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Current or power limits for capacitors

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stevez

Active Member
I would like to improve my understanding of how much power or current a capacitor can handle. The application that drives my interest - impedance matching for RF applications for ham radio gear. I found some info on surface mount capacitors but very little on ceramics, silver mica and so on. Yes, there is stuff on the monster parts used in broadcast transmitters but it's not much help to me. I've seen the stuff on electrolytics for pulse applications. ESR is part of it but not all I need.

All capacitors have some losses which heat the device. Most capacitors have leads and other conductive paths that must have some limit to what they can handle but little is published on those limits. I did manage to get a call from a capacitor manufacturer who have me a rough guide for the heat dissapation of garden variety capacitors in terms of watts per square inch to limit temp rise. The question for me: will the device overheat as the frequency increases before the voltage rating of the capacitor is exceeded.

In the end what I would like to do is construct an antenna "tuner" that uses relays and fixed capacitors and have some assurance that it will withstand the 300 watts that I'll apply with my transmitter. Several manufacturers make them (tuners) with silver mica caps but they all seem to limit power handling to 100 watts. Traditionally the capacitors have been large variables or bread-slicers where temp isn't much of an issue.
 

Scubasteve

New Member
Stevez,

I do not have an answer for you on this, but I do express the same curiousity for what you are trying to learn. If you do get an answer from another source, could you please post it here?

The frequency increases, the heat will go down because of the capacitor's reduced reactance. I believe it is a function of the average current going through the capacitor which causes the heating, which means that there are purely resistive and capacitive losses dissipated as heat. Perhaps the higher wattage capacitors just dissipate heat better and have larger electrodes to deal with increased current demands. As I said, I am pretty lost when it comes to this subject..

Steve
 

Gene

New Member
Since 1000 watts is common among hams, I would look to the Radio Amatures Handbook published by ARRL for an antenna tuner circuit. An alternative might be to find a library that carries QST magazine or an on-line summary of published articles. Companies making 100w tuners are probably interested in the 2 meter crowd - there are others.
 

stevez

Active Member
I'll share what I can as I put a story together. I've not been able to find much at all in ARRL stuff - got several of the handbooks and have done searches. Call is WA2EKL by the way.
 

stevez

Active Member
I've looked thru info on hand as well as some new info - getting closer to understanding more.

For the ideal capacitor the capacitor looks like an open circuit at DC and gets closer to behaving like a short circuit as frequency increases - so current a capacitor could handle increases with frequency.

It appears that ESR (Equivalent Series Resistance) and DF (Dissapation Factor, a cousin of ESR) are used to represent an undesireable characteristic of capacitors. ESR or DF result in power loss (as heat). ESR, among other characteristics, is not flat or constant but changes with temperature and frequency (at least).

In many applications the losses aren't that important. The voltages applied are combinations of DC and AC and the proper capacitor is often determined by the reactance required and the max voltages that will be seen as well as frequency (with due regard for stability, temperature curves, etc).

In some applications the capacitor sees AC at high frequency and the losses can overheat the capacitor long before the voltage limits are exceeded. There are some good rules of thumb related to the surface area of a common capacitor (disc ceramic, silver mica) that suggest limits of power loss. We put ESR and DF to work in determining the heating or power loss for a given application.

So, I am getting closer to understanding how to establish reasonable limits. I am going to crank thru some of the math eventually and will see what I see. Will share with anyone who contacts me - [email protected] It remains to be seen if my work will be worth "publishing" in some small way. At best it will be a simplified version of what's already been done by others.

Please, if this has already been done and published, point me to the article, document, whatever.
 
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