# What causes the series resistance in capacitors?

Especially in high frequency circuits there often seems to be the problem of ESR of capacitors. ESR apparently is a characteristic of non-ideal capacitors, and is essentially a resistance appearing in series with the capacitance of the component.

But after doing some Google searching I haven't been able to find much info on what actually causes ESR. I understand that caps have wires, which always have some resistance, but if wires are the cause of ESR, wouldn't all other wirings of the circuit usually cause much more resistance to be considered? Since caps are basically just plates with a dielectric in between, the only other source of resistance I can think of is the dielectric. But let's consider electrolytic capacitors for example: The dielectric is an electrolytic liquid, shouldn't the liquid conduct electricity quite well? Why would there be much resistance? Or if we consider ceramic caps: What causes resistance here?

I understand ESR much better in the context of inductors, which often have many loops of thin wire, which can be expected to introduce lots of resistance. But what really causes ESR in capacitors?

• Your question in the title doesnt match the question in the body. Why its relevant: just do the math for any source of voltage with a series resistance when you draw significant current. What causes it is a lot broader question that I suspect will vary a lot by capacitor type. And also yes, wires and traces resistance have resistances that need to be considered, thats why it doesnt make sense to put a capacitor with thin traces far away. Commented Feb 22, 2019 at 12:20
• First hit on search engine on "ESR capacitor" avnet.com/wps/portal/abacus/resources/engineers-insight/article/… "The electrolyte’s temperature-dependent conductivity is a major contributor to ESR" Commented Feb 22, 2019 at 12:21
• @WesleyLee Why do you feel the questions don't match? My point was that why the series resistance of a cap is relevant compared to the resistance of all other wires in the circuit, which implies there probably is something else happening inside the cap that makes its ESR more relevant to the circuit than other resistances in the circuit, which are often not relevant to the calculations. Or am I using the word "relevant" in a wrong way (my first language is not English)? Commented Feb 22, 2019 at 12:36
• @Huisman I did find that link but I didn't find it very informative. I did mention that evaporation or drying of the electrolyte causes conductivity to drop, but it seems that only is one piece of the puzzle. Commented Feb 22, 2019 at 12:40
• 'Why it is relevant' is a diferent question than 'what causes it', that is all I'm saying. It is relevant because if you draw current there will be voltage drop, power dissipation, etc. What causes it: type of capacitor, its age, etc. Totally different answers. Commented Feb 22, 2019 at 12:45

I understand that caps have wires, which always have some resistance, but if wires are the cause of ESR, wouldn't all other wirings of the circuit usually cause much more resistance to be considered?

While the traces on your board are probably at least 20 microns thick (and maybe more like 50 microns), the metal layers inside an MLCC are probably only a micron or so thick. This means the resistance can be significant even for very short lengths of conductor inside the MLCC.

Also, the ESR doesn't only account for metal losses. It also accounts for any other power loss in the capacitor, such as dielectric loss. It's a simplified model for a lot of complex behavior, including other effects beside actual resistive loss.

However, there certainly are situations where you should consider your trace resistance in addition to the capacitor ESR.

On the other hand, if your concern is the self-heating of the capacitor and how much ripple current it can handle without overheating, then only the internal power loss, characterized by the ESR, is relevant, and the resistance of the traces connecting to it will have little impact.

But let's consider electrolytic capacitors for example: The dielectric is an electrolytic liquid, shouldn't the liquid conduct electricity quite well?

If you used a liquid that conducted electric current well, you would have built a resistor rather than a capacitor. You must use a strongly insulative liquid rather than a conductive one to build a capacitor.

Or if we consider ceramic caps: What causes resistance here?

There isn't much resistance. For an 0402 cap with a good dielectric, it might be in the 10's or 100's of milliohms. But the resistance isn't zero, and it's high enough that we need to consider it in many circuits, so manufacturers find they get more customers if they specify it.