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Values of inductors and capacitors

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olcal

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Value of an inductor or capacitor is measured at a specific frequency.
When a capacitor has a value of 20 microfarads, at what frequency is this value measured? If I use the capacitor in a circuit operating at a different frequency wont the capacitance change ? I assume the same applies to inductors. Can somebody clarify?
 

stevez

Active Member
I can tell you with a level of confidence that the value listed for a capacitor isn't intended to be frequency dependent - HOWEVER the precise value (capacitance, not reactance) of many capacitors is quite dependent on frequency. That leads me to wonder about tolerances - are the tolerances listed inclusive of frequency changes or is the value at some accepted standard frequency? Sometimes a question begets more questions - sorry.

Data on a Cornell-Dubilier website that I pulled recently shows how capacitance changes with frequency. As I recall it increase, peaks then decreases with rising frequency. The change may not be huge but it's obvious that someone cares because so much info is published.

I would expect inductors to be the same because the cores are somewhat frequency dependent in their behavior.
 

bogdanfirst

New Member
hey, Stevez, can you give me the link on that website you spoke about?
i am quite interested about this.
also, doesn't he refer to reactance?
 

Ravi

Member
I think he is referring to capacitive reactance also.

Olcal - the reactance of a capacitor is inversely proportional to both the value of capacitance and the frequency of the applied voltage.Therefore, the reactance of a cap is defined as the ratio of applied voltage to current and,like resistance, it is measured in ohms.

On the other hand, the reactance of an inductor is directly proportional to both the value of inductance and the frequency of the applied voltage. This is also measured in ohms.
 

stevez

Active Member
Bogdan - will try to see if I can find the link. Ran into it when I was trying to understand current carrying limit of capacitor which isn't much of an issue at DC or low frequencies but at RF the heating can be a problem. Give me a day or two.
 

olcal

Member
I was referring to capacitance and inductance since these are the parameters I am interested in. Is there any difference in capacitors & inductors fabricated for RF and non-RF circuits or are they the same?
 

pebe

Member
Ravi said:
Olcal - the reactance of a capacitor is inversely proportional to both the value of capacitance and the frequency of the applied voltage.Therefore, the reactance of a cap is defined as the ratio of applied voltage to current and,like resistance, it is measured in ohms.
That is not strictly true. Applied voltage/current defines impedance. Capacitors like silvered mica and those using a poly dielectric have a low power factor so for all practical purposes, impedance = reactance.

But when we look at electrolytics, these have a relatively high power factor because the electrolyte has a finite resistance, and this is in series with the capacitive reactance. It also has inductance due to the way the plates are wound. So electrolytics have a poor performance at RF when used as rail decouplers. This is why you often find a 100nF cap in parallel in those conditions.

The impedance of an electrolytic varies with frequency because the reactance changes but the effective series resistance does not.
 
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