"P dissipation" for caps?

Caps don't have to dissipate power. There is one other consideration that being max operating temp.

--
It is, in some instances.

For example, ESR (Equivalent Series Resistance) in electrolytics
puts a limit on the ripple current which the cap can handle since
that ripple current is flowing through the ESR and will dissipate
real power, which raises the temperature of the cap.

The short answer is that capacitors do not turn electrical energy into heat like resistors do, but simple store and release the energy.

The longer answer is that no capacitor is perfect and some energy gets turned into heat on its way to being stored or released from a capacitor. This is especially true if the capacitor is charged up or dumped very quickly, because those processes require that large currents pass through the capacitor connections and internal conductive structures. Since these conductive structures have some electrical resistance, those parts are heated by the current passing through them. The total effective series resistance of all that conductive stuff the current passes through as the capacitor charges and discharges is called the ESR (for equivalent series resistance). And that resistance does have a power rating. This rating is normally expressed as an RMS current rating for the capacitor. Exceeding that current rating will cause damaging internal temperature rise to occur because of the heat being produced by the capacitor current passing through its own ESR and the limited ability of the capacitor to conduct that heat to its surface, and then to get rid of that heat from its surface.

Because - in theory at least - they don't dissipate any power.

In practice electrolytic caps are rated for ripple current (this does cause them to warm) and certain speciality caps typically used in pulse circuits are rated for voltage 'rise time' (V/us).

Graham

Some RF capacitors are designed with power dissipation and self-heating in mind. I think ATC may have some app notes; they make some spiffy, very high-Q, beautiful looking, very expensive porcelain caps for high-power apps.

In high-power RF, regular ceramic caps have been known to unsolder themselves and fall off boards.

Google "water-cooled capacitor"

Power electrolytics are limited in their ripple current capability pretty much by power dissipation. But you're right, caps are seldom characterized for thermal properties, so people wind up experimenting a lot.

John

them

The limiting factor is the AC current they can handle, along with maximum voltage across the plates. I've seen a lot of overheated or destroyed capacitors in high power RF equipment These are usually the metal cased Mica bypass capacitors used in broadcast transmitters.

--
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Central Florida

them

yeah, i've seen caps blow up at Semco in the test environmental unit. subjected them to 1 mhz with a few watts in side a metal box. These were silver mica dipped caps. I must say, they do handle high currents before opening the case :) but when that happens, you have epoxy ceramic all over the inside.

--
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As a "side issue" to the thread: resistors also have a voltage rating. Not a factor in low voltage circuits, but when when the voltage is > 200 you need to look at the specs.

Ed

I tested some high-ohm 0603's. They typically arced over at around

1600 volts, so I figured they were safe to use at 250 or so.

And, interestingly, an 0603, an 0805, and a 1206 can all dissipate about the same amount of power if you solder their ends to big copper pours.

John

"Stephanie"

** They all are - but you have to delve deeper into the maker's data.

It is normally too detailed for them to print on the cap itself.

** Then it is almost guaranteed to be UNSUITABLE as a replacement.

** Not true at all.

** It is.

First you have to work out what the " I " through the cap is going to be.

Maybe using the formula I = C dv/dt.

Look at what uses the maker says the cap is suitable for.

There is a * LOT * to know about capacitors.

....... Phil

rating,

them

rated

Absolutely. As dV/dt increases, the current too increases.

The ones I've experienced wihere dV/dt needs to be considered are in resonant switching supplies. They were polypropylene film types. And yes, they can get quite warm. The metal foil and film parts can withstand a fair bit more repetitive dV/dt (i.e. current) than metallised film on account of the lower electrode resistance and hence lower temp rise from I2R heating.

Graham

Notably wrt to that limitation check out the 'start up' resistor in most SMPS designs.

Graham