>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 17 July 2006 21:08
>To: Microcontroller discussion list - Public.
>Subject: [EE]: Cheapest regulation circuit I've ever seen
>
>
>I just got a board from a dead de-humidifier, which was 
>powered straight from 220VAC. What at first looked like a 
>custom DIP18 chip certainly seems to be a PIC judging by the 
>pinout. No transformer for power input. No 7805. No nothing. 
>The regulation consisted of a big 1uF unpolarised cap, some 
>(smallish) inductors, a large resistor, diodes, and a couple 
>Zener diodes (which I assume did the "regulation"). Is this 
>reliable/safe? Certainly the most simplistic regulation 
>circuit I've ever seen (+5V was tied almost straight to 
>Neutral!). Wonder if it's cheaper than an off-the-shelf 
>switched 5V supply though.
>
>Any thoughts on this? Is this actually in common use, and if 
>so, is it safe or reliable? Were the designers completely 
>crazy doing this? :)


Odly enough I have been asked to fix a dead de-humidifier with what sounds like exactly the same circuit.  A nearby lightning strike took out both of the zeners (a 5v1 and a 5v6 IIRC), though not got around to fixing it yet.

Cheap and basicly crap, but it works "well enough" and lasts "long enough" for the manufacturer to save a few pennies on each unit.  It's not "safe" in that there is no isolation from the mains, but on a double insulated appliance this shouldn't be a problem.  Just don't go clipping your 'scope ground onto the circuit.  They can be reliable is properly designed, but using a capacitor as the main dropping element means that the circuit is idealy designed to accept noise and transients which really punish the zeners, so there is usualy a series resistor to drop some of the voltage and limit current during transients.

Regards

Mike

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