Methods for Measuring NTC Thermistors with a Multimeter
Measure the nominal resistance value Rt
The method of measuring NTC thermistor with a multimeter is the same as that of measuring a regular fixed resistor, that is, selecting the appropriate resistance level according to the nominal resistance value of NTC thermistor can directly measure the actual value of Rt. However, due to the sensitivity of NTC thermistors to temperature, the following points should be noted during testing:
(1) According to the definition of the nominal resistance Rt, this value is measured by the manufacturer at an ambient temperature of 25 ℃. So when measuring Rt with a multimeter, it should also be done when the ambient temperature is close to 25 ℃ to ensure the reliability of the test.
(2) The measured power shall not exceed the specified value to avoid measurement errors caused by current thermal effects. For example, the MF12-1 NTC thermistor has a rated power of 1W and a measured power of P1=0.2mW. Assuming the nominal resistance value Rt is 1k Ω, the test current is:
Obviously, using the R × lk gear is more suitable, and the full current Im in this gear is usually tens to hundreds of microamperes. For example, the widely used 500 type multimeter has an Im of 150uA for the R × 1k gear, which is very close to 141uA.
(3) Pay attention to correct operation. During testing, do not use it to pinch the thermistor body to prevent human body temperature from affecting the test.
Estimate temperature coefficient α t
First, measure the resistance value Rt1 at room temperature t1; Using an electric soldering iron as a heat source, approach the thermistor Rt1 and measure the resistance value Rt2. At the same time, use a thermometer to measure the average temperature t2 on the surface of the thermistor RT at this time. Enter the measured result into the following equation:
αt≈(Rt2-Rt1)/[Rt1(t2-t1)]
The α t of NTC thermistor is less than 0.
Notes:
1. When heating a thermistor, it is advisable to use a low-power soldering iron of around 20W, and the soldering iron tip should not be directly in contact with the thermistor or too close to it to prevent damage to the thermistor.
2. If the measured α t is greater than 0, it indicates that the thermistor is not NTC but FTC.
