if I'm building up a pi filter (or whatever low pass filter for that matter), and need 1200uF at the input is it fine to just put... .say 3 x 400uF caps in parallel? I think it's fine, but I just wanna make sure, caps seem a little tricky in this way
much thanks
MICRO-Farads??
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Man, I was about to blithely answer the question without even noticing the units.
Mr Pan:
What are you _really_ trying to do? Filtering at the frequencies that such capacitor values seem to imply is usually done actively using op- amps, much smaller capacitor values, and much higher resistances than would be 'normal' in a power signal path -- then once the signal is the way you want it, you amplify the snot out of it.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
Putting aside concerns about the wisdom of using electrolytics in a pi filter, yes, you can parallel capacitors. If not with impunity, then often with success, and often (particularly when you're on the upper boarder of the range of a particular capacitor technology's commonly available values) to the benefit of cost and/or performance.
If you're using the caps at the upper end of their frequency range then odd things can happen, but generally you wouldn't be paralleling caps in that case, unless you have made some very unwise decisions about the impedances of the circuit you're designing.
-- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
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It's for the output of a DC/DC converter, I'm following a design similar to the one here
there's a pi filter at the 5V output
reasons I asked about paralleling caps is cause I'm trying to avoid using electrolytics, im thinking i'll put some ceramics in parallel to meet this capacitance
thanks
Software
Paralleling reduces parasitic resistance and inductance (ESR and ESL) so works better than a single cap.
That's a lot of capacitance to get from ceramics. Take a look at polymer electrolytics: they have very low ESR and don't dry out like wet caps.
-- John Larkin, President Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
It's a power supply filter. Microfarads make sense, although 1000 is maybe a little overkill for a 1.7 amp switcher.
-- John Larkin, President Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
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But for this use he really needs to use low ESR caps. Panasonic FMs or Nichicon HZs. Organic polymers?
G=B2
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ncottdesign.com
I hope it's OK if I hijack some of your thread. I wanted to ask a silly question.
I've always thought of pi filters as 'filtering both ways'. That is you've got an L (or R) and C, putting a C on the other end snuffs out signals coming back into the circuit. In the circuit panfilero posted this doesn't seem to be the case. So what's the first cap doing? (C8 in app note) If it's part of the output filter, what impedance is it working against?
George H.
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