Fluke 8842a Applications of the Ohms Functions, Testing Diodes, Testing Electrolytic Capacitors

Measurement Tutorial 4

RESISTANCE MEASUREMENT

NOTE

In the 2 MΩ and 20 MΩ ranges of 4-wire ohms, the voltage across the unknown resistance is sensed between the HI SENSE and LO INPUT terminals. Accuracy is not affected as long as the resistance of the LO INPUT lead is less than 10Ω in the 2 MΩ range, and less than 100Ω in the 20 MΩ range.

4-9. Applications of the Ohms Functions

The 2-wire and 4-wire ohms functions can be used for a variety of purposes in addition to measuring resistance, as the following applications show.

4-10. TESTING DIODES

The 2-wire ohms function can also be used to test diodes.

1.Select the 2-wire ohms function and the 2 kΩ range.

2.Measure the resistance of the diode. If the diode is good, when forward-biased it will measure about 0.6 kΩ to 0.7 kΩ for silicon (0.25 kΩ to 0.3 kΩ for germanium), and when reverse-biased it will cause the 8842A to indicate overrange. (The forward- biased reading depends upon the range used.)

The 2 kΩ range is used because its 1 mA test current provides a typical operating point, and its 2V full-scale voltage is sufficient to turn on most diodes (even two diodes in series).

4-11. TESTING ELECTROLYTIC CAPACITORS

The 2-wire ohms function can also give a rough test of an electrolytic capacitor’s leakage and dielectric absorption. This test works well for capacitors 0.5 μF and larger.

1.Select the 2-wire ohms function, the 2 kΩ range, and the medium reading rate.

2.Connect the test leads to the capacitor (with the INPUT HI lead to the + lead and the

INPUT LO lead to the - lead). The 8842A attempts to charge it to the open-circuit voltage of the 2 kΩ range (about 6V).

3.Disconnect the + test lead.

4.To test for leakage, select the VDC function and the 20V range (leave the 8842A in the medium reading rate), and measure the voltage that was stored on the capacitor during step 2.

a.If the capacitor is good, the voltage across the capacitor will be about 6V, and will be relatively stable.

b.If the capacitor is leaky, the voltage across the capacitor will be much less than 6V, and the voltage will be decreasing. The rate of change depends on how leaky the capacitor is.

c.With some electrolytic capacitors, the reading will increase. This usually indicates the capacitor is defective.

5.To test the capacitor’s dielectric absorption, briefly short the capacitor’s leads together and then measure the voltage across the capacitor.

a.If the dielectric is good (i.e., has low dielectric absorption), the voltage across the capacitor will be nearly zero volts.

b.If the dielectric is poor (i.e., has high dielectric absorption), the voltage across the capacitor will be significantly above zero.

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