# Resistance measurement

Due to its structural characteristics, resistance has lead inductance and distributed capacitance. When working at low frequencies, the resistance component plays a major role, and the reactance component can be ignored. But when the operating frequency increases, the reactance component cannot be ignored. At this time, the resistance value of the Resistor working in the AC Circuit, due to skin effect, eddy current loss and other reasons, its equivalent resistance varies with frequency. Experiments have proved that when the frequency is below 1kHz, the difference between the AC resistance and the DC resistance of the resistor is no more than 1×10-4. As the frequency increases, the difference between them increases.
1. Measurement of fixed resistance
1.Multimeter to measure resistance
When measuring resistance with the resistance file of the multimeter, first select the multimeter resistance file multiplier or range according to the size of the resistance to be measured, and then short-circuit the two input terminals (called the test pen) to zero, and finally connect the multimeter in parallel to the measured resistance. Just read the resistance value at both ends of the resistance.
Pay attention to the following issues when measuring resistance with a multimeter:
① Prevent the two terminals of the resistance and the two test leads of the multimeter from being pinched together in parallel with both hands, because the resistance measured in this way is the resistance value of the equivalent resistance of the human body resistance and the resistance to be measured in parallel, not the resistance to be measured The resistance value of the resistor.
② When the resistance is connected in the circuit, the power supply of the circuit should be disconnected first, and no live measurement is allowed.
③When measuring resistance with a multimeter, pay attention to the voltage and current value that the measured resistance can withstand, so as not to damage the measured resistance. For example, you cannot use a multimeter to directly measure the internal resistance of a microammeter, because doing so may cause the current flowing through the meter to exceed its capacity (microampere) and burn the meter.
④ When the multimeter measures resistance, the zero point of different magnification gears is different, and the zero adjustment should be performed every time you change the gear. When the zero adjustment potentiometer of a certain gear can not make the pointer return to 0 ohms, it indicates that the battery voltage in the meter is insufficient. , Need to replace the new battery.
⑤Because of the non-linearity of the analog multimeter’s resistance gear dial scale, the measurement error is also large, so it is generally used for rough measurement. The error of measuring resistance of a digital multimeter is smaller than that of an analog multimeter, but when it is used to measure a resistance with a smaller resistance value, the relative error is still relatively large.

2.Bridge method to measure resistance
When the measurement accuracy of the resistance value is very high, the bridge method can be used for measurement. As shown in Figure 1, R1 and R2 are fixed resistors, called ratio arms, and the proportional coefficient K=R1/R2 can be adjusted through the range switch. Rn is the standard resistance called the standard arm, Rx is the resistance to be measured, and G is the current sense. count. When measuring, connect the measured resistance, turn on the power supply, adjust K and Rn, make the bridge balance, that is, the galvanometer indicates zero, read the value of K and Rn, you can get the value of Rx.

Figure 1 Bridge method to measure resistance

3.Voltammetric measurement of resistance
Voltammetry is an indirect measurement method. The theoretical basis is Ohm’s law R=U/I. Apply a certain voltage to the measured resistance, and the applied voltage should not exceed the tolerance of the measured resistance, and then use a voltmeter and an ammeter respectively Measure the voltage at both ends of the resistance under test and the current flowing through it, and then the resistance value of the resistance under test can be calculated.
Voltammetry has two measurement Circuits as shown in Figure 1 (a) and (b).
The circuit shown in Figure 2(a) is called the front connection method of the voltmeter. It can be seen from the figure that the voltage measured by the voltmeter is the sum of the voltage across the measured resistance Rx and the voltage drop of the ammeter’s internal resistance RA. Therefore, the measured value obtained according to Ohm’s law is Rmeasure=U/Ix=(Ux＋UA)/Ix=Rx+RA>Rx

Figure 3 Voltammetry to measure resistance

The circuit shown in Figure 2(b) is called the voltmeter post-connection method.It can be seen from the figure that the current measured by the ammeter is the sum of the current flowing through the measured resistance Rx and the current flowing through the internal resistance Rv of the voltmeter. Therefore, the measured value obtained according to Ohm’s law
R measurement=U/Ix=Ux/(Iv＋Ix)=Rx∥Rv＜Rx
When using the voltammetry method, you should select the appropriate measurement circuit according to the size of the measured resistance; if you cannot estimate the size of the measured resistance in advance, you can try both circuits to see the difference between the readings of the voltmeter and the ammeter of the two circuits In case, if the reading difference of the two circuit voltmeters is smaller than the reading difference of the ammeter, you can choose the front connection method of the voltmeter, that is, the circuit shown in Figure 2(a). On the contrary, you can choose the connection method after the voltmeter, which is the circuit shown in Figure 2(b).
Second, the measurement of the potentiometer
1.Use a multimeter to measure the potentiometer
The method of measuring the potentiometer with a multimeter is the same as the method of measuring the fixed resistance. First, measure the total fixed resistance between the two fixed ends of the potentiometer, then measure the resistance between the sliding end and either end, and constantly change the position of the sliding end, observe The resistance value changes until the sliding end is adjusted to the other end. When adjusting the sliding end slowly, it should slide flexibly, with moderate tightness, no hissing noise can be heard, the resistance indicator changes steadily, and there is no jump phenomenon, otherwise it means that the sliding end is in poor contact or there is a fault in the lead-out mechanism of the sliding end.
2. Use an oscilloscope to measure the noise of the potentiometer
As shown in Figure 3, add an appropriate DC power supply E to both ends of the potentiometer. The size of E should not cause the potentiometer to overpower consumption. It is best to use a battery. Let a certain current flow through the potentiometer, slowly adjust the sliding end of the potentiometer, and a smooth horizontal bright line is displayed on the phosphor Screen of the oscilloscope. With the adjustment of the sliding end of the potentiometer, the horizontal bright line moves in the vertical direction. If there are irregular burrs on the line, it means that there is sliding noise or static noise.

Figure 3 Use an oscilloscope to measure the noise of the potentiometer

Three, the measurement of non-linear resistance
Non-linear resistors such as thermistors, internal resistance of diodes, etc., their resistance is related to the working environment and the size of the applied voltage and current, and special equipment is generally used to measure their characteristics. When there is no special equipment, the volt-ampere method introduced above can be used to measure the DC current value under a certain DC voltage, and then change the size of the voltage, measure the corresponding current point by point, and finally draw the volt-ampere characteristic curve. The resistance value obtained is only indicative The DC resistance value at a certain voltage or current. If the resistance value is related to the ambient temperature, a certain external environment should be created.