MadSci Network: Engineering

Re: Why a certain value of resistor is not available?

Date: Sun Sep 23 01:14:48 2001
Posted By: Donald Howard, Staff, Nuclear Engineering, Retired
Area of science: Engineering
ID: 1000620579.Eg

I don't believe so.  Over the years that resistors have been manufactured, 
every conceivable value in the resistor code has probably been made.  
Whether they are available in your location and at the wattage you require 
is moot.

I checked a few web sites and one belonging to Fairchild Semiconductor 
lists a few circuits they manufacture with "25 ohm output resistors," so 
they do exist.

What is done when a resistor is unavailable is to combine other valued 
resistors in series/parallel circuits.  For example to make a 25 ohm, you 
could use two 50 ohm resistors in parallel, or make a 3.2 ohm resistor 
with a three ohm and two 0.1 ohm resistors in series.  

Most electronic repair shops have a box of miscellaneous resistors, and 
when they need a value they don't have they will use them in 
series/parallel to substitute for it.

And, common resistors, at least in America, are rated at plus or minus 
20%.  Therefore, given a supply of resistors that are rated at 3.0 ohms, 
there will be some that will measure 3.2 ohms when checked with an 
accurate meter.

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