Simple question I guess. What causes capacitors to short out at high frequencies? Not neccessarily “high” but why does the frequency of a signal cause a cap to act as a short?
The formula for capacitive reactance is X[sub]c[/sub]=1/(sqr(2piFC)), where F is the frequency in hertz and C is the capacitance in farads. As you can see, as the frequency increases, the reactance decreases. At very high frequencies, the reactance approaches zero.
In less technical terms, a capacitor is a lot like a one loop transformer made with really wide wire.
When you connect a DC voltage to a capactior, current flows for a short time until the capacitor is charged up, and then the current flow stops. So the capactitor looks like an open circuit. However, an AC signal is alternating so current keeps flowing in and out of the capacitor. So it looks like current is flowing through the capacitor. The higher the frequency the easier the flow, because the capacitor never comes close to being fully charged.
WARNING:Simplistic explanation, which may not reflect reality.
Hook up a capacitor to a DC voltage source. Electrons from the negative side of the cap migrate to the positive side. Eventually there aren’t anymore free to migrate. So for a short time the cpacitor does conduct.
Hook it up to a low frequency AC source. at the beginning of a cycle the cap conducts then stops. On the reverse of the cycle the cap conducts and then stops again (The free electrons migrate back).
If the AC frequency is high enough, (that is, switches back and forth fast enough) the cap never “saturates” and the electrons bounce back and forth and cap conducts.
The resistance of a cap is 1/jwc, or (ignoring phase) 1/(2pif*c).
so as frequency (f) aproches infinity, resistance aproaches zero.
Brian
No, it’s not. What you describe is a very low reactance inductor. inductive reactance is X[sub]L[/sub]=sqr(2piFL). The reactance increases with increasing frequency.
N9IWP, KB2YYR…73 
Oh crap, it’s too early in the morning and I miscounted the number of leads…:smack:
<hijack>
N9IWP and KB2YYR are amateur radio callisigns, and “73” is amateur radio talk for “best wishes”
</hijack>
and I forgot the sqaure root
Brian
Ham radio talk, scr4. 73 means ‘best regards’. i’m assuming his username is a callsign. It looks like one.
In the real world though capacitors show a even lower reactance than that expected from that equations specially the electrolytic ones and especially at higher frequencies.
The voltage-current relationship for a capacitor is that the current is proportional to the time rate of change of voltage. A high frequency voltage has a high rate of change of voltage with respect to time. So, for a given voltage magnitude, the current continually increases with frequency and the capacitor acts more and more like a short circuit.
That’s true. The equation was, of course, used for the sake of simplicity. A real-world capacitor has a small amount (electrolytics are higher in this regard than most other types) of internal leakage, represented by a resistance in parallel; a small capacitance to ground; and a small series inductance (dependent upon plate geometry).
Very true. And this is why there is a practical upper frequency limit for a given capacitor. At very high frequencies, series inductance and shunt capacitance (capacitance-to-ground) can be very problematic.
Probably something of a hijack here, but…
What would cause a capacitor to “bulge?” My SO recently had to replace the mainboard in her 'puter because of “bulged” capacitors. Is this a symptom of excessively high frequency? Or is it too many amps or volts? Or what?
IANAEG (I am not an electronics geek.)
Electrolytic caps will bulge when excessive overvoltage overcomes the oxide insulation on the plates, causing a current flow to heat the electrolyte. This creates water vapor, and the pressure makes the can bulge. This is most often caused by a problem somewhere else in the circuit, so replacing the cap by itself may likely not fix the problem, or do so only temporarily.
Q.E.D. is correct.
Here’s a problem I’ve seen: Manufacturers of switching power supplies often do not pay attention to the ESR rating of electrolytic capacitors. ESR capacitance is a very important parameter in a switching circuit, and excessive ESR could cause an electrolytic cap to overheat and bulge over time. Manufacturers often just pick the cheapest caps.
There were some articles posted to slashdot about some motherboards using bad electrolytic capacitors. It seems many manufactures got junk capacitors which were leaking.
Actually, one of the IEEE periodicals (don’t recall which one but I could probably find it if pressed) stated that this was a case of corporate espionage gone wrong. It seems a corporate spy (engineer moving to the competition IIRC) did not copy the full or correct formula for the electolyte. The new company not only started manufacturing capacitors with the incorrect formula, but it sold the formula to other manufacturers. This event (while denied by the company that allegedly originally received the stolen formula) has hurt the entire capacitor manufacturing industry in Taiwan. Many of the Taiwanese companies are saying that the whole story is made up by the Japanese capacitor industry to try to regain market share…