I see what you mean. The problem as I see it is that the SCR cathode point is floating when the SCR is off and basically you need to generate a voltage drop somewhere. What about another cap - resistor-diode network connected to the neutral mains input. This seems to work on pspice! It does double the number of caps etc in this part of the circuit however and makes the cost savings more attractive. Note that diode D6 - the zener - can be used as the return to the anode of D5. This will save you a diode anyway - you might be able to do without the series R as well. Regardless, anything connected to these lines should not be exposed to accidental contact. Phase and neutral connections can be reversed, and even though the item will still work in this mode, the DC neg. line would be at full phase voltage. Put it in an earthed case and screw it up tight! The series cap should be an "X" rated variety to ensure it will withstand the AC mains and spikes etc. Richard ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Swichtec Power Systems Limited, | Phone: (++64) 3 343-3314 39 Princess St, PO Box 11-188, | Fax: (++64) 3 348-8871 Christchurch, New Zealand. | Email: rprosser@swichtec.co.nz Visit us at http://www.swichtec.co.nz +++++++++++++++++++++++++++++++++++++++++++++++++++++++ -----Original Message----- From: Jim Ruxton [mailto:cinetron@PASSPORT.CA] Sent: Tuesday, May 18, 1999 10:57 AM To: PICLIST@MITVMA.MIT.EDU Subject: Re: Driving Back to Back SCR's from PIC or Where do I Find a Cheap Hi Folks , Sorry to drag this thread on but I have a question with respect to Peters circuit. http://www.vertonet.com/members/pvh/scr.pdf In my application I am doing as Glen suggested. I have another diode on the anode side of D6 going to the -ve rail. I have found ,and it seems to make sense, that my circuit will work fine when the SCR is on however when it is off there is a problem. If one removes the SCR from Peter's circuit there is no longer a current path for the DC portion of the circuit if the input going to the load is -ve. I put a large (47K) resistor in parallel with the SCR which seems to work but I don't know if this is the best solution. I am using a 5 Mohm resistor going to the PIC to look for zero crossings so it is important that the PIC be referenced to one side of the AC line. If anyone has any better solutions than the 47K resistor across the SCR I'd be interested to hear it. Thanks Jim