Author Topic: Measuring Capacity of Capacitors?  (Read 1363 times)

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Offline voyager2Topic starter

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Measuring Capacity of Capacitors?
« on: July 05, 2010, 10:21:14 PM »
Hi All
I've got a really dumb question:how do you measure the capacity of a capacitor?
Is it something to do with the numbers on the side?
 Please Help!
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Offline Alfa_Zulu

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Re: Measuring Capacity of Capacitors?
« Reply #1 on: July 06, 2010, 12:24:34 AM »
umm, well electrolytic caps which I believe are the most common ones,
they are black with numbers on the side such as 2uf, 100uf, 330uf, 470uf and so on.
then there can be some ceramic capacitors and they don't all say, but look for a
number and Google it like "34m ceramic cap" and see if you find info that way

but they electrolytic caps should always say unless their really small (physical size) and
the number accidentally didn't find on the side of the label, I have had a couple like this.

however there are also other types of caps I haven't mentioned.

if your still unsure upload a pic of the cap you have an I can tell you what type it is.

Offline Choco_liger

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Re: Measuring Capacity of Capacitors?
« Reply #2 on: July 06, 2010, 04:23:54 AM »
Well the amount of farads (uF) is a measurement of the capacitance of a capacitor.

As said before you can get the readings on usually electrolytic caps but not on ceramic.

You can also get a multimeter with a special farads function.

For example this one:
http://www.bkprecision.com/products/model/2704B/tool-kit-dmm-05-w-cap.html

I wish I had one of these...  :(

EDIT: Whoops, I forgot to say you can get them on the datasheets too.
« Last Edit: July 06, 2010, 04:26:22 AM by Choco_liger »

Offline billhowl

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Re: Measuring Capacity of Capacitors?
« Reply #3 on: July 06, 2010, 06:46:46 AM »


You can make this is a simple but very useful capacitance meter kit.
http://www.seeedstudio.com/depot/capacitance-meter-kit-p-268.html?cPath=104_108
It can measure range from 1pF - 500uF  with cost USD$14.50
http://www.jyetech.com/Products/CapMeter/eCapMeter.php
Schematic

The measuring principle

A proven method of measuring capacitance is to form an RC network, which consists of the unknown capacitance and a known resistor. When a voltage is applied to the series connection of these components, the voltage accross the capacitor will slowly rise. So, by measuring the time constant the capacitance can be calculated easily.


how to read capacitance values

Like resistors, capacitors are generally manufactured with values to two significant digits. Also, small capacitors for general purposes have practical values greater than 1 pf and less than 1 f. As a result, a useful convention has developed in reading capacitance values. If a capacitor is marked "47," its value is 47 pf. If it is marked .047, its value is .047 f. Thus, whole numbers express capacitance values in picofarads while decimal fractions express values in microfarads. Any capacitor manufactured with a value of 1 f or greater is physically large enough to be clearly marked with its actual value.
A newer nomenclature has developed, where three numbers are printed on the body of the capacitor. The third digit in this case works like the multiplier band on a resistor; it tells the number of zeros to tack onto the end of the two significant digits. Thus, if you see a capacitor marked "151," it is not a precision component. Rather, it is an ordinary capacitor with a capacitance of 150 pf. In this nomenclature, all values are given in picofarads. Therefore you might well see a capacitor marked 684, which would mean 680000 pf, or 0.68 f.

Offline voyager2Topic starter

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Re: Measuring Capacity of Capacitors?
« Reply #4 on: July 09, 2010, 07:56:29 PM »
Big help guys!
So ceramic cap with 224 marked on it is 224pf?
And one with 103 is 10000 pf?
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Offline billhowl

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Re: Measuring Capacity of Capacitors?
« Reply #5 on: July 09, 2010, 08:32:06 PM »
Big help guys!
So ceramic cap with 224 marked on it is 224pf?
And one with 103 is 10000 pf?

ceramic cap with 224 marked on it is NOT 224pf is 220000pF or 0.22f.
103 is 10000 pf or 0.01f.


Offline Alfa_Zulu

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Re: Measuring Capacity of Capacitors?
« Reply #6 on: July 09, 2010, 08:52:28 PM »
thanks billhowl lol, i wasn't aware of that :)

Offline voyager2Topic starter

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Re: Measuring Capacity of Capacitors?
« Reply #7 on: July 09, 2010, 09:37:38 PM »
Big help guys!
So ceramic cap with 224 marked on it is 224pf?
And one with 103 is 10000 pf?

ceramic cap with 224 marked on it is NOT 224pf is 220000pF or 0.22f.
103 is 10000 pf or 0.01f.
Slip of the keyboard(the dog ate my homework)
I understand...
But how many uf is one pf?
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Offline billhowl

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Re: Measuring Capacity of Capacitors?
« Reply #8 on: July 09, 2010, 10:13:56 PM »
http://en.wikipedia.org/wiki/Pico-

pf = picofarad =  10-12farad
f = microfarad = 10-6farad

1f = 1000000pf = 1000nF

some big value capacitors given in millifarads
etc. 22000F = 22mF



Offline Alfa_Zulu

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Re: Measuring Capacity of Capacitors?
« Reply #9 on: July 10, 2010, 01:12:41 AM »
i saw a 1Farad capacitor at our local electronics store the other day lol, it was over a foot long  ;D

Offline cyberfish

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Re: Measuring Capacity of Capacitors?
« Reply #10 on: July 10, 2010, 01:47:48 AM »
For a school project we used a 555 in oscillator mode to measure capacitances down to ~10pF.

It's a linear capacitance to period (1/f) converter. Probably not terribly accurate, but otherwise works pretty well.

You can use a microcontroller to measure the period.

Offline voyager2Topic starter

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Re: Measuring Capacity of Capacitors?
« Reply #11 on: July 10, 2010, 02:32:27 AM »
i saw a 1Farad capacitor at our local electronics store the other day lol, it was over a foot long  ;D
They ain't that big!
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Offline billhowl

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Re: Measuring Capacity of Capacitors?
« Reply #12 on: July 10, 2010, 04:27:52 AM »
i saw a 1Farad capacitor at our local electronics store the other day lol, it was over a foot long  ;D


Are you sure?

Offline voyager2Topic starter

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Re: Measuring Capacity of Capacitors?
« Reply #13 on: July 10, 2010, 06:09:03 AM »
Just the Sparkfun 10f super cap is only 13x33.5mm
http://www.sparkfun.com/commerce/product_info.php?products_id=746
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Offline Alfa_Zulu

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Re: Measuring Capacity of Capacitors?
« Reply #14 on: July 10, 2010, 08:40:43 AM »
yeah it was, but it did also have integrated circuitry that displayed a voltage and stuff, it was defiantly that big lol

Offline Soeren

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Re: Measuring Capacity of Capacitors?
« Reply #15 on: July 11, 2010, 09:15:23 AM »
Hi,

That's for large car stereo powers (to get a better bass response).
The small 1F caps are made in either 2.5V or 5V - the large one you saw has to safely handle around 15V and it has to have a low impedance, hence a larger plate area.
Regards,
Sren

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Offline Gertlex

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Re: Measuring Capacity of Capacitors?
« Reply #16 on: July 11, 2010, 01:58:07 PM »
Some multimeters can measure capacitance (mine can do it up to 200f.  I didn't try anything greater).

Not sure how common that feature is, but I think mine's just a $20 one.
I

Offline Admin

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Re: Measuring Capacity of Capacitors?
« Reply #17 on: July 12, 2010, 07:02:01 AM »
Typically the multimeters can measure no more than 100uF-200uF . . . so make sure it can do what you want before buying.

And that foot long cap would be an electrolytic, while the tiny 1F caps are supercapacitors. Supercaps, as Soeren pointed out, have really high ESR - making them fairly useless for frying kittens (high current applications). :'(

 


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