simple low pass RC filter noob question - Page 2 - diyAudio
Go Back   Home > Forums > Design & Build > Parts
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 21st July 2006, 03:58 PM   #11
mr.duck is offline mr.duck  United Kingdom
diyAudio Member
 
Join Date: Jun 2006
I have one more question.. if the low pass filter was used in a power supply, would the capacitor only be involved in filtering or would it also be able to supply power when it is needed?

For example, if the resistor was not there and you just had the capacitor smoothing out the DC, the capacitor would also be able to provide a large amount of current on demand. But when you put the resistor in to make the filter, does the capacitor loose this dual funtion and only be involved with filtering?


Does that make sence??
  Reply With Quote
Old 21st July 2006, 04:20 PM   #12
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
your RC filter does both jobs.

With or without the R the cap will try to keep the voltage at the same level until it's charge changes.

i.e. when the current demand increases the input voltage falls, but as soon as the voltage has fallen below the voltage level of the cap, the cap starts to discharge into the increased current demand.
Similarly if the current demand reduces then the input voltage will increase and the capacitor will absorb current (charge) until it's stored voltage matches the new voltage supply.
__________________
regards Andrew T.
Sent from my desktop computer using a keyboard
  Reply With Quote
Old 22nd July 2006, 01:48 AM   #13
diyAudio Member
 
I_Forgot's Avatar
 
Join Date: Jan 2005
Location: Phoenix, Az.
Simple RC networks like this a extremely easy to simulate using spice. You might consider downloading SwitcherCAD from Linear Technology (FREE!) and learn how to use it. You will be able to answer all your own questions in a matter of seconds, in far greater detail than you can imagine.

Example:
Attached Images
File Type: jpg rc1.jpg (60.7 KB, 99 views)
  Reply With Quote
Old 22nd July 2006, 02:16 AM   #14
diyAudio Member
 
I_Forgot's Avatar
 
Join Date: Jan 2005
Location: Phoenix, Az.
Quote:
Originally posted by mr.duck
I have one more question.. if the low pass filter was used in a power supply, would the capacitor only be involved in filtering or would it also be able to supply power when it is needed?

For example, if the resistor was not there and you just had the capacitor smoothing out the DC, the capacitor would also be able to provide a large amount of current on demand. But when you put the resistor in to make the filter, does the capacitor loose this dual funtion and only be involved with filtering?
The cap can supply current far beyond the regulated supply's nominal capability. I recently made a stroboscopic illuminator for a microscope that uses a 5W LED. I am driving the LED with 12A pulses even though the power supply in the strobe is a 15V, 5A regulated supply. I simply added a 10,000 uF cap on the supply rail at the FET that switches the current through the LED. This works fine for this application because the duty cycle is kept below 1/15. The high current discharge is brief (variable from 5-250 us) and the recharge time relatively long, so I can flash the LED up to about 250 Hz and still maintain the pulses at 12A.

I_F
  Reply With Quote
Old 4th October 2015, 02:00 PM   #15
oculi is offline oculi  Australia
diyAudio Member
 
Join Date: Sep 2014
Location: Australia
thread dig rather than starting a new thread as OP has posted up the LP filter type i'm planning on using on a rough project.

I've been calculating values required for the ballpark cutoff frequency I'm after and experimenting with the handful of capacitors I have on hand (don't want to use inductors as they are "expensive" relative to what i'm building) which has taught me enough to know what value caps to order.

here are my thoughts and some questions:

Using a Higher value resistor means you can use a lower value capacitor, but this is at the cost of attenuation

Using a low value resistor means you need a higher value capacitor (not really a problem in my case) without much attenuation of the desired low frequency noises.

Will using too low a resistance (I tried 0.5 ohms, then omitting the resistor altogether) present an unsafe (low impedance) load to the amplifier?

It seems to me that the obvious solution to this issue would be to put the resistor between the capacitor and "ground", but there must be a reason not to do this.
(EDIT: I've just drawn this out and the answer is pretty simple, with a resistor in series with the capacitor the filter won't "block" high frequency signals, just attenuate them as there will be a voltage divider between the additional resistor and the loudspeaker)

If the low load isn't a problem (I think it probably would be) then I'm surprised this method isn't more common, one source suggested you can get nth order filters by cascading filters, but almost all passive low pass filters I've seen use inductors or inductors and capacitors.

I think I'll just learn by doing, the amplifier is inside an ebay $30 headunit, and I don't like it much anyway.

sorry if this is poorly worded, I did study this stuff 15 years ago, will try to add some diagrams to better explain what i'm getting at.

Last edited by oculi; 4th October 2015 at 02:09 PM.
  Reply With Quote
Old 13th October 2015, 09:45 PM   #16
mr.duck is offline mr.duck  United Kingdom
diyAudio Member
 
Join Date: Jun 2006
I'm not that knowledgeable but I'll have a go at answering.

Capacitors would be like a short circuit to higher frequency signals, so a really big cap and low R would be a problem for an amplifier I'm guessing.

Cascading filters doesn't change the cut-off freq, only the steepness of the filter. So unless you want to channel out a specific frequency portion, you don't need this. I don't know if there is some other undesirable problem around the cut-off freq like a phase shift too which is made worse by cascading filters. Or simply you don't get the expected results with simple passive filters, where the output impedance of one affects the performance of the next.
  Reply With Quote
Old 14th October 2015, 01:20 AM   #17
diyAudio Member
 
Join Date: Sep 2007
Quote:
Originally Posted by poobah View Post
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?

Explained well, but incorrectly drawn. If lead length is critical, then both top and bottom leads should be equal length before terminating at the "first" cap. As drawn, the leads are equal between caps, and it's good that two caps share being "first" in line, but the upper lead is drawn much longer than the bottom lead.

Or am I not seeing this correctly?

Peace,
Tom E
  Reply With Quote
Old 15th October 2015, 10:23 PM   #18
mr.duck is offline mr.duck  United Kingdom
diyAudio Member
 
Join Date: Jun 2006
I think "drawn much longer" is quite an exaggeration. Are the lengths of the leads not at all critical if we assume the resistance of the wires to be zero?
  Reply With Quote
Old 18th October 2015, 10:37 AM   #19
oculi is offline oculi  Australia
diyAudio Member
 
Join Date: Sep 2014
Location: Australia
Quote:
Originally Posted by mr.duck View Post
I'm not that knowledgeable but I'll have a go at answering.

Capacitors would be like a short circuit to higher frequency signals, so a really big cap and low R would be a problem for an amplifier I'm guessing.
yeah that's what I figured. I ended up just series wiring the two inductors in some OTS crossovers I had. I figured that the inductors will knock out a fair bit of the signal power so the capacitors won't have to sink as much. my stupid woofer boxes have a really strong resonant peak at "annoying" Hz, so I need to fix that before worrying too much about the filter. might throw the caps on just to see what happens anyway.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Building a simple high pass filter mjg100 Subwoofers 2 19th March 2009 03:38 PM
Low Pass Filter question synthius Chip Amps 18 28th February 2008 12:13 PM
Low pass filter for simple sub on single rail? tubee Chip Amps 9 21st February 2008 02:28 AM
PASS LABS D1 - filter question macsaif Pass Labs 1 6th December 2002 05:37 PM
Filter noob Question Re Diy headphone. setmenu Headphone Systems 2 8th July 2002 08:17 PM


New To Site? Need Help?

All times are GMT. The time now is 02:36 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki