Reading Resistors and Capacitors

Copyright © 2001, Ian Juby

On this page:
The resistor colour code and how it works
Pop quiz on resistors
About that fourth band...?
Ceramic capacitor value codes and how they work
Pop quiz on capacitors

    After teaching electronics to many age groups over the years, this has been a basic necessity to learn.  So, here goes!

As one camper described it, they're "peanuts with stripes", AKA resistors.  An essential electronic component ( I doubt very much you will EVER build an electronic circuit without one), their values are indicated by the stripes painted on them.  One needs to know both the colour code (which is also used extensively elsewhere in electronics) and how to use the colour code.  Here's a picture to reference to:

Each colour first represents the following numbers:

Violet (purple)

    Now how you use these numbers is a little bit more complicated.  First, orient your resistor so that the Gold or Silver stripe is to the right.  If your resistor has only three stripes, orient it with the stripes closest to the left hand end, as shown in the drawing.  If your stripes are perfectly centered.... well then the people who designed the resistor are bozos who should never have been in the business in the first place - don't buy from them anymore :-)  If your resistor has that fourth gold or silver stripe, just ignore that last stripe for the moment.

    The numbers work left to right, first and second numbers.  The third number is referred to as the "multiplier".  Think of it this way:  This is how many zeroes you add onto the end of your number.  The value is in ohms.  Now remember, resistor values can be from 1 or 2 ohms to 1 or 2 MILLION ohms, hence the reason for the multiplier stripe.

    In our above example, the first stripe is Red so the first number is... 2!  (easy or what?)  The second stripe is purple (believe it or not - the colours on resistors can be very hard to differentiate sometimes), so the second number is... 7!  (have you won the lottery so far?)  The third stripe, the multiplier, is yellow which is 4.  This means we add four zeroes onto our final number which is ...... 270,000 ohms!  Get it?

    Now don't forget, scientific numbers are indicated just like the metric system:  If you have 1,000 meters, you have... 1 km, right?  In this case, we would say this is a 270 k resistor.  We substitute the last three zeroes with a k for 1,000.  We represent millions by M for Meg.  a 2,700,000 ohm resistor would be written on a schematic as 2.7 Mohms.

Try your new found skills out:  Figure out what resistor value the following colour codes represent.  To see if you're right, click and drag your mouse pointer between the arrows >< beside each question for the correct answer.

Red, Purple, Red
>2,700 or 2.7k<
Brown, Black, Black
>10 ohms<
Yellow, Purple, Orange
>47,000 or 47k<
Brown, Grey, Green
>1,800,000 or 1.8 M<

Now go the other way:  What are the colour codes for the following values of resistors?  Again, highlight between the arrows to see the correct answer:

>Brown, Red, Red<
470 ohms
>Yellow, Purple, Brown<
>Brown, Black, Yellow<
1 M
>Brown, Black, Green<

Now about that fourth band....?

    The fourth band is what is referred to as a "tolerance" band.  This stripe tells you the accuracy of the number.  You see, the manufacturing process isn't perfect and it doesn't need to be.  An electronic circuit will work just fine with a 9k resistor in place of a 10k resistor.  The ratings aren't that accurate and the fourth stripe tells you how accurate they really are.  If it's gold, the value will fall somewhere within 5% of the colour code value.  If it's silver, it'll be within 10 %.  If there is no stripe, it means the actual rating of the resistor will be within 20% of the colour code value.

    For example, if you have a 1k resistor with a silver stripe, 10% of 1k is 100 ohms.  That means that that resistors measured value was somewhere between 900 and 1,100 ohms.  (1,000 +/- 100 ohms).  Get it?  Generally, you wind up ignoring the tolerance band as it is seldom important.

Moving along to ceramic capacitors:

    Now that you've become an expert on resistor colour codes, we'll move along to ceramic capacitors.  They're coding system, although written in numbers, works much the same way resistor codes do, and for the same reasons.  They're values can be huge - but in the exact opposite direction:  below the decimal point instead of above it.

    In scientific jargon, larger numbers are represented like this:
G Giga
M Mega
k kilo
no desgn. ones, tens, hundreds
m mili
µ micro
n nano
 p pico

    We'll be working downscale, below the decimal point as capacitors of any type tend to be extremely small in value.

    Capacitance is measured in FARADS.  Very, very seldom will you see a capacitor with a size in the milifarads.  They are almost always microfarads or smaller.
    Smaller capacitors come in a variety of shapes and sizes.  It will have its value written on it either in a three digit code (similar to resistors) or in absolute value, presumed to be either in microfarads or picofarads.  I can only say that experience will tell you what scale it will fall in.  Odds are if it is the tantalum type and its number is written as absolute (i.e. 0.1) its in microfarads.  If it is a ceramic type and written in absolute form, it'll most likely be in picofarads.

    Here's a picture of a ceramic and mylar capacitor and the type of code you're looking for:

    If they are coded, like the resistor the first and second number are just that:  your first and second number.  The third is the multiplier (how many zeroes you add onto the end) and the value is in picofarads.

    Looking at our chart above, the first beige ceramic capacitor would have a value of 1000 picofarads.  Simple enough, but its value can be written on a schematic in several different forms, all of which are correct:

1k pico  (1000 picofarads)
1n  (1 nanofarad)
0.001 µ (.001 microfarads)

    Just to make it more interesting, a capacitor with a value of 4,700 picofarads (that's a code of 472) is sometimes written like this:  4n7  Read it like "4.7 nanofarads".

    Here's some more examples to test your skills on.  Once again, figure out the value of the capacitor by its code and then highlight between the arrows >< with your mouse pointer to see the real answer.

>470 p or .47 n<
>.1µ or 100n<
>1.2n or 1n2 or 1,200 p<
>10 p<

    Once again, reverse the procedure so can find a capacitor the schematic calls for.  Figure out the code for the following values:

0.1 µf
>100 or 10<