Post by ChrisK on Jun 11, 2009 11:11:15 GMT -5
If you need to measure a resistor that is higher in value than what your meter can read (you get the "OL" indication), you can make use of the parallel resistor rule to bring things "down to Earth".
For instance, if you want to measure a 3 M Ohm resistor and your meter only goes to 1.999 Meg Ohms, you do the following;
Get and measure the largest resistance that your meter CAN measure. If my meter could only read up to 1.999 Meg, I'd use a 1 Meg since they are common, or a 1.5 Meg since it's bigger, but not over-range. This is Rref.
Add the unknown resistance in parallel with this reference resistor. measure the combined resistance. This is Rpar.
1/ (1/Rpar - 1/Rref) = Runknown
For example, if Rref measures 1.07 Meg and Rpar measures 0.792
eg, the unknown resistance is:
1/ (1/0.792 Meg - 1/1.07 Meg) = 3.05 Meg
The same approach works for capacitors in series.
1/ (1/Cser - 1/Cref) = Cunknown
1/ (1/792 uF - 1/1,070 uF) = 3,050 uF
For instance, if you want to measure a 3 M Ohm resistor and your meter only goes to 1.999 Meg Ohms, you do the following;
Get and measure the largest resistance that your meter CAN measure. If my meter could only read up to 1.999 Meg, I'd use a 1 Meg since they are common, or a 1.5 Meg since it's bigger, but not over-range. This is Rref.
Add the unknown resistance in parallel with this reference resistor. measure the combined resistance. This is Rpar.
1/ (1/Rpar - 1/Rref) = Runknown
For example, if Rref measures 1.07 Meg and Rpar measures 0.792
eg, the unknown resistance is:
1/ (1/0.792 Meg - 1/1.07 Meg) = 3.05 Meg
The same approach works for capacitors in series.
1/ (1/Cser - 1/Cref) = Cunknown
1/ (1/792 uF - 1/1,070 uF) = 3,050 uF