How to Know If Resistors Are Good or Bad

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Before testing a resistor to see if its good or bad, you must know its value. Resistors decrease the current flowing through a circuit. Each is made to work under a limited load, with the ability to decrease a specific amount of current. The bands around the components indicate resistor values with a color code. Determine the value of a resistor and then test it to see if it meets that standard.


Step 1

Turn off the power running through the circuit containing the resistor you need to test. Exhaust the stored power from any capacitors in the circuit by touching both capacitor terminals with the metal blade of an insulated screwdriver to short out the power.

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Step 2

Look at the color bands around the barrels of the components for the code to determine resistor values. The bands grouped close together represent numbers with the last one being a multiplier. The band slightly separated indicates a tolerance variable. Consult an electronics catalog or use the link in the resources section and click on the resistor color chart to determine the numeric value for each color.

Step 3

Check resistor values with the color codes. A resistor with red, green and black bands followed by a yellow multiplier band and a silver tolerance band would indicate 2, 5 and 0, or 250 multiplied by 10,000, which equals 2,500,000. The reading could vary by 10%, being plus or minus 250,000.


Step 4

Remove one end of the resistor from the circuit board to get an accurate reading. Use a solder iron to melt the solder, holding one end of the resistor. Pull that end up until it is no longer in contact with the board.


Step 5

Set a digital multimeter for "ohms." Set an ohms range that is equal to or higher than the value of the resistor to be tested. Touch the red lead from the meter to one side, and the black lead to the other. Resistors do not hold a polarity, so it doesn't matter which side gets the red or black lead. Check the reading to determine if the resistor is good. The two and a half million value resistor is good if the meter reads between 2,250,000 and 2,750,000 due to the silver tolerance band figure of plus or minus 10 percent.



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