The influence of the output impedance of the test object and the input impedance of the voltmeter on the test value

1. Influence of output impedance and input impedance

In order to test the voltage correctly, great attention needs to be paid to the output impedance of the test object (voltage source) and the input impedance of the voltmeter. Although the output impedance of an ideal voltage source is 0Ω, in reality there must be an output impedance of several Ω (Figure 1). Large batteries sometimes reach several mΩ, and small batteries sometimes reach hundreds of Ω. When testing the output voltage of a bridge such as a resistance strain gauge (Figure 2), or the output voltage of a thermometer (temperature-sensitive) resistor or thermistor, it is necessary to assume an output impedance of hundreds of Ω to hundreds of kΩ. Even for a test object with a small test output impedance, when contacting it with a test probe, it is necessary to consider in advance the contact impedance of several kΩ caused by the aging of the test probe. (image 3).

When the output impedance of the test object is large, it will divide the voltage with the input impedance of the voltmeter, resulting in attenuation of the test value (Figure 4). For example, when using a voltmeter with an input impedance of 10 MΩ to test a coil-type battery with an output impedance of 1 kΩ and an open circuit voltage of 3 V,

There is an error of about 0.01%. In order to not be affected by the output impedance during testing, a voltmeter with a large input impedance needs to be used. For a general digital multimeter, the 100V range and 1000V range are about 10MΩ. For ranges below 10V, you can choose an input impedance of 10MΩ or above 10GΩ (Table 1). When testing voltages below 10V, the input impedance can be set to more than 10GΩ to greatly reduce the impact of the output impedance of the test object.

2. Input impedance of digital multimeter

3. Things to note when testing

A general voltmeter has a capacitor at the input part to remove high-frequency component signals. This capacitor is charged when the test object is connected (Figure 5). A voltmeter with a low input impedance (10 MΩ, etc.) will allow the capacitor to be discharged due to the input impedance. On the other hand, when the input impedance is high (>10 GΩ, etc.), the capacitor of the test object will hardly discharge even if it is separated. Therefore, even if the test object is separated, a value similar to the voltage value just measured will continue to be displayed (Figure 6).

If you can confirm the test values ​​and perform work at the same time, the above problems are not a problem. However, you need to be careful when using relays to automatically switch multiple test objects while testing (scan test). Figures 7 and 8 show the situation when two batteries are tested through relay switching. Assume that a 3.7 V battery is normally connected to Ch. 1, and a 3.6 V battery fails to make normal contact with Ch. 2 due to wear of the test probe. When testing Ch.1, because the test probe is in contact, a normal test value of 3.7 V can be obtained (Figure 7). On the other hand, the test probe was not in contact when testing Ch. 2, but because the capacitor was charged when testing Ch. 1, the test value around 3.7 V was displayed (Figure 8). This phenomenon can be avoided by reducing the input impedance of the voltmeter (10MΩ, etc.). But as mentioned before, if the contact impedance of the test probe increases, the test error will also become larger.

3. Actual test values

Three lithium-ion batteries of 18650 size were prepared here  , and then scanned 5 times from Ch. 1 to Ch. 3 according to the test system in Figure 9.

Figure 10 shows the test results after all channels are connected normally. Among them, Ch. 2 is relatively stable, and the original test value of 3.845 V was obtained. Figure 11 shows the test results after Ch. 2 is opened. In Ch. 2, the test voltage value before and after 4 V, which is similar to the test value in Ch. 1, was measured.

5. Countermeasures for poor contact

For poor contact, it is more effective to use a voltmeter with a contact check function such as DM7276. Using the same wiring method as in Figure 11, set the contact check function of the DC voltmeter DM7276 to ON and then perform the test. The results are shown in Figure 12. The test value of Ch. 2 is not displayed, but an error is reported and the quality cannot be judged. Using the contact check function, you can prevent erroneous testing and realize inspections with high reliability.

6. Summary

-1. In voltage testing, when the output impedance of the test object is large, or the contact between the test probe and the test object is not good, a voltmeter with a large input impedance needs to be used.

-2. When using a voltmeter with a large input impedance, the test value will not change to 0 V even when the test probe is open circuit, but will tend to display the test value just measured.

-3. In order to prevent misjudgments caused by poor contact, it is very effective to use a voltmeter equipped with a contact check function (HIOKI DM7276, etc.).

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