Detailed Operating Procedures for Resistance Measurement Using a Multimete
The principle of detecting resistance is different between a digital multimeter and a pointer multimeter. The pointer multimeter has a current type header, while the digital multimeter has a voltage type header. Moreover, when a pointer multimeter detects resistance, the black probe outputs a positive voltage and the red probe outputs a negative voltage. However, when a digital multimeter probe detects resistance, the polarity of the output voltage is opposite to that of a pointer multimeter.
From the figure, it can be seen that when measuring resistance with a multimeter, whether it is a pointer multimeter or a digital multimeter: both are equivalent to a battery connected in series with a resistor and then connected to the measured resistance Rx outside the multimeter. In the internal circuit of a multimeter, a pointer type multimeter uses the change in current after series connection to display the resistance value on the ammeter head; A digital multimeter sends the voltage drop across its internal resistor to the meter head, which displays the data.
The result we see is actually the number generated by the voltage drop or current across its internal voltage divider resistor.
In other words, when measuring resistance with a multimeter, it uses its internal battery and resistance to form a circuit with external resistance. The current in this circuit is provided by the battery inside the multimeter. For this reason, when using a multimeter to detect resistance, the measured resistance or circuit cannot operate with power, otherwise measurement errors may occur, and more importantly, there is a possibility of damaging the multimeter or the measured circuit. Because there will be unexpected mutual interference and unforeseeable consequences between two circuits.
According to the size of the measured resistance, the range of a multimeter for measuring resistance is generally divided into four.
Some multimeters may be divided into 5 zones, namely 200 Ω, 2000 Ω, 20k Ω, 200K Ω, and 2M Ω.
When the measured resistance is greater than the maximum value of the range, it will display "1.1". At this time, we can expand the range and conduct the test. Until it is possible to display a reading. When in the 200 Ω resistance range, the multimeter has high accuracy and can display a resistance change of 0.1 Ω. For beginners, the unit of resistance is as follows:
1M Ω=1000000=10OOK Ω.
For example, in the 20K Ω resistance range, when the detection data is 5.6, it means that the current detected resistance is 5.6K Ω, which is equivalent to 5600 Ω.
The specific operation steps are as follows.
1. Pull the multimeter to the resistance range and estimate the value based on the measured resistance, which can range from 200 Ω to 2M Ω.
2. Short circuit the multimeter probe, and under normal circumstances, it will display around 0.5 Ω in the 200 Ω resistance range. Some advanced multimeters can automatically zero when detecting resistance, and when short circuiting the probe, it will display 0.0 Ω. This is a normal phenomenon, indicating the contact resistance between the internal and external probe wires of the multimeter and the socket.
3. Confirm that the measured resistance or circuit can only be detected when it is not powered on. Connect the positive and negative probes of the multimeter to the measured resistance and read the data. Subtract the data from step 2 to obtain the true resistance value of the measured resistor.
Attention
When testing resistance, it is important to note that the circuit of the airbag system cannot be tested using resistance mode, as the voltage provided by the multimeter may trigger the airbag. In order to ensure that maintenance personnel do not make mistakes in testing, the wires of the airbag system are protected with yellow wire tubes to distinguish them, and this rule is followed by vehicles all over the world.
