AC power on front panels

Dave Brown tractorb at
Sat Jan 14 13:59:54 CST 2006

----- Original Message ----- 
From: "Chuck Guzis" <cclist at>
To: <cctalk at>
Sent: Sunday, January 15, 2006 7:45 AM
Subject: Re: AC power on front panels

> On 1/14/2006 at 11:09 AM gordonjcp at wrote:
>>Surely a transformer would need to be powered all the time?
> So what?  Your central heating system and doorbell have one.  Much, 
> if not
> most, modern electronic equipment, such as PCs, televisions, etc. 
> all have
> some tiny draw from the AC mains even when powered off.    If all 
> you
> needed was a very small DC supply you could do away with the 
> transformer
> and perhaps use a couple of capacitors in series with each side of 
> the
> line, followed by a rectifier and a zener--it wouldn't provide quite 
> the
> isolation from the line that would be ideal, but it wouldn't need a
> transformer.
> Cheers,
> Chuck

For a practical implementation of this sort of arrangement see the 
circuit here-

I threw this together to get round an issue with under-rated wafer 
switch contacts being used for mains switching.

DaveB, NZ

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