May 24 2019

# How to Measure Resistance and How to Detect Resistance?

## 1.1 Structure and Principle of Ohmmeter

Its structure is shown in Figure 1. It consists of three components: G is an ammeter with internal resistance of Rg and full bias current of Ig. R is a variable resistor, also called a zero-regulated resistor. The battery has an electromotive force of E and an internal resistance of r.

The principle of the ohmmeter is made according to the closed circuit Ohm's law. When the red and black test pens are connected to the resistance Rx to be tested, here can get according to the Ohm's law of the closed circuit:

It can be known from the expression of the current that although the current through the galvanometer is not proportional to the resistance to be measured, there is a one-to-one correspondence between the current measured and the corresponding resistance, which is the basic principle of measuring resistance by the ohmmeter.

Figure 1 Structure of Ohmmeter

1.2 Precautions for using

The larger the pointer deflection angle of the ohmmeter is, the smaller the resistance value to be measured is, so its scale is opposite to that of the current meter and voltmeter, which means the left is big and the right is small; the scale of ammeter and voltmeter is uniform, but the scale of ohmmeter is not uniform, because the relationship between current and resistance is neither proportional nor inverse.

The red and black terminals on the multimeter represent the + and - poles. The black pen is connected to the positive pole of the battery, and the red pen is connected to the negative pole of the battery. The current always flows from the red pen into the black pen out.

When measuring resistance, every shift should be adjusted to zero.

When measuring, the pointer should be as close as possible to the center of the full scale. (generally in the 1/3 area of the median scale)

When measuring, the measured resistance should be disconnected from the power supply and other components.

When measuring, you can't touch the test pen with both hands at the same time, because the human body is a resistor. After use, turn the selector switch away from the ohmic gear, generally to the highest or OFF position of the AC voltage.

II Volt Ampere Method

2.1 Principle

according to Ohm's law of partial circuits.

2.2 Selections of Control Circuit

There are two kinds of control circuits:

one is a current limiting circuit (Figure 2);

the other is a voltage dividing circuit(Figure 3).

(1)The current limiting circuit connects the power supply and the variable resistor in series, and changes the resistance of the resistor to change the current of the circuit, but the current changes have a certain range. The advantage is to save energy; generally, when both control circuits can be selected, the current limiting circuit is preferred.

(2)The voltage dividing circuit connects the total value of the power source and the variable resistor in series, and then leads the wires from the two terminals of the variable resistor. As shown in figure 3, its output voltage is determined by the resistance between aps, so that its range of the output voltage can be changed from zero to near the electromotive force of the power supply. In the following three cases, the voltage divider circuit must be used:

The measurement value is required to change from zero or to draw lines in the coordinate diagram.

The total value of the sliding rheostat is much smaller than that of the resistance to be measured.

## ③The range of the ammeter and voltmeter is smaller than the voltage and current in the circuit.

2.3 Measuring the Circuit

Since there are resistances in the voltmeter and the ammeter, there are two kinds of measuring circuits: amperometer internal connection and amperometer external connection.

(1)The circuit diagrams of the internal connection of the ammeter and the external connection of the ammeter are shown in figure 4 and figure 5, respectively.

Figure 4 Internal Connection of the Ammeter

Figure 5 External Connection of the Ammeter

(2)The selections of the amperometer internal connection and the amperometer external connection:

When knowing the approximate resistance of RV, RA and the resistance to be tested RX:

If   , choose the amperometer internal connection

If   , choose the amperometer external connection

When not knowing the approximate resistance of RV, RA and the resistance to be tested RX, adopt the trial method, as shown in figure 6. When one end of the voltmeter is connected to the point a and the point b respectively, if the number of ammeter has obvious change, adopt the internal connection method; if the number of voltmeter has obvious change, adopt the external connection method.

Figure 6

(3)Error analysis

The error in the internal connection is caused by the partial pressure of the ammeter, and the measured value is too large

The external error is caused by the voltmeter shunt, and the measured value is too small.

2.4 Improvement of the Circuit for Measuring Resistance by Voltammetry

The circuits of the two resistances shown in figure 7 and figure 8 can eliminate the error caused by the internal resistance of the meter.

III Several Special Methods for Measuring Resistance

3.1 Use Only Voltmeter, No Ammeter

(1)If only one voltmeter is used, the resistance of the unknown Rx can be measured by the circuit shown in Figure 9. The specific method is to use a voltmeter to measure the voltage at both ends of Rx as Ux, and then use this voltmeter to measure the voltage at both ends of the fixed resistance R0 as U0. According to the measured voltage value Ux, U0 and the resistance value R0 of the fixed value resistor, the value of Rx can be calculated as follows:

When measuring resistance in this way, a voltmeter should be connected twice.

(2)If only one voltmeter is used and it is required to connect only once, the circuit shown in Figure 10 can be used to measure the resistance of the unknown Rx.

The specific method is to first close S1, the reading of the voltmeter is U1, and then close S1 and S2 at the same time, and the reading of the voltmeter is U2.

Calculate the value of Rx according to the partial pressure formula:

(3)If only one voltmeter is used and it is required to connect only once, the circuit shown in Figure 11 can be used to measure the resistance of the unknown Rx.

The specific method is to first adjust the slider P of the sliding rheostat to the B terminal, close the switch, and record the voltage number U1; adjust the slider P of the sliding rheostat to the A terminal, and record the voltage number U2.

According to the measured voltage values U1, U2 and the resistance value R0 of the fixed value resistor, the value of Rx can be calculated:

In the above methods, it is necessary to measure the voltage twice, so it is also called “volt-volt method”; according to the given devices, there is a voltmeter and a resistor R0 with a known resistance, so it is also called “volt-resistance method”.

3.2 Use Only Ammeter, No Voltmeter

(1)If only one ammeter is used, the resistance of unknown Rx can be measured by the circuit shown in Figure 12. The current through R0 and Rx measured by ammeter is I0 and Ix, respectively.

According to the shunt formula, the value of Rx can be calculated:

(2)The resistance of the unknown Rx can also be measured with the experimental circuit shown in Figure 13. First close the switch S1, record the number of ammeter as I1, then open S1 and close S2, and record the number of ammeter as I2.

According to the shunt formula, the value of Rx can be calculated:

(3)If only one ammeter is used and it is required to connect only once, the resistance of the unknown Rx can also be measured with the circuit shown in Figure 14.

First close the switch S1, record the number of ammeter as I1, then close S1 and S2 at the same time, and record the number of ammeter as I2.

Let the power supply voltage be U, and when close the S1 only, according to the deformation formula U = I (Rx+R) of Ohm's law, we can get:

U = I1 (Rx + R0)

When S1 and S2 are closed at the same time, R0 is short-circuited, and it can be concluded:

U = I2 Rx

Finally, we can calculate:

(4)If only one ammeter is used and it is required to connect only once, the resistance of the unknown Rx can also be measured with the circuit shown in Figure 15.

Adjust the slider P of the sliding rheostat to the terminal A, and record the number of the ammeter as IA. Then adjust the slider P of the sliding rheostat to the terminal B, and record the number of the ammeter as IB.

Then we can get the value of Rx:

The above method needs to measure the current twice, so it is also called "Ampere Method". According to the equipment given, there is an ampere meter and a resistance R0 with known resistance value, so it is also called "Ampere Resistance Method".

3.3 Equivalent Method

(1)It refers to making the current (or voltage) through the resistance to be measured equal to the current (or voltage) through the resistance box in the course of measurement. The circuit is shown in Figure 16. Set the single-pole double-throw switch to a, close S1 to adjust R, make the ammeter read I0, keep R still, hit the single-pole double-throw switch to b, adjust R0 to make the ammeter read I0, then the resistance box reading is the value of the resistance to be measured.

(2)Measuring principle: figure 17 is an experiment done with a voltmeter.

(3)Note: The main components are resistance box and single-pole double-throw switch. The voltage divider control circuit can be used in the dotted wire frame.

## 3.4 Measuring Resistance by Formula Calculating Method

It mainly applies the characteristics of series-parallel circuit and the knowledge of the whole circuit to analyze and calculate the value of the resistance to be measured. Figure 18 is a circuit for measuring resistance Rx. Rx is the resistance to be measured, R is the protective resistance, and its resistance value is unknown. R1 is the fixed resistance known. The power supply electromotive force is unknown. S1 and S2 are single-pole double-throw switches. A is a current meter with no internal resistance.

(1)Measuring Rx: S2 closes to d, S1 closes to a, and record the ammeter reading I1; then S2 closes to c, S1 closes to b, and record the ammeter reading I2.

(2)The formula for calculating Rx is

When S2 is connected to d and S1 is connected to a, the voltage of Rx is: Ux=I1Rx.

When S2 is connected to c and S1 is connected to b, the voltage U1=I2R2 on R1 does not change the resistance R, Ux=U1

So I1Rx=I2R1

So

## 3.5 Measuring Resistance with Bridge Circuit

(1)Principle:

The circuit shown in Figure 19 is called a bridge circuit. Generally, a current flows through the galvanometer, but when a certain condition is met, no current flows through the galvanometer. In this case, it is called bridge balance. When the bridge is balanced, the two potentials of A and B are equal, so the circuit structure can be regarded as: R1R2 and R3R4 are connected in series and then connected in parallel; or R1R3 and R2R4 are connected in parallel, and then connected in series.

the condition of the bridge balance: R1×R4=R2×R3

(2)The measuring method:

As shown in Figure 20, connect the circuits, take R1, R2 as a fixed value resistor, R3 is a variable resistance box (can directly read the value), and Rx is the resistance to be tested. Adjust R3 so that the reading in the ammeter is zero, and apply the equilibrium condition to get the value of Rx.

## 3.6 Measuring Resistance with Semi-bias Method

(1)Measurement method:

The circuit diagram of the ammeter half-bias method is shown in Figure 21. R is a sliding rheostat, R0 is a resistance box, and G is the internal resistance of the current meter to be tested. In the experiment, first close S1, disconnect S2, and adjust R to make the pointer of the ammeter fully biased; then close S2, adjust R0 to make the current meter read half of the full scale. At this time, the value of resistance box is the internal resistance of galvanometer.

(2)Measurement Principle: when S2 is turned on, set the current meter to full bias current:

IV Detection Methods of Most Resistors

4.1 Detection of the Fixed Resistor

(1)The actual resistance value can be detected by connecting the two test pens (not positive or negative) to the two ends of the resistor. In order to improve the measurement accuracy, the range should be selected according to the nominal value of the measured resistance. Due to the nonlinear relationship of the ohmic scale, its middle section is finer. Therefore, the pointer indication value should be lowered to the middle part of the scale as far as possible, in the range of 20%-80% radian at the beginning of the full scale, so as to make the measurement more accurate. It varies according to the error level of resistance. The errors between the reading and the nominal resistance are allowed to be (+5%), (+10%) or (+20%) respectively. If not, beyond the error range, it means that the resistance value has changed.

(2)Note: during testing, especially when measuring resistances with resistance values above tens of kΩ, do not touch the conductive parts of the pen and resistors; the detected resistance is soldered from the circuit, at least one head must be soldered to avoid other components in the circuit. It affects the test and causes measurement error. Although the resistance of the color ring resistor can be determined by the color circle mark, it is better to test the actual resistance value with a multimeter when using it.

4.2 Detection of the Cement Resistor

The method and precautions for testing cement resistance are exactly the same as those for testing ordinary fixed resistors.

4.3 Detection of the Fuse Resistor

In the circuit, when the fuse resistor is melted and disconnected, it can be judged according to experience: if the surface of the fuse resistor is found to be black or burnt, it can be concluded that its load is too heavy, and the current passing through it exceeds the rated value many times; if the surface is open without any trace, it means that the current flowing is just equal to or slightly larger than its rated blown value. The judgment of the fuse resistor with no trace on the surface can be measured by the Rx1 gear of the multimeter. To ensure accurate measurement, one end of the fuse resistor should be soldered from the circuit. If the measured resistance is infinite, it means that the fuse resistor has failed open circuit. If the measured resistance value is far from the nominal value, it indicates that the resistance value is not suitable for reuse. In the maintenance practice, it is found that there are also a few blown resistors that are short-circuited in the circuit, so attention should be paid to the detection.

4.4 Detection of the Potentiometer

When checking the potentiometer, first turn the handle to see if the rotation of the handle is smooth, whether the switch is flexible, whether the “click” sound is clear when the switch is turned on or off, and listen to the internal contact point of the potentiometer and the friction of the resistor body. If there is a "rustling" sound, it means that the quality is not good. When testing with a multimeter, first select the appropriate electrical blocking position of the multimeter according to the resistance of the potentiometer to be tested, and then perform the detection as follows.

(1)Use the ohmic gear of the multimeter to detect the "1" and "2" ends. The reading should be the nominal resistance of the potentiometer. If the pointer of the multimeter does not move or the resistance value is different, it indicates that the potentiometer is damaged.

(2)Check if the movable arm of the potentiometer is in good contact with the resistor. Detecting the ends of "1", "2" (or "2", "3") with the ohmic gear of the multimeter, and turning the axis of the potentiometer counterclockwise to the position close to the button “off”, the smaller the resistance value, the better.

4.5 Detection of the Positive Temperature Coefficient Thermistor

(1)Room temperature detection (indoor temperature is close to 25 ℃): the actual resistance value of the two pins in contact with PTC thermistor is measured, and compared with the nominal resistance value, the difference between the two is normal within ±2 Ω. If the actual resistance value is too different from the nominal resistance value, the performance of the actual resistance value is poor or damaged.

(2)Heating detection: on the basis of normal temperature test, the second step of test-heating detection can be carried out, and a heat source (such as electric soldering iron) can be heated near PTC thermistor. At the same time, the multimeter is used to monitor whether the resistance value increases with the increase of temperature. If the thermistor is normal and if the resistance value does not change, it means that its performance becomes worse and can not be used further. Be careful not to keep the heat source too close to or directly in contact with the PTC thermistor to prevent it from being burned.

4.6 Detection of the Negative Temperature Coefficient Thermistor

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

Rt is measured by the manufacturer at an ambient temperature of 25 ° C. Therefore, when measuring Rt with a multimeter, it should also be carried out when the ambient temperature is close to 25 ° C to ensure the reliability of the test.

The measured power shall not exceed the specified value to avoid measurement errors caused by current thermal effects.

Pay attention to the correct operation: when testing, do not hold the thermistor body with your hands to prevent the body temperature from affecting the test.

(2)First, the resistance value Rt1 is measured at room temperature t1, and then the electric iron is used as a heat source, and the resistance value RT2 is measured near the thermistor Rt. At the same time, the average temperature t2 of the surface of the thermistor RT is measured by a thermometer.

4.7 Detection of the Varistor

Set the multimeter in 10K gear and connect the pen to both ends of the resistor. The multimeter should show the resistance value indicated on the varistor. If the value exceeds this value, it indicates that the varistor has been damaged.

The varistor can be changed from MΩ (megaohms) to mΩ (milliohms) as the voltage applied to it increases. When the voltage is low, the varistor works in the leakage current region, exhibits a large resistance, and the leakage current is small. When the voltage rises into the nonlinear region, the current changes within a relatively large range, and the voltage does not change much. It exhibits better voltage limiting characteristics; the voltage rises again, and the varistor enters the saturation region, exhibiting a small linear resistance. Due to the large current and the long time, the varistor will overheat and burn or even burst.

4.8 Detection of the Photoresistor

(1)The black light film is used to cover the light-transmitting window of the photoresistor. At this time, the pointer of the multimeter is basically kept, and the resistance is close to infinity. The larger the value, the better the photoresistor performance. If this value is small or close to zero, the photoresistor has been burned through and can no longer be used.

(2)A light source is aligned with the light-transmitting window of the photoresistor, and the pointer of the multimeter should have a large amplitude swing, and the resistance value is significantly reduced. The smaller the value, the better the photoresistor performance. If the value is large or infinite, it indicates that the open circuit of the photoresistor is damaged and cannot be used any more.

(3)The light-receiving window of the photoresistor is aligned with the incident light, and the small black paper is shaken on the upper part of the light-shielding window of the photoresistor to be intermittently received by the light. At this time, the pointer of the multimeter should swing left and right with the shaking of the black paper. If the multimeter pointer is always stopped at a certain position and does not oscillate with the paper shake, it indicates that the photosensitive material of the photoresistor has been damaged.

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