How to select proper sized/value capacitors ?

[Foxhunter]

Hey there all I am wondering how to go about selecting a capacitor(s) for noise-filtering purposes? Anybody familiar with how to select one (or more) with the proper values?

Right now I'm about to experiment with using a high-quality 24V battery charger hooked to two 12V batteries wired in parallel (24V). From there I will use specific-length cables to allow me an approximate 4V to 6V drop to result in around 18V-20V of power.

My question is, how do I select some caps for noise-filtering purposes? I know how they work and all and why but I don't really know how someone "picks out the right one". Is it critical that they be a specific value (voltage and farad)? Or will any basic large sized one do?

I see them in all ranges both in voltage and capacity. My gut instinct is to select one rated at or above what I am looking to run through it. What will happen? For instance I know someone who has a whole bin of short fat Mallory 40V caps (unsure of pF value). Can basically any caps be used? Just wondering for both myself and others who might be wondering now or in the future---how you select the right ones?

I also have one 1.5-Farad and two 1-Farad 13.8V car audio capacitors sitting here on the bench but am doubtful about using them for 18V-20V and I don't want "a large firecracker" to go off around my gear. They worked well at filtering in other installs but this power supply experiment with higher DC voltage is a little different.

Anyone familiar with the basics of selecting a proper cap would you please share a little of what you know about choosing them in general and also for what I am trying to do? It would also be of value to other members and Forum visitors. Thank you in advance and I would appreciate some advice.

[linearone]

The filter capacitor size can be approximated from C=IT/E where I is the current, T is the half cycle time (8.33 mS at 60 Hertz), and E is the fall in voltage that will occur during one half cycle.

8000uf per amp is sufficient

[Foxhunter]

Wow so the calculation and selection of them it would appear is very important then. Should I (or is there) just get big caps that are very high in value ("more than enough")? On the other hand, can they be "too big" and be harmful ?

I am hoping to use some good, big "general purpose" caps that I can reuse in other applications or experiments-----as they won't be going into a "fixed" environment (like being permanently installed in an amplifier or power supply). Car audio amplifier capacitors are the limit of my experience. You can tell by my questions I have almost no idea at all what I am doing in selection, just a general idea what they are and are often used for.

Specifically, I have an electric wheel chair 24VDC 8A battery charger hooked to two (2x) 880CCA 12V batteries wired in series. To further "filter" AC what would you recommend for caps both minimum and/or maximum that I should use for caps then? How many? Specifically 24V caps? Something I could reuse in other applications or no?

Thanks Linearone for answering my post and to anyone else who might have something for advice to add.

[linearone]

how many amps are you drawing?

bigger is ok.

[madsage]

your caps will also prevent a change in voltage, so your voltage drop yuo expect 4-6v might be negated.
you'd want 35v caps for a 24v application... they are typicaly built in 15v, 20v, 35v. 20v is most common in mobile audio applications. your 40's would be fine.

Your charger will put out more than 24volts also, if its designed to charge 24v batts. Its problably near 26-28volt. (like a 12v charger is typicaly 14.x)
Also make sure your rectifier is full wave, not all battery chargers are. you can get by with a 1/4 wave to charge a battery, this not ideal for much anything else.
The rectifier should have 4 leads, or 4 diodes for full wave. or multiples of 4 depending how its designed.

i believe your idea is a good one. What you would want is a regulator like what is used in a motor maul, they also operate on 24volt.
This would give you the ability to regulate 24 - 12volts on a dial. Might be worth looking into that before plugging in aprox 26volts with very high value caps to your equiptment. hope this helps..

The 933 RiffRaff
in the stickerz

[Foxhunter]

This is why this forum is good, to bounce ideas off of members like this and I appreciate the responses. Another member recently put it as this forum being one giant "think tank" for ideas and I agree with the way he put it----it's only possible with members like this.

Linearone I'm only planning on casually running at most my NJ DaveMade 2x8 pulling around what, 200A. Maybe either of the 2x4's also, depending on how well it works.

Madsage also has some ideas. I've used and advocated the car audio farad capacitors in a prior radio/small amp install I did and felt they did have value. They do lessen voltage drop as well as cannot pass AC resulting in noise-filtering qualities. I've got a consumer-grade 1.6-farad 16-20V cap, as well as two 1-farad 20-24V caps sitting on the bench here. Because of the increased voltage of 24V+ I didn't want to even try using them

What I am asking about is one of several power-supply schemes I want to have going for alternative DC supply systems. I have the components for both linear and switched-mode AC to DC power supplies. These are both "direct" AC to DC of course.

I also want a 3rd indirect system utilizing a small current charger continually feeding/replenishing a pair of 12V automotive batteries for a separate mainly DC battery-powered system. The charger is mainly for redundancy. Since I am not usually extremely long-winded in 75% of my contacts I figured I could use a battery/charger system in those instances.

True, capacitors could "help" negate/fight the planned voltage drop. Although most installations try and combat "voltage drop" in a lead, I was counting on trimming power leads to a specific long-length to achieve the desired drop by design. I was thinking by locating the caps close to the batteries/charger source, then with the longer supply leads still causing some drop down to the 18-20V mark at the amplifier +/- feedpoints. In the planning and gathering stages for this (and several other things going) and wanted to run this by you guys to see if it is a faulty idea or what other things are needed to be incorporated into the setup.

I like the idea of using capacitors, as so many big amps and big linear power supplies utilize them. Although this little setup looks still promising, as an alternative I could always skip the capacitors and use "brute force" type noise filters instead. I have two big high-quality filters of that type, both are rated at 150A each maximum. I'd hate to tie them up with this as I have other plans for them, for use combined with some switching power supplies------but the big filters could always be a "plan B" I'd guess.

[linearone]

if you got some 1 farads laying around they will work just watch the voltage thing, math wise it says you need 1.6farads fo properly filter. the reality is if you had like 200000UF youd be fine

[Foxhunter]

Thank you Linearone and I didn't see that you wrote and answered until just now. I am going to post another somewhat related topic thread in the same vein but more properly titled "BUILDING YOUR OWN RF NOISE FILTER" for some hopefully increased exposure. It'll be under the: AMPLIFIERS / POWER SUPPLIES section. I'd appreciate you contributing what you would to it, as all can see here on the CBRT Forum that you are quite knowledgeable in this arena and your experience & input is highly valued.

I don't think I'm the only one thinking about trying to solve these type issues or could learn from this and think many other members would be interested in building something along these lines-----not to mention countless people out there doing random online searches who turn up as new CBRT Forum vistors or new members as a result.

['Doc]

I would suggest one additional precaution. It doesn't hurt if those capacitors have a higher voltage rating than is required. It's a very good idea for that voltage rating to be at least 2 or 3 or more times the supplied voltage. That will typically handle any surges. As far as I'm concerned 1.5 times the supply voltage is NOT adequate, especially with larger capacitors. With ones as large as 1 F, you ain't talking 'firecracker' any more. More like a hand grenade. And that is NOT an exaggeration.
- 'Doc

[Mikey B]

Hand grenade is right! This leans more to AC power, but, the other day I was doing some refrigeration maint. at a local store. I opened up the condensor for an ice machine and saw a run capacitor with the top blown off and all kinds of black stuff covering the inside of the unit. When they pop, they pop!

[Foxhunter]

Yes I blew one in a noise filter 9 months ago right beside me and it scared the heck out of me and startled me. I accidentally cranked the voltage up from 15V to 36V and ka-pow! The filter is a goner.

I appreciate the response and am still trying to figure out the best method. Would you also please consider another thread I have also on a capacitor "banked system" I've just written? I'm interested in going from "grenade level" up to "claymore anti-personnel mine" level----if at all possible.

BUILDING YOUR OWN RF / AC NOISE FILTER
http://www.cbradiotalk.com/vi ... 19&t=27133

Here will be my next noise filter attempt if I don't get some good detailed advice:

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Here's a video of some of the smaller value caps blowing up, even these here pack a punch. Can't imagine the bigger ones.
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[Mikey B]

That video of the caps is pretty funny! Don't mean to offend you but I got a kick outta that!

[Foxhunter]

That video of the caps is pretty funny! Don't mean to offend you but I got a kick outta that!

Thank you but which one?

[Mikey B]

The one where the capacitors are blowing up. I don't have any experience with what you are trying to build, but a local tried to run his setup on a battery and charger and it not only gave him some noise, but he would tx noise as well. I've tried looking for info, and I'm sure you have, on this type of thing (and your 400A filter) and it's not talked about much as far as I can tell.

[Crusher]

LOL< reminds me of my first day in DC circuitry analysis. The teaching showed the class why WVDC (Working Volts D.C) was important> he put on welding gloves and took a standard 35V capacitor and stuck it into the wall outlet. LOL scared the hell out of me.