AU>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Thursday, July 26, 2001 4:14 AM
Subject: Re: [EE]: Transformerless PSU


>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
>
>--
>http://www.piclist.com hint: PICList Posts must start with ONE topic:
>[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads
>
>
>

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads