ESR confusion, what is good and bad esr

[odgaf]

im confused over esr ratings, if i have a cap that should have an esr of 0.6, but it has an esr of 0.20, is that cap good or bad, how far out of spec can esr go before a cap is bad,
also if i have a cap thats esr is supposed to be 0.20, but shows as 0.1, would this cap be ok and if not why

[mariushm]

Re: ESR confusion, what is good and bad esr

It depends on the volume of the capacitor and the capacitance.

A 10uF 400v capacitor could have an esr of 0.6-2 ohm and it would be OK, because it's a small capacitor both in value and capacitance.
A 270-470uF 400-450v primary capacitor could have an esr of around 0.4-0.8 ohm and it would be acceptable.
However, from a larger volume capacitor like a 2200-3300uF capacitor rated for 6.3-16v, low esr would be considered a value under 0.1 ohm

With some experience you get used to what to expect from various capacitance/voltage combinations.
For computer switching power supplies and power supplies in monitors, which usually run at high frequencies (something like 40-150 kHz), you can just look up the impedance at 100 kHz column in datasheets of various capacitors to know in what range the esr should be for that capacitor.
At 100kHz or around that frequency, impedance is pretty much the same as the esr value.

[odgaf]

Re: ESR confusion, what is good and bad esr

hi buddy thx for helping, im a complete noob at this
i get that the esr depends on capacitance, but on a 220uf 16v caspacitor rated at an esr of 0.3 im getting a reading of 0.30, im using a peak esr 70 to take the measurement, and im unsure at what point a caps esr is defective ?

[stj]

Re: ESR confusion, what is good and bad esr

0.3 = 0.30

[momaka]

Re: ESR confusion, what is good and bad esr

0.3 = 0.30

But are you sure?

@ odgaf: like stj said, 0.3 is the same as 0.30. So you cap is in spec. Also 0.6 (i.e. same as 0.60) is more than 0.20. So your caps appear to be in spec... or at least from what you give us from your spec vs. measurement values.

[Mint Waxed]

Re: ESR confusion, what is good and bad esr

im confused over esr ratings, if i have a cap that should have an esr of 0.6, but it has an esr of 0.20, is that cap good or bad, how far out of spec can esr go before a cap is bad,
also if i have a cap thats esr is supposed to be 0.20, but shows as 0.1, would this cap be ok and if not why

To start, I think a definition of ESR would be useful for those who might not know. "ESR is the sum of in-phase AC resistance. It includes resistance of the dielectric, plate material, electrolytic solution, and terminal leads at a particular frequency. ESR acts like a resistor in series with a capacitor (thus the name Equivalent Series Resistance)". A very informative (pdf) article by Mark Gebbia can be found at this link:

[PeteS in CA]

Re: ESR confusion, what is good and bad esr

When an electronics teacher or professor tells students, "Assume an ideal capacitor," ESR is one of several real-world things he's telling students to ignore. In that sense, ESR is an imperfection, but always present to some degree.

ESR was defined well by "Mint", above, except that ESR does not include the resistance of the dielectric, which should be as close to infinite as possible. Being a resistance, Ohm's Law applies, and if - as in the case of a P/S or VRM O/P cap - the cap is conducting significant AC current (aka "ripple current") power is dissipated (which hurts efficiency), and heat is generated (which can accelerate electrolyte evaporation). Basically, there is no such thing as "good", i.e. desirable, ESR.

In the real world, if the ESR of a cap is higher than its datasheet rating, it's either gone bad, or was never good. In that same real world, a control circuit, such as in a P/S, has design elements that compensate for the effects of ESR in that control loop. If the ESR is too high or too low it will cause performance problems and/or instability in the control loop.

So, in the context of an entire circuit design, it is possible for ESR to be too low. This is the reason, in many threads here, there is frequent discussion about whether a particular series cap is a suitable replacement for a cap of another series. The replacement needs to have specs similar to those of the original(s).

HTH