Resistance measurement method
Bridge method
The method of applying the bridge balance principle to measure the DC resistance of the winding is called the bridge method. There are two types, one is the single-arm bridge (Wheatstone bridge), and the other is the double-arm bridge (Kelvin bridge). ).
1. Single arm bridge
The measurement principle of the single-arm bridge is shown in Figure 2-1, and xR is the resistance to be measured. When the voltage drop across Rx is equal to the voltage drop across R3, there is no potential difference between A and B, that is, there is no current in the galvanometer. At this time, 1I flows through xR and 2R, and 2I flows through 3R and 4R. Achieve balance.
2. Double arm bridge
The wiring of the double-arm bridge is characterized by separating the current and voltage lines of the connection and the Rn test. The voltage drop is used to bring the voltage drop across Rx and Rn to the bridge balance, so that the contact resistance of the lead and the wiring through the current is made. The upper voltage drop is not introduced into the bridge, which eliminates the influence of contact resistance and lead resistance, and is suitable for measuring small resistances with high accuracy requirements.
The schematic diagram is shown in Figure 2-2. In the figure, P is the galvanometer, XR is the measured resistance, R3, R4, 'R3 and 'R4 are the bridge arm resistance, NR is the standard resistance, and C1 and C2 are the measured resistance. The current connector, P1 and P2 are the voltage connectors of the resistor under test.
3. Current voltmeter method
The current voltmeter method, also known as voltammetry, is the most primitive and simple method. Its principle is to pass a DC current in the winding to be tested, thus generating a voltage drop across the resistance of the winding, as long as the current I in the circuit is measured. And the voltage drop U across the load, according to Ohm's law R = U / I, you can calculate the resistance R. The measurement method of the resistance is shown in Figure 2-3. According to the requirement of measuring the small resistance, the voltammetry method is an external connection method. The accuracy of the measuring instrument should not be lower than 0.5. The voltmeter should be as large as possible to select a 4-bit high-precision digital multimeter with large internal resistance. The current meter should use a relatively small internal resistance. The power supply (battery) in the multimeter is connected to the resistor through the test leads, and the voltage and current at this time are measured inside the multimeter. It can be seen from the figure that the actual measured resistance value of the multimeter includes the measured resistance XR and the test lead feeder resistances RL1 and RL2.
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