Large capacitor have the value printed plainly on them, such as 10.uF (Ten Micro Farads) but smaller disk types along with plastic film types often have just 2 or three numbers on them?
First, most will have three numbers, but sometimes there are just two numbers. These are read as Pico-Farads. An example: 47 printed on a small disk can be assumed to be 47 Pico-Farads (or 47 puff as some like to say)
Now, what about the three numbers? It is somewhat similar to the resistor code. The first two are the 1st and 2nd significant digits and the third is a multiplier code. Most of the time the last digit tells you how many zeros to write after the first two digits, but the standard (EIA standard RS-198) has a couple of curves that you probably will never see. But just to be complete here it is in a table.
What these numbers don't tell us is the ESR rating of a capacitor. Despite popular belief capacitors will often still have the correct value of capacitance when they fail.
milli, micro, nano, pico
1 mili Farad (or any other unit) is 1/1,000th or .001 times the unit. (10-3)
1 micro = 1/1,000,000 or 0.000 001 times the unit (10-6 )
1 nano = 1/1,000,000,000 or 0.000 000 001 times the unit (10-9 )
1 pico = 1/1,000,000,000,000 or 0.000 000 000 001 times the unit (10-12 )
Now for an example: A capacitor marked 104 is 10 with 4 more zeros or 100,000pF which is otherwise referred to as a .1 uF capacitor.
Most kit builders don't need to go further, but I know you want to learn more. Anyway, Just to confuse you some more there is sometimes a tolerance code given by a single letter. I don't know why there were picked in the order they are, except that it kind of follows the middle row of keys on a typewriter.
So a 103J is a 10,000 pF with +/-5% tolerance
Now to be really complicate things there is sometimes a
letter-number-letter (like Z5U) code that gives information. Table 3 shows
how to read these cryptic codes. A 224 Z5U would be a 220,000 pF (or .22 uF) cap
with a low temperature rating of -10 deg C a high temperature rating of +85 Deg
C and a tolerance of +22%,-56%.
There are some Capacitor color codes - the last dot is the tolerance code where brown is +/-1% red +/-2% as in the resistor color code with two exceptions black is +/- 20% and white is +/- 10% going backward the three dots to the left of the tolerance dot form the value in pF There will be two or three more color dots before the value but they mean different things about temperature range and coefficient depend which one of three systems is used - so I will leave it out for now unless some one asks.
There are two more number systems seen on caps. The first one can be recognized as the EIA because it starts with an R.
R DM 15 F 471(R) J 5 O (C)
The above number means the following
This next one is the Military code
CM 15 B D 332 K N 3
Cog or NPO refer to caps that don't have any temperature drift (at least in theory <g> they all have SOME amount of drift.)