Introduction to the measurement method of component or branch resistance in circuit

Many electronics enthusiasts often need to measure the resistance of certain components or branches in the circuit when assembling and repairing electronic devices. Beginners usually think that only by removing the components can the measurement be accurate. In fact, in many cases, it can be measured directly on the circuit, and there are many techniques to measure the resistance of electronic components.

1. Roughly determine the resistance value of the component when it is open or short-circuited

When testing a circuit, if you suspect that a component is damaged (open circuit or short circuit), you can roughly measure its in-circuit resistance value to make a judgment. For example, some diodes and transistors are more easily damaged when working in high current and high voltage situations. When inspecting, in order to make a quick judgment, you can use a multimeter to directly measure the forward and reverse resistance of the diode after the circuit is powered off. If the resistance value is large, it means that the circuit is open; if the resistance value is small, it may be short circuited. At this time, you should also check whether there is a component with a very small resistance connected in parallel with it. If so, you need to solder one end of the diode before testing. The transistor can also be tested in the same way.

Some insurance and current limiting resistors in the circuit generally have very small resistance values ​​and are easy to burn out and cause an open circuit. When the circuit is powered off, these resistors can be directly tested in the circuit because they are generally connected in series between the load and the power supply. If the measured resistance value is less than or equal to

2. Precision on the road

Measuring component resistance

Some components can be directly measured in circuit to get their precise resistance. For example, R1 and R2 in Figure 1 are separated from other parts of the circuit by capacitors, switches and other components with infinite DC resistance. Measuring them in circuit after power off is the same as taking them down. Although some components in the factory cannot be measured directly, their accurate resistance can be measured indirectly. As shown in Figure 2, component 1 and component 2 are connected in series. If the resistance value R1 of component 1 is known, it is only necessary to turn on the power to measure the terminal voltage U1 and U2 of component 1 and component 2. According to the characteristics of the series circuit 11=12 and Ohm's law, it can be known that the terminal voltage of the component is proportional to its resistance value. Therefore, it is only necessary to accurately measure the values ​​of Ul and U2 to obtain the resistance value of component 2 (Note: both component 1 and component 2 should be linear components).

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3. Impact of capacitor on measurement value

1. Effect of uncharged capacitor on measurement

As shown in the circuit in Figure 1, when measuring the resistance values ​​of R1 and R2, the internal battery of the multimeter will charge C1 and increase the current through the meter. The deflection of the multimeter pointer is large, and the resistance value read at this time is large. Since the capacitance of C1 is large, the charging time constant z is large, and the charging is slow. During the charging process of C1, the deflection angle of the multimeter pointer will gradually decrease. The multimeter pointer must stop before a more accurate resistance value can be read.

2. The impact of charged capacitors on measurement. For example, some circuits have a high working voltage and contain large capacitors. Their discharge speed is very slow, which will cause large errors in the rough measurement of component resistance values ​​in the circuit, causing misjudgment. Sometimes, other components and measuring instruments in the circuit are even damaged during detection, expanding the scope of the fault and causing unnecessary losses. Therefore, these capacitors should be discharged first when detecting the resistance value of components in the circuit.

Figure 1 is a part of the equivalent circuit of a switching power supply. When testing the circuit, C1 should be discharged first, because C1 is charged during operation, and its terminal voltage can be as high as 300V, and the discharge time constant is very large, "(R1+R2)C1" 175s. It takes (3-5) seconds for the capacitor to be discharged, about 10 minutes. The discharge is very slow, and voltage drops U1 and U2 are generated on R1 and R2 respectively during discharge. At this time, measuring the resistance values ​​of R1 and R2 in the circuit will produce large errors, and may even burn out the multimeter and other components.

IV. Example Analysis

1. The measured resistance value is smaller than the actual resistance value

For example, to measure the resistance of R1 (see Figure 3), after powering off the circuit, connect the red probe of the multimeter to point A and the black probe to point B. Ul can be regarded as a power source, which is connected in series with the battery in the meter, increasing the current passing through the meter head (that is, the direction of the current Ic provided to the meter head is consistent with the direction of the current Ie provided by the battery E in the meter). If the deflection angle of the multimeter pointer increases, the measured resistance value of R1 will be smaller. When Ic is larger and Ic;Ie, the deflection angle of the multimeter pointer will be large, and sometimes even short circuits between points A and B may be suspected. When Ic is too large (U1 is too high), and the current passing through the meter head is much greater than its full-scale current, the meter needle will swing rapidly and exceed the full scale, thus damaging the meter needle. In serious cases, if the meter head is not equipped with a protection circuit, the meter head will be burned out.

2. The measured resistance value is greater than the actual resistance value

When the red and black test leads in the above measurement are swapped, that is, the black test lead of the multimeter is connected to point A, and the red test lead is connected to point B (see Figure 4), U1 and E are reversed in series, and the direction of the current Ic provided by U1 to the meter head is opposite to the direction of the current Ie provided by the battery E in the meter, which reduces the current passing through the meter head, and the resistance value of R1 is measured to be too large. When the size of U1 is close to E, the pointer hardly deflects, and R1 will be suspected of being broken. If U 1.>E, the current passing through the meter head will be much greater than its full-scale current, which will damage the pointer of the multimeter at the least, and burn the meter head at the worst. Sometimes, a high voltage will be added to a certain component through the multimeter and damage it. This shows the importance of discharging high-voltage capacitors during detection.

I believe that everyone has mastered a lot of techniques for measuring the resistance value of electronic components!