Resistor serves to regulate the amount of current flowing in an electrical circuit. Resistors create resistance, or impedance, in an electrical circuit and reduce the amount of current that is allowed to flow. Resistors are also used for simple signal conditioning and to protect active electrical devices that could be damaged by receiving excessive current. To perform all of these functions, resistors must be properly measured and in good condition. Use these tips to find out how to test resistors.
Step
Step 1. Remove power from the circuit containing the resistor
This can be accomplished by unplugging it from the mains power source or by removing the battery if the circuit is a portable device. Keep in mind, some appliances may remain exposed to potentially damaging voltage for several minutes after they have lost power!
Step 2. Isolate the resistor from the circuit
Measuring the resistors still connected to the circuit can give inaccurate results, as part of the circuit may also be measured.
Remove one end of the resistor from the circuit. It doesn't matter which end is removed. Remove it by pulling on the resistor. If the resistor is already soldered, melt it with an electric soldering tool and pull out the resistor using small pliers. Soldering tools are available at electrical and hobby supply stores
Step 3. Check the resistors
If the resistor shows signs of blackening or scorching, it may have been damaged by excessive current. Resistors that appear blackened or charred should be replaced and discarded.
Step 4. Read the resistor value visually
The value will be listed on the resistor. Smaller resistors may list values marked with color-coded bands.
Pay attention to the tolerance of the resistor. There is no resistor whose value is exactly the same as listed. Tolerance indicates how much the listed value can vary and is still considered an appropriate resistor value. For example, a 1000 ohm resistor with a tolerance of 10 percent is still considered accurate if it produces a measurement of no less than 900 ohms and no more than 1,100 ohms
Step 5. Set up a digital multimeter (DMM) to measure resistors
DMMs are available at electrical and hobby supply stores.
- Make sure that the DMM is on and that the battery is not low.
- Set the DMM scale to the next setting that is higher than the approximate resistor value. For example, if the DMM can be set to a scale of 10 and it is measuring a resistor marked 840 ohms, set the DMM to a scale of 1,000 ohms.
Step 6. Measure the resistance
Connect 2 DMM probes to the 2 pin resistors. Resistors have no polarity. So it doesn't matter which DMM probe is connected to which leg of the resistor.
Step 7. Determine the actual resistance value of the resistor
Read the result shown on the multimeter. To determine whether the resistor is within the allowable range for the resistor or not, don't forget to take into account the tolerance value of the resistor.
Step 8. Reassemble the resistor that produces an accurate reading
Reconnect the resistors in the circuit by screwing them back into place if you tug on them with your finger. If the solder joint needs to be melted and the resistor has to be removed using pliers, melt it with a soldering iron and use pliers to screw it back into place.
Step 9. Replace the resistor that gives a measurement result outside the allowable value range
Remove the old resistor. Resistors are available at electrical and hobby supply stores. Note that replacing a malfunctioning resistor does not always solve the problem, if the resistor breaks again, the source of the problem must be looked for elsewhere in the circuit.
