Re: Shielding a non-grounded transformer
From: Ben Bradley (ben_nospam_bradley_at_mindspring.com)
Date: 07/26/04
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Date: Mon, 26 Jul 2004 12:54:50 -0400
On Wed, 21 Jul 2004 23:26:21 -0400, Circuit Breaker
<bagboy6437@ship.aol.com> wrote:
>Hello all
>
>I know this can be done. They do it to VCRs, TVs, DSS receivers... there
>must be a way I can do this, but Google isn't helping, nor is ask.com.
>
>I have a variable voltage, 450 milliampere, regulated DC power supply.
>Without going into too much detail, I used a 2-conductor power cord. I
>didn't think I'd need a grounding cord, because the box is entirely
>plastic and the secondary electronics are physically and electrically
>separated from the transformer (save but for the two secondary leads,
>of course). So, I didn't think I'd need a three-conductor grounding
>cord. If I *HAVE* to, I've got about a half dozen old computer supply
>cords I can cut up, but I'd rather stick with the cord I have (since I
>KNOW this can be done).
>
>I am trying to get rid of a hum I get any time I use this supply for
>radio-frequency projects (I have an FM-broadcast-band transmitter (less
>than quarter watt input, and that's mostly the caps, LED, and other
>non-transmitting components) (range is only a couple hundred feet), plus I
>have played with receiver projects as well. For some reason, it never
>dawned on me until a few days ago, that whether the hum is actually in the
>power output or not, the source is the transformer. I could probably move
>my projects a couple feet away using a long power feed wire (which I
>vaguely recall having a positive effect before, indicating the hum was
>picked up inductively as opposed to through the power leads), but that
>becomes inconvenient compared to having them right there with the supply.
I think there are several possible sources of the hum, and you
haven't narrowed it down enough to find the easiest and most
appropriate fix. I'm going to assume you have adequate power supply
filtering so that the 60 or 120Hz ripple on the output is
insignificant (though some projects might be unusually sensitive to
power supply hum).
The problem being with RF circuits makes me suspect RF generation
at the rectifiers. Put a 0.1 uF cap across each rectifier and see if
that fixes it. While you're at it, put one across each electrolytic in
the power supply as well. Check to see if this fixes it.
>...
>Directly connecting the case of the transformer to the "ground" side of
>the DC circuitry (I guess "common" would be a better term -- the "negative
>terminal", if you will) won't have any effect, will it? Could it be that
>simple?
This 'should' be okay and yes, it may help. Try it if the caps
don't reduce things adequately. Transformer cases are grounded on most
metal chassis anyway.
>Any advice? Thanks in advance, you'll be finishing a two-day search on
>the subject -- which is more than I usually waste on something like this.
>
>CJ
>
-----
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