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From: John Woodgate
Subject: Re: Wrestlemania Royale: Tantalum vs. Electrolytic Vs. Polys, etc.
Date: Sat, 26 Oct 2002 20:51:04 +0100
Organization: JMWA Electronics Consultancy
Reply-To: John Woodgate
NNTP-Posting-Date: Sat, 26 Oct 2002 20:46:13 +0000 (UTC)
X-Newsreader: Turnpike (32) Version 4.01 <5Z8C9wtxbnpWyFnyfFzqmVF739>
I read in sci.electronics.design that The little lost angel wrote (in <firstname.lastname@example.org
t.sg>) about 'Wrestlemania Royale: Tantalum vs. Electrolytic Vs. Polys,
etc.', on Sat, 26 Oct 2002:
>On Fri, 25 Oct 2002 20:17:40 +0100, John Woodgate
>>>Say for a low voltage 10~20v, low current 0.2A, what would be the best
>>>type of capacitor with smallest physical size?
>>Frequency? And where is this current going? Through the capacitor or
>Past it, I noticed that while fixing the LED widget for those PC SMPS
>that on some low quality PSU, the voltage appears to spike/dip enough
>to notice momentarily flickers.
What 'LED widget' do you mean? While the 12 V output of a PC SMPS isn't
tightly regulated, it is very unlikely to spike or dip enough to result
in LED flickering.
>Though I'm quite confused about the capacitor rating to use
>The thing was going to shop revealed that I either made a serious
>maths error or they aren't stocking the type of caps I need, hence the
>question for smallest physical size.
You calculation is wildly wrong, unless this 'widget' is something very
>C= I* Dt/Dv
That would make a mathematician's toes curl! You can't safely invert
differentials as if they were ordinary fractions. However, in this case,
if you get the numbers right, it works.
>The nominal input is 12V, I'll think a momentarily drop to 9v
>shouldn't cause a noticeable flicker.
Well, it's the LED current that matters directly, not the supply
voltage. Unless your SMPS is very poor indeed, you won't get anywhere
near 3 V variation without any additional capacitance. If you are
getting flicker, I strongly suspect that the reason is more obscure that
just poor regulation.
>And if I can see a flicker, it's
>got to be quite a long dip, say 0.04 sec (since the eye perceives
>smooth motion, thus no flicker at 24 to 30fps)
>C= 0.2 * 0.04 / 3 = 0.00266
So you have 200 mA flowing through your LED(s)? Lots of LEDs?
>That's about 2660 uF... and the caps are quite large relative to the
>LED... the small round ones barely hit 1 uF. That's why I'm kinda
>confused since I didn't expect to need a too big a cap for just a few
No, it can't be right, although your cap value calculation is correct
for 200 mA and 40 ms. But your calculation shows the capacitance you
would need to sustain the voltage to 9 V minimum for 40 ms **with the
SMPS switched off**. Unless the SMPS voltage really dips to zero for 40
ms or more (and you have a diode in series to stop the cap discharging
back into the SMPS!), that's pessimistic.
Do you have an oscilloscope? If not, steal one and look at the 12 V DC
output on its screen and the flickering LED simultaneously.
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
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