# Capacitors...Insulation Resistance Can Be Confusing

## Technical Bulletin No. 04

Confusing? Yes - it can be - but doesn't have to be! An understanding of the basic principles in­volved in this concept of "Insulation Resistance" should help to dispel this confusion.

When a capacitor is charged from a DC energy source, an initial high current flows from the energy source into the capacitor. This current flow rapidly decreases toward zero as the capacitor absorbs it. At the same time, the voltage charge on the capaci­tor starts from zero and rapidly increases toward the energy source voltage value (see Figure 1 ).

Once a steady state charge condition is reached, the current flow into the capacitor should be zero, and the capacitor has a voltage charge equal to the source voltage value. Now, if we had an "ideal" capacitor, no further current would flow in the cir­cuit. Unfortunately, there is no "ideal" capacitor obtainable, and a very small "leakage current" does flow in the circuit. This "leakage current" is a result of electrons physically making their way through the capacitor. In a correctly designed and manu­factured unit, the "leakage current" is composed of electrons that make their way through the dielec­tric itself, around the edges and across the surfaces of the dielectric, and between the leads. Usually, the flow of electrons through the dielectric is far greater than the total of the other paths, and there­fore the other paths can be ignored.

This "leakage current" through the dielectric is us­ually converted to the expression "insulation resistance" by using Ohm's Law.

"Insulation Resistance", then, is a measure of the ability of the dielectric to withstand the passage of electrons through itself and should not be con­fused with the inherent "series resistance" of the capacitor. For ease of identification, this "insula­tion resistance" is also referred to as the "parallel" or "shunt" resistance of the capacitor (see Figure 2).

It should be noted here that for comparison pur­poses, Rs is usually infinitesimal compared to Rp. The magnitude of the leakage current for any cap­acitor is primarily controlled by the type of dielec­tric used, the temperature, the capacitance rating, and the time of electrification prior to making the measurement. The thickness of the dielectric and the magnitude of the charging voltage have a com­paratively minor effect on the leakage current.

TYPE OF DIELECTRIC

Each dielectric medium has its own inherent in­sulation resistance characteristic which largely de­pends on the chemical and molecular structure composition of the material.

TEMPERATURE

Insulation resistance properties of all dielectrics will decrease with increased temperature. This in­crease in temperature causes an increase in the orbital velocity of the electrons which, in turn, results in a higher flow of electrons through the di­electric.

CAPACITANCE RATING

Inasmuch as the capacitance rating in effect reflects the total area (square inches) of dielectric in the capacitor, it can (within design limits) be used as a direct measure for insulation resistance. In general, if we double the area of dielectric, we also double the number of paths for electrons to flow through the dielectric, and the final result is double the leakage current (one-half the insulation resis­tance).

Now, however, this inverse ratio between the ca­pacitance and insulation resistance for any given dielectric provides the capacitor manufacturer with a handy tool for designating a single value of insulation resistance as a guarantee to cover all capacitance values for that line. This is done by multiplying the insulation resistance (ohms) times the capacitance (farads) to arrive at a constant val­ue of (ohms x farads) or, more commonly (meg­ohms x microfarads).

This use of a limiting value became necessary as a convenience when plastic films made their appear­ance as capacitor dielectrics. These plastic films have such high inherent insulation resistance that very small values of capacitance ratings would re­quire instruments that could measure in the mill­ions of megohms area. Since present standard measuring equipment is not capable of reasonable accuracy above approximately 500,000 megohms, this limitation is used.