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Capacitor Characteristics/Specifications- Explained

There are many characteristics and specifications which appear on a capacitor's datasheet which holds significant value to the nature of the capacitor.

These include terms such as the temperature coefficient, the capacitor's equivalent series resistance (ESR), insulation resistance, dielectric absorption and so on.

What do all of these terms mean? Well, in this article, we will go over each practically so that you can understand what each specification is and how it determines whether the capacitor suits and can function appropriately in a particular circuit.

Capacitor Specifications

Temperature Coefficient

The Temperature Coefficient of a capacitor is a specification that tells us how much the capacitance varies with temperature.

We must take into account the temperature coefficient of a capacitor for a circuit that is intended to operate in extreme conditions.

The temperature coefficient (also sometimes referred to as tempco) may be expressed as the percentage variation in value over the working range of temperature, or as the variation in parts per million per degree Celsius.

Equivalent Series Resistance (ESR)

The Equivalent Series Resistance of a capacitor is the a mathematical construct, expressed in ohms, that allows all capacitor losses (resistance in capacitor leads, electrodes, dielectric losses, and leakage) at a single specific frequency to be expressed as a single series resistance with the capacitance.

A high ESR causes a capacitor to dissipate more power (loss) when high AC currents are flowing. This can degrade the capacitor and cause serious performance consequences at RF and in supply decoupling carrying high ripple currents. Using capacitors with low ESR is important in high-current, high-performance applications, such as power supplies and high-current filter networks. The lower the ESR, the higher the current-carrying ability. A few capacitors with low ESR include both mica and film types.

Dielectric Absorption

The dielectric absorption of a capacitor is the inability of a capacitor to completely discharge to zero.

It is sometimes referred to battery action or capacitor memory, because of this charge retention, and this is due to the dielectric of the capacitor retaining a charge after it is supposedly discharged.

The effect of dielectric absorption is that it reduces the capacitance value of the capacitor. This is because since it isn't able to discharge all of its charge, it has less effective working charge. Even though the capacitor can still charge up to a certain charge, it cannot discharge all of it.

High dielectric absorption is a negative, undesired trait of a capacitor, while low dielectric absorption is a positive, desired one.

All capacitors have some dielectric absorption, but electrolytic capacitors have the highest amount. Dielectric absorption has an undesirable effect on circuit operation if it becomes excessive.

Insulation Resistance

The Insulation Resistance is the measure of the resistance of a capacitor to DC current flow through it under steady-state conditions.

Insulation resistance is an important parameter because it signifies how well a capacitor can block DC signals. A capacitor with a high insulation resistance can function well as a coupling capacitor (to pass only AC signals), since it has high resistance to DC current flow and, thus, will block DC signals. On the other hand, a capacitor with a low insulation resistance is poor at blocking DC signals, since it has a low resistance to DC current, so it will function poorly as a coupling capacitor.

Capacitor Leakage

Capacitor Leakage is the amount of current (and, thus, also voltage) that a capacitor leaks after being charged up.

Even though capacitors are storage devices, they aren't perfect charge retainers. So after they are fed voltage, they don't perfectly keep that voltage forever. They leak current and voltage over time.

Electrolytic capacitors generally have high leakage rates. Therefore, they are not good capacitors to be used for AC coupling. This is because they will leak too much DC voltage and current onto the AC signals, not allowing for good AC coupling. Capacitors that have low leakage rates lend themselves better to AC coupling. They won't leak as much DC voltage and current to the AC signal, so they allow more effective AC coupling. Capacitors that have low leakage include film-type capacitors such as polypropelene and polystrene. These capacitors have insulation resistance of 10(sup)6 MΩ. Film capacitors make for very good capacitors for AC coupling, when you want to only pass through AC signals and block DC.

Capacitor Shelf Life

Capacitor shelf life is the amount of time a capacitor can last while stored away during a period of disuse.

Except for electrolytic capacitors, capacitors do not deteriorate with age while stored, since there is no applied voltage. Electrolytic capacitors, however, like dry cells, should be used fresh from the manufacturer because the wet electrolyte may dry out over a period of time. Therefore, electrolytic capacitors are the worst capacitors in terms of shelf life, but the others shouldn't show any deterioration at all.

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