A series capacitor can be used along with a shunt zener (rated at the desired voltage plus 0.6V). This is then followed with a series diode (like a 1N4004) and a shunt filter capacitor. It looks something like a voltage doubler circuit with the first diode replaced with a zener. For a positive supply the cathode of the zener is connected to the series cap and the anode of the 2nd diode is connected to this same point. The output voltage is taken across the 2nd filter cap (100uF or so). The series cap must be rated for the AC peak voltage used. The circuit uses energy from both positive and negative cycles and since the capcitor is reative, little energy is dissipated in it. It's value depends on the current requirements and line frequency. I've used a 0.1 uF at 240V RMS 60Hz to supply about 15 mA or so. Be careful: there is no isolation from the AC line as would be with a transformer! Also, its a good idea to have a resistor in series with the 1st cap to limit current on startup. Maybe 20 Ohms or so. Dan Mulally ----- Original Message ----- From: "William Chops Westfield" To: Sent: Saturday, May 20, 2000 1:09 AM Subject: Re: PIC PSU - Multiple Zener Diodes > >> I need to build a PIC Circuit which will draw a total of 500mA, > >> and I need a transformerless PSU to convert 230VAC to 5VDC. > > > > At 500mA the power dissipatioj in the resistor will be 315 * > > 0.500A = 157 watts !!!! (!!) > > This is not a practical way of getting 500mA at 5V from the mains! > > What he said. > > > A 3 VA transformer is small, cheap and much much safer. > > You didn't specify why you wanted a "transformerless" design. Perhaps > you'd be more interested in some of the somewhat new switching power > supply chips designed to operate directly from rectified AC wall power. > (ie Motorola/ONSemi MC333622) By operating the transformer at high > frequencies, the physical size of the iron shrinks considerably - I > seem to recall an press release describing a 5V/1A supply that fit in > the space of a standard 3-prong plug... > > BillW