Capacitors What Value To Use, Polarized Or Not???

[WLS]

Hi everyone;

I recently got hooked into building pedals and I try to select projects that I can learn from. And I have gained a pretty good understanding of the common components involved. The one that still seams to mystify me are capacitors.

How do you know what value to use?

When to you use a polarized capacitor?

What is their basic function in a circuit  besides filtering?

Thanks,

Bill

[Ripthorn]

The value should be marked on the schematic, so that should be easy enough.

Polarized caps (usually electrolytics) as a rule of thumb should only be used when called for (electrolytics especially have a limited life span, so use them only where necessary).  The symbol will have either a plus sign at the positive side or it will have one straight and one curved line (as opposed to two straight lines) and the curved line indicates the negative side of the polarized cap.

The basic function is filtering and storing charge (though the filtering is accomplished by storing charge).  They create low pass or high pass filters to make the signal have a desired response.

[WLS]

The value should be marked on the schematic, so that should be easy enough.

Polarized caps (usually electrolytics) as a rule of thumb should only be used when called for (electrolytics especially have a limited life span, so use them only where necessary).  The symbol will have either a plus sign at the positive side or it will have one straight and one curved line (as opposed to two straight lines) and the curved line indicates the negative side of the polarized cap.

The basic function is filtering and storing charge (though the filtering is accomplished by storing charge).  They create low pass or high pass filters to make the signal have a desired response.

I know how to read a schematic. Maybe I did not make myself clear.

I am speaking in terms of building a circuit from scratch where their is no schmatic involved.

Thanks for your reply.

Bill

[jacobyjd]

Basically, from what I've picked up around here and elsewhere, use an electrolytic when you just need 'enough' capacitance. The tolerances of electros just aren't tight enough to use when precision is necessary.

When you need precision (and for smaller values), use non-polarized mylar/poly/whatever caps.

[armstrom]

it really depends on what you want the capacitor to do. In most of the circuits used in pedals capacitors usually take on four "roles".

1) Power supply filter: These are always electrolytic and usually fairly large (47uF - 200uF). The capacitor serves to "smooth out" the power supply voltage to prevent noise in the circuit.

2) Signal Filters: This is very common. We use capacitors as part of RC filters to roll-off high or low frequencies. The value of the capacitor is very critical here. Google "RC Filter calculator" to find out how various combinations of capacitor values and resistor values filter frequencies. These caps are always low value non-electrolytics.

3) AC Couping/DC blocking: One handy property of all capacitors is that they block DC voltage. Only AC signals are allowed to pass through them. You often find coupling caps between gain stages to avoid passing the DC offset from stage to stage. The sizes here can vary as well and cover both ceramic/poly caps as well as electrolytics.

4) "Bypassing" components: There's any number of reasons why you might want to bypass a component. Maybe you want to let certain frequencies to "bypass" a circuit segment. Another common use is to increase the gain of an amplification stage like a common source JFET gain stage. In this configuration you bypass the source resistor with a capacitor to increase the gain.

Keep in mind that a single series capacitor anywhere in the signal chain is performing BOTH roles 2 and 3. This is where careful selection of capacitor values becomes very important. If your primary goal is to simply block  DC voltage then any cap will do... but you have to take into consideration how the selected value will affect your signal. If you pick the wrong value the frequency response of your circuit can be greatly affected. A good general rule here is to pick as large a value as you can get away with, up to a reasonable limit of about 10uF.  Sometimes the design of the circuit prevents you from choosing an electrolytic cap as it may cause a reverse voltage across the polarized cap.

[MikeH]

Another interesting tidbit about caps: For series and parallel calculations they're the opposite of resistors.  In fact, they're sort of the opposite of resistors in many ways.

[WLS]

... Sometimes the design of the circuit prevents you from choosing an electrolytic cap as it may cause a reverse voltage across the polarized cap.

Then this is why I see in many circuits that when a polorized capacitor "electrolytic capacitors" are being used they usually follow a rectifiier or something from the diode family.

Thank you for your explanation. It takes away some the mistery.

Thanks to everyone for there input. If anyone else would have more to add about capacitors useage and sellection.

Please elaborate!

Bill

[Ice-9]

Great explanation there Armstrom.
1 obvoius point to make as well, polarised electros and tants need to go in the circuit the correct way around, whereas non polarised caps can go anyway round.

[R O Tiree]

... of course, you can buy non-polarised electrolytic caps. Or just put 2 ordinary ones back to back, which achieves the same result...

[col]

Don't know if you're UK or not but the last time I bought a big order from Rapid I ordered non polar electos as they were on offer and I have had no problems whatsoever with them. In fact it makes things easier as I don't have to bother with installing them the right way around. I've not noticed any difference in sound quality and I think that's the way I'll go in future.
Other than that I'm not too fussy about the type of cap I use as long as the value and voltage rating is right. I did a mini test and couldn't tell the difference in sound and have never had any problems through doing this.

BTW my youngest son (22) had a nasty accident last week with a cap exploding. He noticed smoke coming from his TV digiatal box and opened it up to have a look. One of the caps exploded and burned into his eyeball. Luckily they have now said at the hospital that there is unlikely to be any permenant damage but he still can't use that eye and he has to contantly put drops in to prevent it from bleeding. Anything that takes a risk like that away has to be worth doing!

Col

[WLS]

Don't know if you're UK or not but the last time I bought a big order from Rapid I ordered non polar electos as they were on offer and I have had no problems whatsoever with them. In fact it makes things easier as I don't have to bother with installing them the right way around. I've not noticed any difference in sound quality and I think that's the way I'll go in future.
Other than that I'm not too fussy about the type of cap I use as long as the value and voltage rating is right. I did a mini test and couldn't tell the difference in sound and have never had any problems through doing this.

BTW my youngest son (22) had a nasty accident last week with a cap exploding. He noticed smoke coming from his TV digiatal box and opened it up to have a look. One of the caps exploded and burned into his eyeball. Luckily they have now said at the hospital that there is unlikely to be any permenant damage but he still can't use that eye and he has to contantly put drops in to prevent it from bleeding. Anything that takes a risk like that away has to be worth doing!

Col

I have also used the non-polarised electrolytic caps and you're right they are a lot easier to work with. I picked up a bunch of then from a web site called Dan's special deals or something like that. I too could not hear any difference in sound quality. I also picked up some polarised electrolytic caps that are designed for audio. I have not tried them out yet.

Has anyone used the polarized electo audio caps and if so how was their performance.

As far as location I live across the pond and do most of my orders through mousers.

Somethings are a little high in price, but you have to watch out for that anyways.

I do like their  sellection and customer service.

Sorry to hear about your son's unfortunate accident. Do you know what caused the capacitor to fail in such a manner?


Bill

[MohiZ]

... of course, you can buy non-polarised electrolytic caps. Or just put 2 ordinary ones back to back, which achieves the same result...

This is fine when the voltage is the same on both sides of the caps and you want to transfer a small signal, for example. But polarized electrolytics can't tolerate even a relatively small reverse voltage for a long time before they fail. So putting ordinary ones back to back is not as foolproof as using non-polarised ones.

WLS: have you read this page? There are many good articles on the basic components, and they're all made with the DIY effects maker in mind.
http://gaussmarkov.net/wordpress/category/parts/

[col]

Bill,

I assume that something had gone wrong with the power supply meaning that the voltage rating of the cap had been exceeded or the charge across it had become reversed somehow. I have heard that they explode but never actually seen the results before. I have had a pot set on fire though, and at only 9v!

Col

[devnulljp]

it really depends on what you want the capacitor to do. In most of the circuits used in pedals capacitors usually take on four "roles"...snip

Great post. Probably the single most informative thing I think I have read anywhere in years. So much good information, well explained, in such a small space. Thanks.

[WLS]

Bill,

I assume that something had gone wrong with the power supply meaning that the voltage rating of the cap had been exceeded or the charge across it had become reversed somehow. I have heard that they explode but never actually seen the results before. I have had a pot set on fire though, and at only 9v!

Col

Yes, another great link!

I started to read this once before and for some reason stopped.

I like his sight it contains some nice schematics a long with informative information.

This thread as I hoped is turning out to be very iformative not only for myself but for others that need a deeper understanding to capacitors and their function.

Again, thanks to everyone for their input and if anyone else has more to add to this thread then please enter it.

Bill

[armstrom]

I re-read my post and thought it might be helpful to point out something that may or may not be obvious. Previously I mentioned that any series capacitor in the signal path is both blocking DC and acting as a frequency filter. I think I should clarify that further. Series capacitors are acting as as "high pass" filter meaning they attenuate frequencies lower than their "knee" frequency.  However, you also need to look for capacitor values from your signal path to ground as these act as "low pass" filters attenuating frequencies above their knee frequency. The good thing is that when you see a cap to ground from your signal path it was usually placed there to deliberately act as a filter so the value is usually carefully chosen. The issue with series caps is that the designer may only have been considering one "role" when adding the capacitor (usually they want to block DC) and ignored the side effect of rolling-off low frequencies.

When designing a circuit you cannot avoid these situations, the trick is to decide ahead of time how you want the frequency response of your circuit to "look". If you want a particular circuit segment to be "transparent" (flat response) then you have to pick capacitor values that will form filters that only roll off frequencies outside of the normal audio range (I try to prevent affecting frequencies above 50Hz and below 15kHz, but that's just a personal choice).  Because of this you usually see "larger" capacitors (in the nF or uF range) in series with the signal and smaller capacitors (in the pF range) from the signal to ground. This is not a hard and fast rule but it's pretty common.

One last "gotcha" to watch out for is caps near pots. A pot set up as a variable resistor can affect the frequency response of the circuit in drastic ways. Obviously, if this is a tone control pot then that's what you want, but it's possible to have this effect in other areas of your design where it may not be desirable. The most obvious example that comes to mind is a variable gain pot in an op-amp gain stage.

[WLS]

I assume that something had gone wrong with the power supply meaning that the voltage rating of the cap had been exceeded or the charge across it had become reversed somehow. I have heard that they explode but never actually seen the results before. I have had a pot set on fire though, and at only 9v!


Col

Col,

Thank you for mentioning about the caution needed in selecting the proper voltage rating for the capacitor you are going to use. I usually when building a pedal I try to always double the voltage of what the circuit requires to operate. For instance if the circuit operates off 18 volts the minimum voltage rating I will use is 35 volts.

I pray for your son's quick recovery!

... helpful to point out something that may or may not be obvious. Previously I mentioned that any series capacitor in the signal path is both blocking DC and acting as a frequency filter. I think I should clarify that further. Series capacitors are acting as as "high pass" filter meaning they attenuate frequencies lower than their "knee" frequency.  However, you also need to look for capacitor values from your signal path to ground as these act as "low pass" filters attenuating frequencies above their knee frequency. The good thing is that when you see a cap to ground from your signal path it was usually placed there to deliberately act as a filter so the value is usually carefully chosen. The issue with series caps is that the designer may only have been considering one "role" when adding the capacitor (usually they want to block DC) and ignored the side effect of rolling-off low frequencies.

When designing a circuit you cannot avoid these situations, the trick is to decide ahead of time how you want the frequency response of your circuit to "look". If you want a particular circuit segment to be "transparent" (flat response) then you have to pick capacitor values that will form filters that only roll off frequencies outside of the normal audio range (I try to prevent affecting frequencies above 50Hz and below 15kHz, but that's just a personal choice).  Because of this you usually see "larger" capacitors (in the nF or uF range) in series with the signal and smaller capacitors (in the pF range) from the signal to ground. This is not a hard and fast rule but it's pretty common.

One last "gotcha" to watch out for is caps near pots. A pot set up as a variable resistor can affect the frequency response of the circuit in drastic ways. Obviously, if this is a tone control pot then that's what you want, but it's possible to have this effect in other areas of your design where it may not be desirable. The most obvious example that comes to mind is a variable gain pot in an op-amp gain stage.

Armstrom,

Thanks, for another well written detailed explanation on capacitors. It definitely opened my eyes.

Thanks Guys,

Bill

[tempus]

Here's some must read pages:

http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html
http://my.execpc.com/~endlr/index.html
http://www.humblehomemadehifi.com/Cap.html

Supposedly certain types sound better than others, but I don't know if I've ever found this to be the case. This article is classic:

http://www.reliablecapacitors.com/pickcap.htm

If you read the whole thing, he actually claims that the type of cap used in power supply filtering (given the same value and rating) will affect the sound.

I have a hard time believing that.

[WLS]

Believe it or not I lean towards that it does make a difference as to the type of capacitor that is used. My first build was a Foxx Tone Machine using GGG's schematic. I had a friend that also wanted a decent fuzz. So I order two PCB' from GGG and built one with using just any old capacitor, manly price dictated the sellection. Then on the secound one which was for myself I used Orange Drops and everthing else was the same. You could hear the difference. The Orange Drops where able to achieve a deeper tone than the saturday night specials.

This is a very good point to bring up and thanks for your input

Bill

[MannequinRaces]

This is an awesome post!  I wish there were posts like this for resistors, transistors, and diodes!     Lots of great stuff.  I learned a lot about the roles that capacitors play!