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NTC thermistor detection method

baijiahao | 21/06/2021

Several important parameters of the thermistor must be tested when applying the thermistor. Generally speaking, the thermistor is highly sensitive to temperature, so it is not appropriate to use a multimeter to measure its resistance. This is because the working current of the multimeter is relatively large, and when it flows through the thermistor, it will generate heat and change the resistance. But for confirming whether the thermistor can work, a multimeter can also be used to make a simple judgment. Specifically: set the multimeter to the ohm block (depending on the nominal resistance value), use the fish clip instead of the test pen to clamp the two feet of the thermistor respectively, and record the resistance value at this time; then pinch the heat with your hand. Sensitive resistance, observe the multimeter, you will see that as the temperature slowly rises, the pointer will slowly move to the right, indicating that the resistance is gradually decreasing. When it decreases to a certain value, the pointer stops. If the ambient temperature is close to the body temperature, this method will not work. At this time, you can use an electric soldering iron to approach the thermistor, and you will also see the hands move slowly to the right. In this way, it can be proved that only negative temperature coefficient thermistors are good.

Pay attention to the following items when testing negative temperature coefficient thermistors with a multimeter.

(1) The battery in the multimeter must be newly replaced, and the ohm zero point should be adjusted before measurement.

(2) The electrical barrier of ordinary multimeters is non-linear. In order to reduce the error, it is important to read the method correctly, that is, the line of sight is facing the needle when reading. If there is a mirror on the dial, the hands that the eye sees should coincide with the shadow in the mirror.

(3) The nominal resistance on the thermistor is not necessarily equal to the reading of the multimeter. This is because the nominal resistance is measured with a special instrument at 25°C, and there is a certain current when measured with a multimeter. Heat is generated by the thermistor, and the ambient temperature cannot be exactly 25°C, so errors inevitably occur.

Take the MF11 type negative temperature coefficient thermistor as an example to estimate its resistance at a certain temperature. The temperature coefficient of the MF11 type negative temperature coefficient thermistor is d25=-(2.23~4.09)%/℃ (based on the benchmark The temperature is 25°C as the starting point, and the resistance of the thermistor will increase (2.23%~4.09%) every time the temperature rises by 1°C. For simplicity, d25 can be taken as -3%/"°C, so the estimation is very Simple, at a certain temperature (t℃), the resistance of the thermistor is equal to the resistance of the previous temperature multiplied by a coefficient of 0.97 (ie 100%-3%=97%=0.97). For example, a certain MF11 The resistance of the negative temperature coefficient thermistor is 250Ω at 25°C, then 250Ω×0.97=242.5Ω at 26°C.

(1) Measure the nominal resistance value R. The method of measuring NTC thermistor with a multimeter is the same as the method of measuring ordinary fixed resistance, that is, the actual value of Rt can be directly measured by selecting the appropriate electrical barrier according to the nominal resistance of the NTC thermistor. But because NTC thermistor is very sensitive to temperature, the following points should be paid attention to when testing.

1) From the definition of the nominal resistance value R, we can see that this value is measured by the manufacturer when the ambient temperature is 25°C. Therefore, when measuring Rt with a multimeter, it should also be carried out 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, so as to avoid the measurement error caused by the heating effect of the current. For example, MF12-1 NTC thermistor, its rated power is 1W, and the measured power is P1=0.2mW. Assuming that the nominal resistance value Rt is 1kΩ, the test current is

Obviously, it is more appropriate to use the R×1k block, which is full of current I. It is usually tens to hundreds and tens of microamperes. For example, the R×1kΩ block of a 500-type multimeter has Im=150μA, which is very close to 141pμA.

3) Pay attention to correct operation. Do not pinch the thermistor body with your hands during the test to prevent the body temperature from affecting the test.

(2) Estimate the temperature coefficient at. First measure the resistance value Rt1 at room temperature t1; then use an electric soldering iron as a heat source, close to the thermistor R1, measure the resistance value Rt2, and use a thermometer to measure the average temperature t2 of the thermistor Rt surface at this time. Substitute the measured result into the following formula

At <0 of NTC thermistor

The matters that should be paid attention to in the measurement are as follows

1) When heating the thermistor, it is advisable to use a low-power soldering iron of about 20W, and the tip of the soldering iron should not directly touch the thermistor

Or get too close to prevent damage to the thermistor.

2) If the measured a>0, it means that the thermistor is not NTC but PTC.

(3) Simple test method for thermistor in air (One). In the absence of special NTC thermistor test instruments, meters and measurements

In the case of a fixture, it is necessary to measure the NTC thermistor. Although this method cannot reach the professional level of accuracy,

The resistance and accuracy of the thermistor can be roughly measured.

The measuring tools and materials are as follows.

1) Four and a half digit multimeter: two pieces.

2) Test fixture: one set.

3) ±0.1℃ thermometer: one

4) Stainless steel alligator clip: one pair

The measurement method and procedure are as follows.

1) First, use an air conditioner to control the temperature of a room at (25±0.1)°C.

2) Put the thermometer in the room to monitor the indoor temperature

3) Connect a pair of alligator clips to the test end of the multimeter respectively.

4) Clamp the two lead ends of a standard thermistor to the alligator clip, insert the other end of the multimeter pen into the multimeter, and turn on the multimeter

Set it to the appropriate gear for resistance measurement. At this time, the multimeter displays the resistance of the thermistor at this room temperature (Note: Nominal resistance accuracy

(Within 0.1%, the accuracy of B value is ±1%)

5) Place the alligator clip near the thermometer. This multimeter is used to monitor the temperature change in the room.

6) When the display value of the multimeter that monitors the room temperature is relatively stable, it indicates that the room temperature is relatively stable. At this time, the resistance of the room temperature can be tested, such as the resistance and accuracy of the thermistor used for monitoring on the alligator clip. When the resistance accuracy is the same, comparison measurement can be performed.

7) The thermistor to be tested can be judged as a qualified product according to the standard resistance value ±5% range tested at room temperature.

The matters needing attention in the measurement are as follows.

1) The temperature change in the room should be controlled within (25±1)℃. Be sure to close the doors and windows during measurement to reduce air flow to avoid indoor people entering and exiting and walking back and forth.

2) When the constant temperature cannot reach the control accuracy, the thermistor with the same resistance accuracy can be used as the room temperature monitoring resistor for comparison measurement.

3) The two multimeters should be calibrated accurately before measurement, and at least the consistency of the two meters should be checked.

4) The constant temperature of the thermistor to be tested should be no less than 30min at room temperature. If the product to be tested is energized and used, it should be placed for 12h before testing.

5) The thermometer, monitoring thermistor, and resistance to be measured should be placed in the same position indoors to ensure the accuracy of the measurement.

6) When testing the thermistor, the operator's hand should keep a certain distance from the thermistor as much as possible to avoid measurement errors caused by human body temperature.

(4) Simple test method for thermistor in air (2). Measuring tools and materials are as follows

1) Four and a half digit multimeter: two

2) A constant temperature oil pan: the size (500mm×300mm×100mm) can be customized, but it should not be small.

3) 0.1℃ precision thermometer: one piece.

4) Stainless steel alligator clip: one pair

5) Salad oil: appropriate amount (limited to full oil pan).

The measurement method and procedure are as follows.

1) Fill the thermostatic oil pan with salad oil (Note: If you use a non-riding steel pan, please put a 3~5m plastic plate on the bottom of the pan as a thermal insulation layer).

2) Put the thermometer into the oil pan (the thermometer should be completely immersed in oil) with the scale up.

3) Change a pair of alligator clips to the test end of the multimeter respectively.

4) Clamp the two lead ends of a standard thermistor to the alligator clip, insert the other end of the multimeter pen into the multimeter, turn on the multimeter and set it to the appropriate gear for resistance measurement. At this time, the multimeter displays the resistance of the thermistor at this room temperature. (Note: The accuracy of the nominal resistance value is within ±o1%, and the accuracy of the B value is ±1%).

5) Put the alligator clip in the oil pan filled with salad oil, so that the alligator clip and thermistor are all immersed in the oil. This multimeter is used to monitor the temperature change of the oil in the oil pan.

6) When the value displayed by the multimeter for controlling the oil temperature is relatively stable, it indicates that the oil temperature is relatively stable. At this time, the resistance of the constant temperature in the oil can be tested, such as the resistance and accuracy of the thermistor used for monitoring on the alligator clip and the temperature to be measured. When the resistance accuracy of the resistors is the same, comparative measurement can be carried out.

The matters needing attention in the measurement are as follows.

1) The temperature change of the oil in the oil pan should be controlled within the range of (25 ± 3) ℃, and doors and windows must be closed during measurement to reduce air flow to prevent indoor people from entering and exiting and walking together.

2) When the constant temperature can not reach the precision according to the system, the thermistor with the same resistance precision can be used as the oil temperature monitoring resistor for comparison measurement.

3) Two multimeters should be calibrated accurately before measurement, and at least the consistency of the two meters should be checked.

4) The constant temperature of the thermistor to be tested in oil should be no less than 2min, and all thermistors should be immersed in oil.

5) The temperature probe, monitoring thermistor, and resistance to be measured should be placed in the same position in the oil pan to ensure the accuracy of the measurement.

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Thermistors - NTC

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