How to test thermistors with a multimeter?
Thermistor in the current electrical appliances used more frequently, it is through the change in ambient temperature and resistance changes, thereby changing the working state of the circuit, is widely used in temperature sensors and control systems.
Thermistor according to its resistance value and the relationship between temperature change, can be divided into positive temperature coefficient and negative temperature coefficient of two kinds. Positive temperature coefficient means that the resistance value of thermistor decreases with the rise of ambient temperature.
The nominal resistance value of thermistor refers to the resistance value of the environment at 25℃, therefore, when measuring the resistance value of thermistor, it is necessary to pay attention to the influence of the ambient temperature on its resistance value. When the ambient temperature is 25℃, the resistance value of thermistor measured by multimeter is its nominal resistance value. If the ambient temperature is not 25℃, it is normal phenomenon that the measured resistance value does not match the nominal resistance value of thermistor.
If you need to test whether the thermistor has a positive or negative temperature coefficient, you can add heat around the thermistor when testing the thermistor, such as using a soldering iron to get close to the thermistor, and then if the measured resistance value increases, it is a positive temperature coefficient thermistor. If the resistance value increases, the thermistor is a positive temperature coefficient thermistor, and if the resistance value increases, the thermistor is a negative temperature coefficient thermistor.
The working principle of multimeter resistance
The working principle of multimeter resistance is to use a sensitive magneto-electric DC ammeter (microammeter) as the meter head. When a small current passes through the head, a current is indicated. However, the head cannot pass large currents, so some resistors must be connected in parallel and in series to shunt or step down the current, voltage and resistance in the circuit.
Because the head is a DC meter, so when measuring AC, you need to add a parallel and series half-wave rectifier circuit to rectify the AC into DC and then through the head, so you can measure the AC voltage according to the size of the DC. The method of extending the AC voltage range is similar to the DC voltage range.
Connect appropriate resistors in parallel and series on the meter head, and connect a battery in series at the same time, so that the current passes through the measured resistor, and according to the size of the current, the resistance value can be measured. Changing the resistance value of the shunt resistor will change the range of the resistor.
