Can you IR test a capacitor?

TYPE OF DIELECTRIC

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Every dielectric material has its own unique insulation resistance characteristics, which are primarily influenced by the chemical and molecular structure of the substance used.

TEMPERATURE

The insulation resistance features of all dielectric materials decline as the temperature rises. Increased temperature accelerates the movement of electrons, leading to a heightened flow of electrons through the dielectric.

CAPACITANCE RATING

Since the capacitance rating essentially represents the total surface area (in square inches) of the dielectric within the capacitor, it can (within design limits) directly indicate insulation resistance. Typically, if we double the dielectric area, we effectively double the pathways available for electrons to traverse the dielectric, resulting in a doubling of leakage current (thus halving the insulation resistance).

This inverse relationship between capacitance and insulation resistance provides capacitor manufacturers a useful guideline for specifying a singular insulation resistance value that applies across all capacitance ratings for a specific product line. This is typically calculated by multiplying the insulation resistance (in ohms) by the capacitance (in farads) to achieve a constant value of (ohms x farads), more commonly denoted as (megohms x microfarads).

The establishment of a limiting value became crucial when filmmakers emerged as capacitor dielectrics since these plastic materials possess extraordinarily high insulation resistance. With small capacitance ratings, the requisite measuring instruments would have to operate in ranges beyond millions of megohms, a feat not manageable by standard measuring tools, which typically lack accuracy above 500,000 megohms.

Note: It is "megohms times microfarad" rather than "megohms per microfarad."

TIME OF ELECTRIFICATION

When discussing insulation resistance, the "time of electrification" is often overlooked yet is prone to significant error. It is essential to acknowledge that all dielectrics exhibit some level of chemical polarity, resulting in "interfacial polarization." A strongly polar dielectric (such as mylar) shows high levels of "interfacial polarization," reflected in relatively high "dielectric absorption." Conversely, a non-polar dielectric (like polystyrene) will exhibit lower degrees of "interfacial polarization," leading to lower dielectric absorption readings.

The duration required to achieve a steady state condition varies based not only on the type of dielectric but also on various other factors. However, all units will generally conform to a common pattern as illustrated. For demonstration purposes, the curves closely represent typical behavior for non-impregnated mylar dielectric capacitors. The influence of temperature on both the insulation resistance value and the time to achieve a steady state condition is also indicated.

It is vital to always specify the "time of electrification" when determining insulation resistance values, whether derived from user specifications or manufacturers’ catalog listings. Generally, a two-minute electrification period is accepted in most applications. Furthermore, the insulation resistance value may fluctuate significantly between otherwise identical units from the same production batch.

The significance of insulation resistance, particularly in relation to its magnitude and fluctuations with time and temperature, becomes especially crucial in circuits where leakage current through the capacitor can lead to malfunctions or other undesirable results. Such situations frequently arise in coupling or decoupling circuits, along with certain blocking, timing, or signal pickup applications.

Insulation Resistance Testing Capacitors