At 09:38 PM 6/8/2004, Harry Pfeifer wrote: >If you use zener diodes, they must be capable of carrying the full load >current of the mains operated device. You may need to allow 10-15 amps, >which >could be a very expensive zener diode What on earth are you talking about? The type of transformerless power supply Matthew is talking about can be approximated as a constant current source feeding the load. A voltage clamp is needed to absorb the excess current not required by the load - a zener diode works well here. The maximum current that the zener has to handle is when there is no load. That current is the amount of current supplied by the approximated constant current source. Using a 1 uF capacitor with a 100R series resistor results in a current of about 45 mA. Assuming this is a half wave supply (the most common), the zener has to handle an average of half that value (about 22 mA). However, don't skimp on the zener size. The above is the steady-state condition. The zener is also subject to a much larger in-rush current when power is first applied. We generally use only 1n473x & 1n474x series zeners in this application - anything smaller may fail in the field. Same with the series resistor - we use at least 1/2W metal oxide resistors that have a *wonderful* peak power rating (as opposed to carbon film resistors that don't). dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 20 years of Engineering Innovation (1984 - 2004) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu