Calculate ESR of Electrolytic capacitor over frequency sweep?

[cupoftea]

Hi,
We are using the Rubycon YXF 1mF, 35V Electrolytic cap.

https://docs.rs-online.com/f8b3/0900766b80029b9d.pdf

Its datasheet (above) says it has D.F. of 0.12.

DF = ESR/XC

But this means at 682Hz it has ESR = 28mR....this sounds way too low?

 

[FvM]

DF is specified for 120 Hz. It's not constant over frequency, you can't extrapolate.

 

[DartPlayer170]

In theory ESR doesn't vary with frequency. It is a DC resistance in series with the ideal capacitance. So, you must calculate the ESR at 120Hz which is how the D.F. is specified. Xc will vary with frequency, but the ESR is still "ideally" the same - 159mR.

 

[Easy peasy]

Z in ohms = 0.048 according to data sheet, 20 deg C 100kHz

for various electro-chemical reasons the resistance is higher at 100/120 Hz

--- Updated Jul 13, 2023 ---

Also tan-delta, or DF, is multiplied by the VA the cap sees to get loss in watts

from this if you know the rms current - you can get an effective ESR for that freq, some of that loss is parallel current loss in the dielectric ... so not strictly ESR

 

[pjmelect]

If you were to measure the ESR of an electrolytic capacitor you will find that it stays mostly constant up to around 150KHz and then the ESR starts to go down until around 2MHz where it then goes crazy due to inductance effects.

 

[FvM]

In theory ESR doesn't vary with frequency.

In practice, capacitors don't behave according to over-simplified theorie. Neither ESR nor DF are frequency independent.

 

[cupoftea]

In theory ESR doesn't vary with frequency. It is a DC resistance in series with the ideal capacitance. So, you must calculate the ESR at 120Hz which is how the D.F. is specified. Xc will vary with frequency, but the ESR is still "ideally" the same - 159mR.

If you were to measure the ESR of an electrolytic capacitor you will find that it stays mostly constant up to around 150KHz.

Thanks, though the DF calculation does indeed give ESR = 0.159R at 120Hz. Though the datasheet gives ESR at 100kHz as 48mR....so there is some unknown function of ESR and frequency.

 

[FvM]

See this TDK curves:

 

[cupoftea]

Thanks, ayk, tan delta = ESR/Xc....So if tan Delta increases with frequency, then so must ESR....but this is not the generally accepted case..

 

[FvM]

So if tan Delta increases with frequency, then so must ESR

No, if tan delta increases less than frequency proportional, ESR still decreases, as in the shown diagrams.

 

[The Electrician]

Thanks, though the DF calculation does indeed give ESR = 0.159R at 120Hz. Though the datasheet gives ESR at 100kHz as 48mR....so there is some unknown function of ESR and frequency.

The values of DF and 100kHz impedance are maximums. Typical values would be much less than those.

I don't have the exact same capacitor you are using, but I have something close. Using a Rubycon 1000 uF, 35V, VX series capacitor, here is a sweep of its DF and ESR versus frequency. The analyzer doesn't show DF greater than 10. These curves are very typical for aluminum electrolytics:

Notice that the value of DF at 120 Hz is about .04, and ESR at 100 kHz is about .030 ohms.