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[Amps] Capacitance Amount Formula

To: <amps@contesting.com>
Subject: [Amps] Capacitance Amount Formula
From: G3SEK at ifwtech.co.uk (Ian White, G3SEK)
Date: Mon Feb 10 10:16:51 2003
Dr. David Kirkby wrote:
>jeff millar wrote:
>>
>> This is a ripple calculation based on the basic capacitor formula
>>
>>     I = C * dV / dT
>>
>> Let's say you want 3% ripple at 4 KV...that's 120V.  The capacitor charges
>> on each line peak, twice in each 60 Hz cycle, or 8.3 msec.  Rearranging
>>
>>     C = I * dT / dV
>>
>> Plugging in
>>
>>     C = 0.5A * 8.3 ms / 120V = 35 uFd
>>
>> jeff, wa1hco
>I think you will find the solution is not quite as easy as that - it 
>depends on type of rectification used - half wave, full wave bridge, 
>full wave two diodes, 3 phase ....

Jeff's solution is the one given in Horowitz & Hill, 'The Art of 
Electronics' and probably many other practically-oriented college texts. 
It is a good first cut at the problem, and applies quite well to the 
kinds of power supplies most AMPS users build.

>There's a classic paper on this published many years ago - I'm sure a 
>search on Google will find it.

However, that has now been superseded by computer modeling - it's 
faster, it's easier and it's more accurate. Probably the simplest 
program is Duncan Munro's 'PSU Designer' because it specializes in power 
supplies and has point-and-click selection of circuit blocks like 
transformers, rectifiers and filters.

If you download the program via my web site, you can pick up some 
examples for high-voltage supplies, and a substitute file of rectifier 
data including high-voltage rectifiers:
www.ifwtech.co.uk/g3sek/in-prac/best-of.htm#psud

You still have to learn to use the program, and a number of features 
will not be what you first imagine - like the value to use for a 
transformer's 'secondary' resistance, or when to click and when to 
double-click. It's all in the Help, so do read it.

If you've ever built a power supply, and only then found out that you 
chose the wrong transformer or other components, this program is for 
you! Highly recommended.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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