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DC Filter Choke on low voltage power supplies

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Edward R Cole

DC Filter Choke on low voltage power supplies

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).

73, Ed - KL7UW, WD2XSH/45
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Don Wilhelm-4

Re: DC Filter Choke on low voltage power supplies

Ed,

Using a large filter choke in a relatively low voltage supply is
difficult because of the voltage drop across the choke - and that
voltage drop will change with current draw changes - so instead of
improving the regulation, it will actually worsen. The problem is in
the size wire used to wind the choke. In other words, a 25 volt drop at
3000 volts is a drop in the bucket, but even a 10 volt drop at 50 volts
is likely to be too much.

73,
Don W3FPR

On 2/5/2012 12:54 PM, 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).
>
>

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Steve Kercel

Re: DC Filter Choke on low voltage power supplies

Don and all:

Don is right, but I think that the situation is even worse than Don
describes. The power supply for a tube-type rig need only generate up to
a few Amps, not the 40 or 50 Amps that you might need for a big
solid-state rig. An inductor on the order of several Henrys and rated
for several tens of Amps would be impractically HUGE (and hideously
expensive) for table-top gear.

73,

Steve
AA4AK

On 2/5/2012 1:56 PM, Don Wilhelm wrote:

> Ed,
>
> Using a large filter choke in a relatively low voltage supply is
> difficult because of the voltage drop across the choke - and that
> voltage drop will change with current draw changes - so instead of
> improving the regulation, it will actually worsen. The problem is in
> the size wire used to wind the choke. In other words, a 25 volt drop at
> 3000 volts is a drop in the bucket, but even a 10 volt drop at 50 volts
> is likely to be too much.
>
> 73,
> Don W3FPR
>
> On 2/5/2012 12:54 PM, 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).
>>
>>
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Jim Brown-10

Re: DC Filter Choke on low voltage power supplies

In reply to this post by Edward R Cole

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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