SparkFun Forums

[Riiser]

Hi There!

I've been trying to reverse-engineer a Kill-A-Watt meter latly and It seems like They have this great power supply made up by a simple capacitor and some other basic components:
http://electroschematics.com/5678/capac ... er-supply/
This is really genious, because for alot of projects like home automation controls and such don't require alot of power, but needs to be compact so it can be embedded (almost) everywhere! This regulator Is really efficient too!
I would really like to make this myself, but I don't trust my Knowledge about analog circutry combind with High voltages...

And if you guys decide to make it, be sure to make one compatible of the 230v mains we have here in Europe

[phalanx]

Here is a variation of that design I used on a recent project. I use caps on both AC lines to improve isolation although I'm still very cautious about how I connect to it and take measurments from it. I haven't tested it beyond the circuit it's attached to but more than likely it's overkill since the PIC10F and LEDs together only use about 5mA.

Here is its basic schematic along with the PIC10F circuit.


Here is the actual board it's on. It fits in the space between the pink capacitors (C7, C8) and the electrolytic (C9). The safety resistors R13 and R14 are on the top layer directly under C7 and C8 while the bridge rectifier, diode D8, and the Zener are on the bottom layer inbetween C7 and C8.


-Bill

[Riiser]

Thanks alot for the schematic! It really helped! Did some testing in a simulator and things made more sense now. But is it corrent that there is a constant current through the circuit making it not so efficient? Is there a way to implement a big capacitor that gets recharged by switching on/off the supply when needed? This would increase the efficiency of the circut alot, and then there is no reason for choosing high calues on the caps that are in series with the AC(Pink ones).

[phalanx]

Any power not consumed by the rest of the circuit will be dissipated by the Zener diode. Yes the 1uF caps are larger than necessary but I decided to trade off overall efficiency for substantially less ripple on the output. It's up to you as the designer to determine what your application requires.

-Bill

[n1ist]

Be real careful with non-isolated power supplies. Remember that unless you specifically isolate things, scope grounds, serial and usb grounds, some power supply negatives are all tied to AC safety ground, and to your mains neutral back at the service entrance. Your circuit ground may, however, be at line potential with spectacular results if you connect them. I always use an isolation transformer when powering such circuits under development, programming, and testing.
/mike