Capacitors
- by Dave Jones
Capacitors are made up of conducting materials separated by insulating (dielectric) materials. Like most other passive electronic components they are never pure ideal components. The materials and the method of construction create a number of phantom components inside of them. Placing them in a circuit board adds a couple more phantom components.

What this means is that a capacitor, besides having capacitance, also has resistance and inductance. The equivalent circuit for a real world capacitor is usually modeled as a capacitor with a resistor in parallel with it plus another resistor and an inductor in series with the first two.

The resistor in parallel with the capacitor defines the "leakage" of the capacitor, or how much of the charge drains away on it's own. The resistor in series with it is known as the ESR or Equivalent Series Resistance. That resistance prevents the capacitor from charging or discharging as quickly as a theoretical capacitor would. Then there is also an inductor in series with that resistor known as ESL or Equivalent Series Inductance (inductance is represented by the letter L). That usually includes the leads that connect the capacitor to the real world, and that inductance can change the frequency characteristics of the capacitor. The inductance, resistance, and capacitance all form a sort of passive filter that effects how the capacitor works at various frequencies.

Even these equivalent circuits are generalisations, and the real world capacitor might have a much more complex equivalent circuit. But in general this model is used the most, and the specs for capacitors are created with this model in mind.

Different types of capacitors have different values for each of those phantom components. Those different values in turn make that particular type of capacitor work better in some types of circuits than in others.

At the same time, the type of materials used to make them, and the style of construction (flat layers vs rolled up layers, SMT vs through hole, etc...) also determine the range of values that are available for that type of capacitor. For example electrolytic capacitors are generally high values of capacitance while ceramic are generally small values. Film caps are generally in the mid range. All of those ranges overlap to some degree.

In general aluminium electrolytic are not so good at high frequencies, while ceramics are great for high frequencies. Aluminium and ceramic caps in general are not very accurate values and drift with temperature, except for NP0/C0G ceramics. Film capacitors generally have more accurate values, and in general have low drift with temperature. But then there are a lot of different types of film caps, and each has different characteristics.

The bottom line is that if a circuit designer specifies a specific type of capacitor, they did that for a reason and you should try to use that type. Sometimes they don't specify a type, but assume that a given type will be used because of the value or physical size in the board.

There are many cases where one type can be replaced with another and there won't be any changes in function. There are other times that will cause something about the circuit to work differently. Knowing what will happen when you substitute a part involves understanding exactly what the original part was doing in the circuit and how the new part will change that circuit. In many cases the characteristics that make different types different are not all that important in a circuit. In other cases they are very important.

C0G and NP0
C0G and NP0 (those are zeros, "C" zero "G" and "N" "P" zero) are designations for ceramic capacitors with no (or very little) change in value as the temperature changes.

NP0 stands for "Negative Positive Zero", meaning the value doesn't go negative or positive as temperature changes. It's an industry term that means the same thing as C0G. C0G is a designation given by the EIA (Electronics Industry Association) for temperature drift on ceramic caps. There are two groups of temperature designators in EIA and C0G is the lowest you can get, which is +/-0.003% per degree C of change (so going up 10 degrees C might change the value 0.03%).

Other common ceramic temperature styles are X7R (+/-15%) and Y5V (+22%, -82%). There are lots of others too. If the first letter is A through U then it holds it's value very well with temperature. If the first letter is X, Y, or Z then it drifts a fair amount with temperature.

This tolerance to temperature drift is a separate issue than tolerance to initial value. For example a capacitor might have a value (at any temperature) that is +/-5% of the value it is sold as. But might maintain that value within 0.1% across the range of temperatures.