ESR in capacitors

[guyrosc0]

Sorry if this is a basic question, but I'd rather ask and understand it properly than keep building my knowledge on a shaky foundation. ESR keeps coming up, which I understand stands for effective series resistance, but if it's called resistance, and resistance is what restricts current flow, then in capacitors ESR should be about restricting AC current, right? That's how I've been thinking about it
But then I see people talking about ESR in terms of heat dissipation and power loss, which makes it sound more like it's about energy being wasted rather than just restricting the AC signal and some explanations even make it seem like lower ESR is always better for filtering applications, but if it's resistance wouldn't some resistance be necessary for certain functions?
Appreciate any insight on this

 

[CoolBrad2025]

@guyrosc0, this is not basic at all. so, the main job of a filter cap is to shunt AC ripple to ground. If it has a high ESR, it can't handle the high ripple current well, gets hot, and its lifetime drops dramatically. You definitely want low ESR for power filtering

 

[barfle]

Capacitors are useful in both AC and DC applications. ESR is one of many specifications that real-world components have, although ESR is usually specified for filter or bypass (essentially the same thing - they go across the power supply and reduce unwanted noise) capacitors.

You would figure out your ESR based on your circuit’s need for low-noise power and the changes in the current it draws. Think if there were a resistor between the filter capacitor and your circuit, what’s the maximum resistance your circuit could handle there?

 

[SiteSupporter]

ESR also can mess with a capacitor's impedance when frequencies are high. If the ESR goes up, performance takes a hit in quick-switching circuits, like those power supplies that switch modes.