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Ahad, Mac 03, 2013

How to build Transformerless Power Supply

I have received a few emails asking for a transformerless power supply. Here is such a supply. This supply uses no heavy step down transformer and has an extremely low parts count. The circuit can be built very small and can supply small currents for small projects. The major downfall of this supply is that it is not isolated from the AC line and can only supply small currents.
Circuit diagram

Parts:
C1 0.39uF 250V Capacitor
C2 220uF 25V Electrolytic Capacitor
D1 1N4741 11V Zener Diode (See Notes)
BR1 1 Amp 200V Bridge Rectifier
MISC Line Cord, Board, Wire, Case
Notes:
1. The value of C1 can be increased to increase the amount of current the circuit can supply. With the values shown, the circuit can supply up to about 15mA. Remember to increase the size of C2 also.
2. A different value can be used for D1 to increase or decrease the voltage as needed.
3. Please note that this circuit is not isolated from 120VAC. Because of this, the circuit must be treated with caution and encosed at all times. Do not work on the circuit (or any other circuits attached to it) when it is plugged in.
4. You may want to add a resisor in series with C1 to limit current if the circuit is plugged in and the mains is at its full voltage.
5. If you are running the circuit from 220VAC, then use a capacitor rated at greater than 400V for C1.
6. If you want isolation from the AC line, you can connect up a small isolation transformer at the inputs of the circuit. Small 600ohm:600ohm audio transformers work nicely.

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