step down 240V ac 50Hz

Discussion in 'General Electronics Chat' started by garrettheart, Dec 16, 2009.

  1. garrettheart

    Thread Starter New Member

    Dec 16, 2009
    I would like to know is it possible to step down 240V Ac from a PSU to like about 12 V Dc without using a transformer.

    For eg using a full wave rectifier, zener diode, resistors and capacitors.

    For further detail,
    I have this project where I wanted to build a remote control socket. There would be a receiver to act as a remote to control the relay. Hence, I do not intend to use any battery supply to run it. I was think to use the PSU supply to run the device, but stepping down the voltage without using a transformer. Is it possible?
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    It is a common technique to run small none-isolated gadgets.

    For very low powers, just a high value resistor with a diode plus zener and smoothing cap can be used, eg. 47K 2W
    That will give you a couple of milliamps.

    For higher currents, the normal method is use a mains-rated (eg X type) capacitor, say 0.1 or 0.22uF in series with a low value resistor (47 Ohms).
    You feed that to two rectifier diodes, one out to the zener + smoothing cap and the other back to Neutral (which is also circuit 0V / Common.).

    The 'output' diode is connected with anode to the capacitor+resistor, the cathode out to the zener. The other diode has anode to Neutral/0V and cathode to the capacitor+resistor.

    The capacitor has a defined impedance at the AC mains frequency, 1/ 2 x PI x F x C (in farads). Unlike a resistor, it does not dissipate any significant power, so the circuit can run cool.

    Be VERY careful developing any direct-mains device - you should really use a safety isolating transformer while working on it, or temporarily use a conventional isolated DC supply until the electronics are working, then switch over to the none-isolated system.
  3. garrettheart

    Thread Starter New Member

    Dec 16, 2009
    Thanks for your advice. I think I get what I wanted. I will try build on it and thanks a million again