I had the same reaction when I first saw it. Several long diatribes on the PIClist have railed against the idea for these very reasons. However, I am not the only one to use this trick, most any auto-shut-off steam iron has a similar circuit. The reason is, there is no place to dump heat from a dropping resistor inside a steam iron chassis (it is already hot) and there is no money for a transformer, nor room for it. You will also find this in the electronics of many other cheap appliances. I am not aware of any problems due to the power supply in any of these units. -- Lawrence Lile Martin McCormick Sent by: pic microcontroller discussion list 05/26/2004 10:58 AM Please respond to pic microcontroller discussion list To: PICLIST@MITVMA.MIT.EDU cc: Subject: Re: [EE:] Obtaining 5 Volts from live wire llile@SALTONUSA.COM writes: >There is a common circuit used to drop mains voltage to reasonable levels, >that works generally like this: > >A capacitor is connected to the AC mains on one end and to a full wave >bridge rectifier on the other. It can be called a dropping capacitor. The >output of the full wave bridge can be connected to a zener diode or some >other suitable regulator. The circuit will not work correctly with a >single diode, only a full wave rectifier will do. This is a clever circuit all right but it kind of scares me. It's proper operation as opposed to smoke-venting mode depends upon the reactive resistance of a 1uf capacitor at 120 volts at 60 or 50 HZ. It is possible for something like a brush motor or switching power supply to dump energy back in to the line at all sorts of frequencies. Not only that, but the wave form of all that trash could be anything. Do these circuits get fried very often by sustained radio frequency-like hash on mains wiring? The circuit is basically a high-pass filter being operated at the extreme edge of its roll-off. When one gets in to radio frequencies, that 1uf capacitor will begin to pass most of the signal appearing at the input. If there is any energy there, it's going to heat stuff up until something pops. It is probably that flame-proof resistor, but I am surprised more of these types of voltage dropping circuits don't vaporize more often. Oh well- Who am I to argue with a million steam irons?:-) Martin McCormick WB5AGZ Stillwater, OK OSU Information Technology Division Network Operations Group -- 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