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mr.duck 24th June 2006 05:49 PM

simple low pass RC filter noob question
 
Standard low pass filter:
http://www.muzique.com/images/lpf.gif


Can you use more than one capacitor in parallel in the above circuit? (to give you more capacitance)

I need 10,000uF for the filter. Instead of using one big cap, can I use 3*3300uF caps?

Will the filter work the same? Will it be better/worse? etc


Thanks.

AndrewT 24th June 2006 06:27 PM

Hi,
yes you can parallel caps to increase the value.

What frequency are you trying to achieve? That's a mighty big cap for a low pass filter.

mr.duck 24th June 2006 06:35 PM

Aiming for around 3Hz :)

Its going to be part of a power supply for an amp. The power supply is unregulated but has a low pass filter to get rid of all the noise. The resister value is only going to be around 5 ohm.

poobah 24th June 2006 06:44 PM

There is no harm in paralleling caps... it will often result in a lower ESR & ESL than would be achieved in an equivalent single cap... it's actually a good thing.

Are you laying these on a board or using point to point wiring?

I ask because there is an important detail often overlooked when big caps are wired in parallel.

:)

mr.duck 24th June 2006 06:53 PM

probably be point to point... what's this important detail that gets overlooked?

poobah 24th June 2006 07:11 PM

2 Attachment(s)
I have attached a very poor drawing. This is how you wire the caps... PCB or P to P.

The drawings on the right are the WRONG, but typical, way to do it. The problem is the cap marked "A" will see more of the ripple current and do more than its share of the work.

The idea behind the methods on the left is that each cap sees the same length of connecting wire. There is inductance and resistance in the wiring which is on a par with the ESL & ESR of the caps.

Does this makes sense? Explained well enough?

:)

mr.duck 24th June 2006 07:25 PM

Yeah that makes sense, thanks :) I'll keep that in mind for the future.

poobah 24th June 2006 07:29 PM

Cool... batteries should be wired the same way.

;)

AndrewT 25th June 2006 09:29 AM

Hi Poobah,
I never thought about the ripple capacity of the first cap in the line.
At the 100/120Hz recharge pulse rate, is the cable reactance significant?

poobah 25th June 2006 03:56 PM

Hey Andrew,

It is a fine point at 120 Hz, but it is good practice. Just sketch out the current waveform in a PSU cap... there are lots of higher harmonics present. The current is what causes all the heating and wear, so it makes sense to spread it out evenly.

I build high power inverters (100 kW) with huge banks of caps, at these levels, "zee wiring" is simply a must. I have to use laminated copper sheet for low L busbars... they are a real hassle, but they look cool!

You do this with batteries as well. the small voltage differences have a huge impact on charging efficiency and quality.

:)


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