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From: "Phil Allison"
Subject: Re: PSU ripple current
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Tue, 14 Jan 2003 09:11:36 +1100
NNTP-Posting-Date: Tue, 14 Jan 2003 09:01:05 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"steve c l" wrote in message
> Jim Backus wrote in message
> > The way I've always done it is to assume the peak to peak ripple is a
> > sine wave (clearly it isn't), calculate the equivalent rms voltage,
> > calculate Xc at the frequency in use and then Ir = V / Xc.
> > Obviously it's approximate, but by the time you've added a design
> > margin it should be safe.
> > I'd better check where my flame proof suit is now :-)
> > --
> > Jim Backus
> > bona fide replies to jimb(at)jita(dot)demon(dot)co(dot)uk
> > http://www.jita.demon.co.uk
> It's similar to the approach I was trying, but I was bothered by the fact
> that the value of Ir yieled appeared to be too large relative to a cheapo
> book on power supply design I'd seen, which suggested a rule of thumb
> of twice the max load current. I used the rms value of a triangular wave
> it happens, so your method would actually exceed mine and give an even
> greater margin of safety. I assumed like you a purely reactive
> load, whereas maybe the resistance around the circuit has more
> than I imagined. But what about this ESR? It's supposed to be responsible
> for the care needed in prescribing the right ripple rating. But the
> catalogues I've so far seen suggest piddling values that you'd expect
> be swamped by the reactance. So I'm baffled, and still very keen to
** We cannot see your maths or any figures, you may be making an error.
Say the cap is 4700uF and the voltage is 50 volts DC average. A
typical ripple voltage is 10 % so 5 volts p-p - OK ? For a triangle
wave the conversion factor is 0.57 of the peak so 2.5 x 0.57 = 1.42 volts
rms approx. The reactance of a 4700uF cap is 0.34 ohms at 100 Hz so
dividing gives 4.3 amps.
I = C dv/dt so a 5 volt drop in 5 mS gives I = 4.7 amps average DC
Is this what you did ?
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