Hi, I'm working on a small project with a PIC and am stuck on a problem. I posted it on the Microchip forum - to save copying here, could anyone have a look and let me know if they can help?
http://forum.microchip.com/tm.aspx?m=174006
Thanks!

 

In the circuit you posted as a 2nd post in that forum, reverse the collector and emitter.....

 

Cheers, that helped!

 

You might have better luck if you replace the fullwave bridge with a discrete version that uses four schottky diodes. The lower forward voltage drop associated with the schottky diodes would yield more voltage for use in switching your transistor.

hgmjr

 

So just use 4x 1N4004? Thanks I'll try it out

 

So just use 4x 1N4004? Thanks I'll try it out

Actually, the 1N4004 diode is a standard silicon diode. What I had in mind was something like the MBR340 available from Digi-key.

hgmjr

 

OK thanks, trying to find a local equivalent (Australia)
Cheers
Would something like a 1N5822 do the trick?

 

Well, they are schottkeys.... another option would be to use germanium diodes (if you can find them) as they have a voltage drop of 0.2 - 0.3 volts (double that if you go thru a full wave bridge) like the old 1N34, or the OA90 etc etc.
Another option might be to use half wave rectification.
When you use a full wave silicon diode rectifier with 1N4001-7 or 1N918/4148 etc, you will need at least 1.8 volts of signal to turn on a silicon transistor (0.6 + 0.6 + 0.6) because the signal will pass thru 2 diodes + the B-E junction of the transistor.
If it were me, I'd do it like this.... Half wave germanium rectifier, a little smoothing then switching a silicon transistor

 

Cheers Gadget, I'm working on it

 

I like gadget's half-wave rectification approach. It appears to represent a good compromise for your application.

I considered suggesting germanium diodes but I decided that they might be tough to find so I went with schottky diodes. A drawback to germanium diodes is the tendency of their forward voltage drop to vary with temperature. This is only a problem if the circuit is going to be subjected to wide temperature variations.

I think the use of a half-wave rectifier composed of schottky diodes and some good filter caps should provide you with a solid solution.

hgmjr