Resistors in Series

Adding resistors together to increase their value

You're building your latest project, and a 2.7K resistor is missing from the kit (not that such a thing could ever happen).  You rummage through your box of spare parts hoping to find the resistor you need, but come up empty.  What's an aspiring electronics engineer to do?  Well you find you've got a few 1K resistors, a couple of 690R resistors, and a 10R resistor.  Can you use any of these?  Well, sure you can.

Resistors can be added together in series to create a larger total resistor value.  Take R as the value for a resistor, where:

Rtotal = R1 + R2 + R3 + ... + Rn

In this simple relationship, you can add together in series two 1K resistors in, one 690 Ohm resistor, and one 10 Ohm resistor to get the 2.7K you need:

1K     +   1K   +   690R  +  10R

Rtotal = 1K + 1K + 690R + 10R
Rtotal = 1000 + 1000 + 690 + 10
Rtotal = 2700R = 2.7K

Given that the tolerance is 5%, a 2.7K resistor would have an acceptable range of 2.565K to 2.835K.  This would mean that you could get away with using only the two 1K resistors and the 690R resistor.  You would get a value of 2.69K which is within the accepted tolerance range.

Resistors come in standard values.  If you keep a parts box of some of the most common values, you won't be stalled just when you're completing a project.  When you purchase resistors, it's always a good idea to purchase more than you need.  In this way, over time you will build a valuable collection of spare parts which you can draw upon at any time.  By putting what you have together, you can create what you don't have.