[Elecraft] Capicator Size for filtering across battery power

[David Cutter]

The car stereo situation is different in that the battery is charged from a 
high frequency 3 phase supply ie the vehicle alternator.  The ripple 
frequency is quite high, from low kHz at tickover to 10s of  kHz when you 
are motoring.  In either case it's a lot higher than the 120Hz ripple 
frequency of your home charger, ie it's much longer between current bursts 
injected into the battery.  Your vehicle supply is voltage limited to around 
13.8V average and the peak of this voltage under normal conditions is 
probably around 15V - 16V or so, though it can have considerable nasty 
spikes on it during engine starting and when large solenoids pull in and 
drop out.  That makes a vehicle supply quite aggressive but not hard to 
filter.  Your home supply can have nasty spikes as well but more important 
is the on-load, off-load and line voltage variations you can experience. 
For this reason, you need a much higher voltage rating capacitor than you 
might think at first sight.  I would take a compromise on size and cost and 
go for 25V minimum and feel fairly safe at 35V as big uF as you like. 
Caution: when you first apply volts to your cap, it will suck an enormous 
current, possibly enough to weld the open ends as they touch, so, I would 
charge it up first with a modest resistor.

Ideally, the place to put the cap is near the radio so it absorbs 
fluctuations in resistive drop of the supply cables, but that is sometimes 
inconvenient electrically and mechanically.

David
G3UNA


----- Original Message ----- 
From: "Eric Sluder" <esluder84 at gmail.com>
To: <goldtr8 at charter.net>
Cc: "elecraft list" <elecraft at mailman.qth.net>
Sent: Sunday, October 07, 2012 3:29 PM
Subject: Re: [Elecraft] Capicator Size for filtering across battery power


>I typically use a high uF cap that is rated at 70-80 volts DC.  My current
> installation utilizes a 44,000 uF cap and all is quiet for my DC power
> needs when running off of the battery fed by a charger.
>
> 73,
> Eric
> W9WLW
> On Oct 6, 2012 5:36 PM, <goldtr8 at charter.net> wrote:
>
>>
>> Gents,
>>
>> My setup uses four ()4 12v batteries in parallel for powering my K3 and
>> SGC powercube amp that I float with a battery maintainer.
>>
>> I believe that I have an RF problem with the power setup.   I have put
>> chokes in the power leads to the equipment but at certain times I can
>> hear the ferrites buzzing with either the digital code being sent of
>> matching my voice with SSB.   The chokes are clamp on types.  I am not
>> observing any RF problems or reported problems on the air.  But I think
>> that something is not right as of yet so I want to go after it to make
>> my shack even better.
>>
>> It has been suggested to me by a ham on the air that batteries just pass
>> this thru as they have no filtering capability.   His suggestion is to
>> but a bit CAP across the batteries to kill this phenomenon.
>>
>> So my question as I am not an EE by education is how would I properly
>> pick a CAP size and would different materials that CAPS are made out of
>> make any difference.
>>
>> It would be easy to go out and get a large CAP like used on car stereo
>> systems but I want to have a better shot of fixing it right than just
>> hacking away.
>>
>> As always thanks in advance for your help.
>>
>> ~73
>> Don
>> KD8NNU
>> FH#4107
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