Relationship between the value of a capacitor and the frequency it will filter - Page 1
Forum section: electronics / projects

author: Planobilly
author_rank: Regular Contributor
timestamp: December 30, 2015, 11:57:55 pm
content: Hi guys,I would like to understand the relationship between the value of a capacitor and the frequency it will filter.Take the example of the capacitor often found on the mains AC line going into a transformer in a audio amp. We want to filter line noise coming in from the power company.I don’t know what frequency would likely exist there or what range of frequency would likely need to be filtered out but just say for example I wanted to remove 100hz to 10,000hz.If that was what I wanted to do, how do I decide the value of the cap or caps and I guess what type of capacitor would be needed and why?Thanks,BillyPS: If this is a stupid question requiring pages to answer please feel to say so!!!

author: ataradov
author_rank: Super Contributor
timestamp: December 31, 2015, 12:15:34 am
content: Capacitors on their own don't do any filtering. Capacitors used as parts of filtering circuits do. Specific frequencies depend on a type of circuit. The simples one is an RC-filter. Wikipedia has an excellent article on the subjecthttps://en.wikipedia.org/wiki/RC_circuit

author: Planobilly
author_rank: Regular Contributor
timestamp: December 31, 2015, 12:31:02 am
content: Perhaps I am really confused!!!..lol...Would not be the first time!!So what does the 0.22 cap do in the above?I had assumed something like this:?1?Class X capacitors?An X capacitor is used in Across-the-line application?In this location,a failure of the capacitor will not cause anybody to electrical shock hazards and usually will open the fuse and turnthe equipment off.Therefore;the definition for "X"capacitor application is:"where damage to the capacitor will not lead to the danger of electrical shock".?The X capacitors suppress normal mode RFI noise.?Since,normal mode noise is low-frequency then,X capacitor values are 0.1~1.0?.One would think that a capacitor with a larger constant should be used for suppression against normal-mode noise.However,increasing the X-capacitor's capacitance causes a problem of increased reactive current.Am I missing something here?? Not understanding something?Cheers,Billy

author: ataradov
author_rank: Super Contributor
timestamp: December 31, 2015, 01:05:31 am
content: This is an EMI filter. It does mostly nothing for actual circuit operation and used to pass regulatory requirements. The actual value depends on type of noise your device is leaking.I have no idea if there is a solid theory on selecting a value of those filter capacitors. I'm pretty sure most of the time it is done through selecting some base value and experiment.I was thinking you are asking about actual audio filtering.

author: ataradov
author_rank: Super Contributor
timestamp: December 31, 2015, 01:09:08 am
content: For audio applications, you will still need to do reasonable filtering on the DC part, I'd focus on that and not worry about quality of the AC input, unless you are in a really harsh environment, like machine shop with lots of motors going on and off all the time.Edit: and as far as math goes, this circuit uses the fact that impedance of the capacitor decreases with frequency as Xc = 1 / (2*pi*f*C). So at 50 Hz this capacitor acts as 15 KOhm resistor, but with higher frequencies, this "resistance" drops, so energy of the noise is dissipated in the capacitor and not transferred into the line.

author: Planobilly
author_rank: Regular Contributor
timestamp: December 31, 2015, 01:26:52 am
content: Thanks ataradov,I was assuming that the 0.22 cap and the choke on the drawing was preventing EMI/RFI from getting into the amp and also getting out...I assume.The question was not based on any problem I am having, just wondering if there was a relationship between freq and cap value, or how someone would get to the correct value.Cut and fit...lol...I guess...lolThanks for responding.Now if you can just fix this Vibrato circuit in this Fender Twin Reverb I am working on!!!!!...lol....just kiddingCheers,BillyPS: Just saw your edit which actually answered my question...got it, thanks

author: ataradov
author_rank: Super Contributor
timestamp: December 31, 2015, 01:33:01 am
content: Quote from: Planobilly on December 31, 2015, 01:26:52 amIt works both way. Think of this cap as a frequency-dependent resistor. The value of this resistor is high at low frequencies and gets lower with frequency going up.Quote from: Planobilly on December 31, 2015, 01:26:52 amSee the formula above. There is no "correct" value. It all depends on your goal.Quote from: Planobilly on December 31, 2015, 01:26:52 amYou can do some preliminary estimations, of course, but actual value (and even topology) of the filter depends on the entire system design, not just the power supply.

author: Zad
author_rank: Super Contributor
timestamp: December 31, 2015, 01:34:43 am
content: If it helps, the impedance (effectively the AC resistance) of a capacitor = 1/ (2 * pi * f * c)Where f is the frequency you analysing for, and c is the capacitance in Farads.This of course neglects the effective series resistance, and the impedance of the circuit it is connected to. Obviously you don't want a capacitor in the AC part of the PSU circuit to present a noticeable load at 50/60Hz!

author: Planobilly
author_rank: Regular Contributor
timestamp: December 31, 2015, 02:17:36 am
content: Thanks guys,I am really just getting started in electronics so there is a ton of stuff I don't understand. Plus the fact that I am 69 years old and and of the things I have lost, I miss my mind the most...lolMy advice is to never get old...lol...and for sure never retire because the wife will work you harder than any job ever did..lolActually, electronics is really fun stuff for me.Thanks for putting up with me!!Billy

author: donmr
author_rank: Regular Contributor
timestamp: December 31, 2015, 06:05:21 pm
content: As previously mentioned, the impedance of a capacitor varies with frequency (you can look up the formula).This can act with other impedances in the circuit to reduce signals of various frequencies.The problem is not figuring out the capacitor, it is figuring out what are the other impedances in interacts with.In the circuit posted, what matters is the source impedance of power and any high frequency noise coming in over the power line. That is made up of many small values that are hard to measure and can easily change.You have to use experience or trial and error (or both).Your 0.22 uF is 12k ohms at 60 Hz and 7 ohms at 100k Hz, so if the input impedance is roughly 100 ohms, you can see that the capacitor will have little effect on the 60 Hz power signal and will short out most of a 100k Hz noise signal.

author: bson
author_rank: Unknown
timestamp: December 31, 2015, 06:44:08 pm
content: If the 220uF cap weren't there transient loads would cause voltage drops over the inductors since their impedance is wL (assuming purely reactive/high Q). This would cause voltage drops during current spikes. The purpose of the reservoir cap is to meet these transient needs, which is effectively the same as its impedance dropping. It's actually the inductors that block high frequency transients, not so much the capacitor. The size of the capacitor depends on the transient current needs as well as the series inductance: the larger the inductors, the bigger the cap. The series inductance depends on the desired corner frequency.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 31, 2015, 08:39:57 pm
content: Quote from: Zad on December 31, 2015, 01:34:43 amIn theory, a capacitor never actually dissipates power and in practice, that's usually pretty close. If you connect a 10uF motor run cap across 120V mains, it will draw about 450mA but not heat up by any noticeable degree. Actually, there are energy saver/whole house surge protection/noise filter gadgets that are little more than a motor run cap in a box and while the energy saving part is more or less a scam (not to mention those boxes generally being way overpriced for what's inside), they do help prolong the life of MOVs (by shunting small spikes that would otherwise wear away at the MOV) and do help with noise filtering.

author: Seekonk
author_rank: Super Contributor
timestamp: December 31, 2015, 09:15:45 pm
content: I used to work for a company that made RC networks and took a lot of application calls from people with noise problems. The 10uF motor run cap is a good idea but put a couple ohm resistor in series with it to absorb the noise power. Otherwise it keeps bouncing around. Second, place it nearer to the source of the noise. Filtering at the amp just causes noise currents to be radiated from the power line going to the amp that will be picked up by audio cables. A noise filter is a power distribution box would be effective.

author: Planobilly
author_rank: Regular Contributor
timestamp: January 10, 2016, 11:17:01 pm
content: Thanks guys!!This is a lot of info for me to look into and understand.Thanks for all the feed back.Billy

author: Audioguru
author_rank: Super Contributor
timestamp: January 11, 2016, 02:35:08 am
content: Why are most people talking about a small 0.22uF capacitor when a huge220uFfilter capacitor was circled? its job is to reduce the 100Hz or 120Hz from the rectifiers.

author: ataradov
author_rank: Super Contributor
timestamp: January 11, 2016, 02:36:54 am
content: Quote from: Audioguru on January 11, 2016, 02:35:08 amBecause that what the question was about.Quote from: Audioguru on January 11, 2016, 02:35:08 amI assume this schematic was simply taken from the Internet somewhere with capacitor already circled.

author: Richard Crowley
author_rank: Super Contributor
timestamp: January 11, 2016, 07:26:01 am
content: For several decades now (since before computers) I have found this chart to be a valuable and convenient guide that shows impedance vs. frequency for a particular capacitance/inductance (or vice-versa).http://www.opamplabs.com/products/apps/app-nomograf.html