capacitors in parallel

panfilero · Mar 23, 2012

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

Jim Thompson · Mar 23, 2012

On Fri, 23 Mar 2012 08:58:30 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

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

Tim Wescott · Mar 23, 2012

On Fri, 23 Mar 2012 09:10:30 -0700, Jim Thompson wrote:

On Fri, 23 Mar 2012 08:58:30 -0700 (PDT), panfilero
panfilero@gmail.com> wrote:

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??

Electrolytics make for lousy, inaccurate filters.

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

Tim Wescott · Mar 23, 2012

On Fri, 23 Mar 2012 08:58:30 -0700, panfilero wrote:

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

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

John Larkin · Mar 23, 2012

On Fri, 23 Mar 2012 09:48:39 -0700 (PDT), panfilero
<panfilero@gmail.com> wrote:

On Mar 23, 11:17 am, Tim Wescott <t...@seemywebsite.com> wrote:
On Fri, 23 Mar 2012 08:58:30 -0700, panfilero wrote:
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

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 & Softwarehttp://www.wescottdesign.com

It's for the output of a DC/DC converter, I'm following a design
similar to the one here

http://www.powerint.com/sites/default/files/product-docs/an45.pdf

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

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

John Larkin · Mar 23, 2012

On Fri, 23 Mar 2012 11:14:16 -0500, Tim Wescott <tim@seemywebsite.com>
wrote:

On Fri, 23 Mar 2012 09:10:30 -0700, Jim Thompson wrote:

On Fri, 23 Mar 2012 08:58:30 -0700 (PDT), panfilero
panfilero@gmail.com> wrote:

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??

Electrolytics make for lousy, inaccurate filters.

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.

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

panfilero · Mar 23, 2012

On Mar 23, 11:17 am, Tim Wescott <t...@seemywebsite.com> wrote:

On Fri, 23 Mar 2012 08:58:30 -0700, panfilero wrote:
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

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 & Softwarehttp://www.wescottdesign.com

It's for the output of a DC/DC converter, I'm following a design
similar to the one here

http://www.powerint.com/sites/default/files/product-docs/an45.pdf

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

Mar 26, 2012

On Mar 23, 10:14 am, John Larkin <jlar...@highlandtechnology.com>
wrote:

On Fri, 23 Mar 2012 11:14:16 -0500, Tim Wescott <t...@seemywebsite.com
wrote:

On Fri, 23 Mar 2012 09:10:30 -0700, Jim Thompson wrote:

On Fri, 23 Mar 2012 08:58:30 -0700 (PDT), panfilero
panfil...@gmail.com> wrote:

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??

Electrolytics make for lousy, inaccurate filters.

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.

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 comhttp://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

But for this use he really needs to use low ESR caps. Panasonic FMs or
Nichicon HZs. Organic polymers?

G˛

George Herold · Mar 27, 2012

On Mar 23, 12:48 pm, panfilero <panfil...@gmail.com> wrote:

On Mar 23, 11:17 am, Tim Wescott <t...@seemywebsite.com> wrote:

On Fri, 23 Mar 2012 08:58:30 -0700, panfilero wrote:
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

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 & Softwarehttp://www.wescottdesign.com

It's for the output of a DC/DC converter, I'm following a design
similar to the one here

http://www.powerint.com/sites/default/files/product-docs/an45.pdf

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- Hide quoted text -

- Show quoted text -

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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