Instructables

Reading Surface Mount Resistor codes

Picture of Reading Surface Mount Resistor codes
Unlike Through hole resisors which use colored bands to determine the value, surface mount resistor have numbers and sometimes letters. I will teach you how to take those letters and digits and determine the value.

Looking at the image you see 4 resistors, the first being 391  the seconed 270 and the two at the bottom are the same 30R9. That does not stand for a 391 ohm , 270 ohm, and so on. The formula for breaking the code down is as follows.
1st value-2nd value-multiplier
3              9               1             = 390 ohm resistor
2               7               0            =  27 ohm resistor
2               3               3            =   23000 ohm or 23K ohm resistor

There are some resistors which have 4 digits. The formula is the same with the addition to a digit.
1st value-2nd value-third value-multiplier
2              3                5                 3 =       235K ohm resistor

You may ask"why some have three diigits and some have four?" three digits typically means that the resistor is 5%  tolerance the four digit typically means the resistor is 1% tolerance. If a letter is used to represent tolerance use the chart below.

B=.1%
C=.25%
D=.5%
F=1%
G=2%
J=5%
K=10%
M=20%
Z=80%

Now you will notice that the last two resistor have an "R" in the equation. The way to look at these is very simple. The R represents a decimal. So 30R9 is a 30.9Ohm resistor.
R23= .23 ohm
5R6= 5.6 ohm
45R3= 45.3 ohm


 Of course you can read the values with a meter, but whats the fun in that? I hope you all learned something from this little write up.
Nightshade7 months ago
What if you have some without any markings at all?
yadavneeraj9 months ago
hi,i am neeraj i m joining a new member in ur electronic world. plz.help me.
mstokes31 year ago
Here's the full link. The part after and including the last % is not needed.

http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773204%7FH%7Fpdf%7FEnglish%7FENG_DS_1773204_H.pdf

Hope this helps.
mstokes31 year ago
There is a completely different code used on some resistors. It has to do with a three number scheme that allows 1% values. I found a good reference here:

http://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1773204%7FH%7Fpdf%7FEnglish%7FENG_DS_1773204_H.pdf%7F1622827-1
pfred22 years ago
I like to put the surface mount resistor on a hard surface then smash it open with a rock. Then like reading tea leaves I divine what it's resistance once was ...
ColorBomb (author)  pfred22 years ago
Pfred i gather from this post ,and others, that you aren't a fan om SMD. :0)
Should I be? When I strip boards with SMD on them I just let those parts float on the top of my solder pot until I skim it. So when I say SMT stinks I mean it literally!
!!! "Perfect" !!!

Exactly what ive been wanting to know to get into SMD soldering !!

Thanks a bunch for taking the time to make this nice and simple 'ible so a complete beginner (started this year!) in electronics can easily digest !

Keep up the GREAT work and hopefully some more write-ups on SMD's !

Oh by the way, how would i measure the capacitance of a SMD capacitor using a DMM ??
many thanks in advance fo a noob question but my friend google hasnt turned up much, or maybe im not searching for the right thing here, lol !!!