How to measure capacitors/Test a Capacitors (8): How can I measure a capacitor’s insulation resistance?
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A. Insulation Resistance (IR) is the extent to which the dielectric material in a capacitor resists leakage current. It is the resistance of the dielectric material itself*1. IR is measured by leakage current. Knowing the leakage current and applied voltage, the insulation resistance can be calculated based on the ohm’s law. There are two basic ways to measure the leakage current. First, apply an ammeter in series with the capacitor and voltage source (see Figure 1).

Second, apply a voltmeter in parallel with a resistor, and then connect in series to the capacitor and voltage source (See Figure 2).

The first method is usually applied to capacitors less than 1uF. Low capacitance capacitors have low leakage current; thus, a low current ammeter can measure the current accurately. If the leakage current is high, the ammeter will not able to measure accurately due to the noise and unstability of the charged capacitor. Therefore, the second method should be used for higher capacitance capacitors*2.

Figure 1: Series ammeter with capacitor.

Figure 2: parallel voltmeter with a resistor and series to a capacitor.
  • *1 “Glossary of Capacitor Terms”, Ray Ostlie (1989)
  • *2 Keithley Switching Handbook, 3rd Edition, 1995, Page 4-12

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