What is the function of insulation resistance in a film capacitor

The range of applications for film capacitors is expanding rapidly, thanks to advantages such as non-polarity, high insulation resistance, excellent frequency characteristics (covering a wide frequency range), and low dielectric loss. Insulation resistance is a key characteristic of film capacitors; so, what exactly is it? Today, I’d like to talk to you about the insulation resistance of film capacitors.

The insulation resistance of a film capacitor is defined as the ratio of the voltage applied across the capacitor to the leakage current—expressed as R = U/IL, where R is the insulation resistance, U is the voltage, and IL is the leakage current. Many people may be unfamiliar with the concept of leakage current in film capacitors; when the rated DC operating voltage is applied, the initial charging current is high but decreases over time, eventually stabilizing at a specific value. This stabilized current value is known as the leakage current.

Therefore, the capacitance of a film capacitor is given by C = Q/U, where Q is the electric charge and U is the voltage applied across the capacitor. Combining this with the formula above yields R = Q/(C × IL); thus, it can be seen that the larger the capacitance, the lower the insulation resistance, and the smaller the capacitance, the higher the insulation resistance.