How to Measure Resistance,Thermistor&Photoresister?(With Images)

Resistors are generally directly referred to as resistances in daily life. It is a current-limiting element. After the resistor is connected to the circuit, the resistance value of the resistor is fixed. Generally, there are two pins, which can limit the current through the branch it is connected to. Resistors usually act as voltage dividers and shunts in circuits. This article mainly introduces the measurement method, steps and precautions for resistance, as well as the measurement of thermal and photosensitive resistors.

How to Measure Resistance

The resistance measurement method mainly uses the ohm block of the multimeter to measure the resistance value of the resistance, and compares the measured value with the nominal value, so as to judge whether the resistance can work normally, whether there is short circuit, open circuit and aging phenomenon (the actual resistance value is different from the nominal value). In the case of a large difference in resistance value).

Measurement of common resistance
Measurement of common resistance

Steps in Measuring Resistance

Visual inspection. Check whether the resistor is burnt, whether the resistor pins are falling off or loose, and rule out the open circuit of the resistor from the outside.

Power outage. If the resistance is in the circuit (that is, the resistance is still welded in the circuit), the power supply in the circuit must be disconnected, and live detection is strictly prohibited, otherwise not only is the measurement inaccurate, but also the multimeter is easily damaged.

Select the appropriate range. According to the nominal value of the resistance, select the range of the multimeter’s electrical resistance, so that the pointer of the multimeter falls in the middle of the multimeter dial (or slightly to the right).

On the road detection. If the measured value is far greater than the nominal value, it can be judged that the resistor has a short circuit or serious aging phenomenon, that is, the resistor is damaged.

Open circuit detection. During circuit testing, if the measured value is less than the nominal value, the resistor should be disconnected from the circuit for testing. At this time, if the measured value is basically equal to the nominal value, the resistance is normal; if the measured value is close to zero, it means that the resistance is short-circuited; if the measured value is much smaller than the nominal value, the resistor is damaged; if the measured value is much greater than the nominal value, the resistor is opening circuit.

Precautions When Measuring

The resistance to be measured must be desoldered from the printed circuit board, and at least one lead wire must be disconnected to avoid reading errors due to the parallel connection of other components.

Do not touch the lead wires at both ends of the resistor or the conductive part of the test pen with your hands at the same time, because the resistance of the human body (hand) has a great influence on resistance of more than tens of kiloohm.

The accuracy of the multimeter should be compatible with the error level of the measured resistance (such as ±5%, ±10% or ±20%, etc). Resistors whose readings are too different from the nominal value or whose readings are unstable should not be used.

Now that we have taken care of things when measuring, let’s look at how thermistors and photoresistors are measured.

Thermistor & Photoresistor Measurements

Thermistor Measurement

While measuring the resistance value of the thermistor with the ohm block of the multimeter, bake the thermistor with an electric soldering iron. At this time, the resistance value of the thermistor increases slowly, indicating that it is a thermistor with a positive temperature coefficient and is OK If there is no change in the measured resistance of the thermistor, the thermistor is bad. When the resistance value of the measured thermistor exceeds the original resistance value many times or infinity, it indicates that the internal contact of the resistor is poor or open circuit. When the measured resistance of the thermistor is zero, it indicates that the internal breakdown short circuit has occurred.

Thermistor Measurement

Photoresistor Measurement

You can use the R×1k block of the multimeter to touch the test leads of the multimeter to the lead pins of the photoresistor. When there is light, see if there is any change in the bright resistance value. Whether the resistance value changes, if there is a change, the photoresistor is good. Or change the intensity of the irradiated light. At this time, the pointer of the multimeter should swing with the change of light, indicating that the photoresistor is also good.

Photoresistor Measurement


A resistor is a circuit element that resists current flow based on Ohm’s law. It plays a wide range of roles in the circuit, and there are many types. In addition to the detection resistor method, there is a lot of related content to learn.


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