Re: DC Filter Choke on low voltage power supplies

Posted by Jim Brown-10 on Feb 05, 2012; 9:15pm
URL: http://elecraft.85.s1.nabble.com/DC-Filter-Choke-on-low-voltage-power-supplies-tp7256286p7256780.html

On 2/5/2012 9:54 AM, Edward R. Cole wrote:
> Recent discussion of use of a filter choke with a battery powered K3
> prompts my question in another application.  I have a 50vdc 50amp
> switching PS which will power a 1100w sspa.  Output filtering may be
> needed to suppress switching impulses on the 50vdc output.  I have a
> large filter choke that was used in a 3000v PS which is probably at
> least 5-8Hy.  Do I need a bleeder resistor across it for protecting
> circuits when powering off the switching PS?  Or is using the large
> filter choke not a good idea at all.  What size filter chokes would
> be recommended?  The 50v PS is a HP PS intended for telephone industry (51v).

As others have noted, the choke you describe is quite inappropriate. Any
filtering required would be RF filtering, not low frequency filtering,
and the most effective choke would be simply winding multiple turns of
the DC conductors around a suitable ferrite core. Winding a SINGLE
conductor forms a differential mode choke, and if you used a fairly
large core, would be fine for the relatively small current needed to
operate the receiver, but would likely saturate with the larger current
on transmit.  That's NOT a problem.  Winding BOTH conductors through the
core forms a common mode choke, and there's no danger of saturation.  
See measured data for various suitable ferrite cores in

http://audiosystemsgroup.com/RFI-Ham.pdf

Note that filtering is often necessary on BOTH the AC power line cord
and the DC line. I use some noisy switchers that I bought cheap at a
hamfest to float-charge big storage batteries that run my station, and
I'm using common mode chokes on both cables.  I also have capacitor
rated for use on the 120V line across the line (that is, line to neutral).

73, Jim Brown K9YC
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