-----Original Message-----
From: William Fuqua <wlfuqu00@uky.edu>
To: Ian White, G3SEK <g3sek@ifwtech.co.uk>; amps@contesting.com
<amps@contesting.com>
To: <amps@contesting.com>
Date: 05 September 2002 16:24
Subject: Re: [Amps] Computer Grade Electrolytic Capacitors vs. Radial
>The 1.414 factor is the maximum assuming zero source resistance. It will be
>less with series resistance.
>
>73
>Bill wa4lav
>At 01:07 PM 9/5/02 +0100, Ian White, G3SEK wrote:
>>Steve Thompson wrote:
>>>
>>>>For an inductive input filter the RMS ripple current is equal to the
output
>>>current if you have a >single phase supply. For a capacitive input
>>>filter the max ripple current will be equal to the >square root of 2
>>>(1.414) times the DC output current.
>>>
>>>
>>>Doesn't it vary with source resistance and C/R values? I use a rule of
>>>thumb of 3x for capacitive input - it was very instructive putting a .1
>>>ohm resistor in the bottom of the capacitor stack and hanging a 'scope
>>>across it. The example circuit in the Duncan software has a cap input PSU
>>>putting 450V across 5k. DC current is 89mA, rms current in the cap is
>>>226mA and 248mA in the transformer winding.
>>
>>I haven't checked that example, but one "gotcha" in the Duncan program is
>>that the default results in the tables include the power-on cycles where
>>current can be very high.
>>
>>The solution is to set the "Report from" time a few cycles in.
Good point Ian, I hadn't spotted that. I changed the delay before analysing
from 100ms to 1000ms - should be well into steady state. Now, dc current is
89mA, capacitor rms current is 202mA and transformer rms current is 221mA.
Ok, it's a bit lower than the original values but either stack up reasonably
with what I measure in practice. When rms current tends to be dominated by
peak I^2 and the current pulse in the cap is a narrow spike varying with
source and load R and cap value, it seems to me that the dc/rms ratio could
vary widely, and readily exceed 1.414. Can someone put me straight here?
Steve
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