ESL value of a capacitor

[Engineer4ever]

Hi,

I am wondering how can I determine the needed value of the ESL (equivalent series inductance) of a certain capacitor?

Thanks,

 

[malli_1729]

what application....Capacitor ESL depends on construction i doubt...

 

[D.A.(Tony)Stewart]

If you can sweep the cap with a small series resistance the voltage drop will maximize at the self resonant frequency (SRF) From this test the ESL and ESR can be calculated.

 

[KlausST]

Hi,

Why are you talking about "needed" inductace?
Often this inductance causes problems (ringing, SRF...)

Klaus

 

[Borber]

He needs to know it and SunnySkyguy described how to measure capacitor series resonance and attenuation at that frequency from where both inductance (ESL) and resistance (ESR) can be calculated.
Capacitor has parasitic L and R series connected with C forming series resonant circuit. Both parasitics can be determined only by measurement it's resonance curve.

 

[D.A.(Tony)Stewart]

All leaded Caps have a significant ESL and a SRF that is often stated in the specs when using the best sources for specs such as Murata.

The series test resistor ought to be the approximate value of your ESR so that when resonance occurs, you get 50% of the applied voltage or some easy to measure ratio of 100% to determine the R/(R+ESR) ratio.

When the external series resistor approaches the internal ESR or less, the Q of resonance increases rapidly which means the current rises sharply to maximum at resonance. and may be as high as 100, meaning if you will get a very low signal across Rs except at resonance where it will be 50% of Vin, if you match the ESR perfectly or 100% if Rs is too high.


Any questions?

 

[Engineer4ever]

If you can sweep the cap with a small series resistance the voltage drop will maximize at the self resonant frequency (SRF) From this test the ESL and ESR can be calculated.

I am sorry but can you please explain it a little more? what do you mean by "sweeping the cap."?

 

[Borber]

sweeping the cap=measure cap impedance at different frequencies to get resonance curve either manually with signal generator (point by point) or automatically by sweeping frequency generator (LF sawtooth signal for FM instead of turning a knob to set frequency)...