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Thread: PIC PSU - Multiple Zener Diodes
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face BY : Dan Mulally email (remove spam text)



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 removed}
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