Hector Martin [PIClist] wrote:
> 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? :)
>   
No surprise. I have two designs that drive straight off of the AC mains. 
You need at least a CAP
(300vDC for 115V,  450vDC for 230VAC) and at least ONE rectifier/diode, 
and a 5.1V zener diode.
It works great to supply a few mA. In order to pass UL, you will need to 
insert a small fuse (50mA)
to protect things from fire if the cap  decides  to short. You will also 
need to protect the circuit from
lightning strikes, 'cause an overvoltage will take that cap out 
instantly. Generally speaking, the caps
need to be plastic, not paper or electrolytics.

While developing the code, using a scope, etc, make sure you use an 
ISOLATION TRANSFORMER
to protect yourself and/or equipment from injury.

--Bob 
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