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# In electronics, how do I figure out how much resistance a resistor has?

Question #63349. Asked by Aprial. (Mar 10 06 11:42 PM)

xfacilitatorx

Preferred values
Standard resistors are manufactured in values from a few milliohms to about a gigohm; only a limited range of values called preferred values are available. In practice, the discrete component sold as a "resistor" is not a perfect resistance, as defined above. Resistors are often marked with their tolerance (maximum expected variation from the marked resistance). On color coded resistors the color of the rightmost band denotes the tolerance:
silver 10%
gold 5%
red 2%
brown 1%.

http://www.reference.com/browse/wiki/Resistor

http://www.reference.com/browse/wiki/Resistor

 Mar 11 06, 1:02 AM
Baloo55th

Something not too widely known is that the 1% ones are taken out of the batch first, then the 2% ones (so the 2% ones are between plus 1 and 2% or minus 1 and 3%), then the 5% (which are between 2 and 5% and so on. This means a 20% resistor will be of a value between plus 10 to 20% or minus 10 to 20%. It's unlikely to be between plus 10 and minus 10%. Non-preferred values can be obtained, but these are usually very close tolerance and for rather specialised uses. You won't find a 420 Ω resistor in 20% tolerance. For a listing of the tables of preferred values, see http://www.electronics2000.co.uk/data/itemsmr/res_val.htm
Tolerances down to 0.1% can be found - but rarely in everyday use.
In addition, some resistors may be marked with numbers such as 472. This is likely to mean 47 times 10 to the power 2 (4700 Ω or 4k7 - the k in the middle being kilo). Be wary - some small capacitors look the same as some small resistors and are often marked in numbers. Test with a meter to see what it is if not sure - but only when it's out of the circuit or you won't get a true reading.

 Mar 11 06, 9:51 AM
eggy103

Look at the color coding on the surrounding bands, or use a multi-meter to test it.

 Mar 11 06, 10:51 AM

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