Exact match. Not showing close matches.
'[EE]: Cheapest regulation circuit I've ever seen'
Hector Martin [PIClist] n/a
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? :)
Hector Martin (marcansoft.com) hector
Public Key: http://www.marcansoft.com/hector.asc
M. Adam Davis
Transformerless power supplies are not uncommon. Microchip has an app
note that sheds some light on the subject:
And there've been countless threads here on the subject.
They are allowed by UL and other safety regulations/organizations as
"safe" as long as the user never comes in contact with any part of the
circuit that's not isolated from the AC line.
And yes, it's vastly cheaper than a switched power supply. The total
cost for the power circuit is likely under $0.50 in small quantities.
On 7/17/06, Hector Martin [PIClist] <hector+marcansoft.com> wrote: piclist
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
to protect yourself and/or equipment from injury.
X-10 devices (at least all the ones I've dismantled) use
this sort of power supply to run the electronics. I'm
not sure how reliable they are when the mains goes out
of spec (lightning strikes nearby, etc) but they work
well enough to be sold to consumers!
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.
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Michael Rigby-Jones wrote:
In my designs, I use a transorber (PK6X) in parallel with a 1 ohm
resistor in series with the whole circuit. The fuse is just for UL, so I
used the smallest possible size.
The cap I used was a 1uF 400VDC non-polarized polyester cap. Mine was
made in Germany. I then used 1N4005's as rectifiers. I used a 100uF @
10V aluminum as a supply filter before applying it to the Zener for 5V
regulation. I placed a small resistor before the zener, I think about
100 ohms,. None ever failed, though the client was in Tennessee, a very
high-lightning area. The load was 5mA max, 2mA typical.
The way UL works here is that if a fault occurs, the source must be
tripped, and the load must not ignite. The PCB has to be FR4, etc.
It was cheap, and dangerous to use if open, but this was completely
potted and worked prefectly.
Alan B. Pearce
>In my designs, I use a transorber (PK6X) in parallel with
>a 1 ohm resistor in series with the whole circuit. The fuse
>is just for UL, so I used the smallest possible size.
Could you sketch this please? I am a little confused by your description.
>The cap I used was a 1uF 400VDC non-polarized polyester cap.
I presume you use an X2 class cap ...
Alan B. Pearce wrote:
>> In my designs, I use a transorber (PK6X) in parallel with
>> a 1 ohm resistor in series with the whole circuit. The fuse
>> is just for UL, so I used the smallest possible size.
> Could you sketch this please? I am a little confused by your description.
>> The cap I used was a 1uF 400VDC non-polarized polyester cap.
> I presume you use an X2 class cap ...
I replied with one of the actual schematics used... If anyone else wants
a copy, I'll send it offline.
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