"P dissipation" for caps?

[Stephanie]

Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

What got me thinking about this is that I looked up a capacitor (via (C) and
(V)) to replace a 20-year-old design and it's tiny compared to the original.
Then I got to thinking that, were it a resistor, I'd presume that the larger
one was rated for greater power dissipation. But capacitors, no: as long as
the (C) and (V) ratings are identical, they are interchangeable (generally;
specific application aside).

So, why is it not important to consider power dissipation and, in turn, (I)
when choosing a capacitor?

Thanks.
Stef

 

[Meat Plow]

On Sun, 14 Oct 2007 16:38:15 +0000, Stephanie wrote:

Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

What got me thinking about this is that I looked up a capacitor (via (C) and
(V)) to replace a 20-year-old design and it's tiny compared to the original.
Then I got to thinking that, were it a resistor, I'd presume that the larger
one was rated for greater power dissipation. But capacitors, no: as long as
the (C) and (V) ratings are identical, they are interchangeable (generally;
specific application aside).

So, why is it not important to consider power dissipation and, in turn, (I)
when choosing a capacitor?

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

 

[John Fields]

On Sun, 14 Oct 2007 16:38:15 GMT, Stephanie <nada@notavail.net>
wrote:

Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

What got me thinking about this is that I looked up a capacitor (via (C) and
(V)) to replace a 20-year-old design and it's tiny compared to the original.
Then I got to thinking that, were it a resistor, I'd presume that the larger
one was rated for greater power dissipation. But capacitors, no: as long as
the (C) and (V) ratings are identical, they are interchangeable (generally;
specific application aside).

So, why is it not important to consider power dissipation and, in turn, (I)
when choosing a capacitor?

---
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.


--
JF

 

[John Popelish]

Stephanie wrote:

Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

What got me thinking about this is that I looked up a capacitor (via (C) and
(V)) to replace a 20-year-old design and it's tiny compared to the original.
Then I got to thinking that, were it a resistor, I'd presume that the larger
one was rated for greater power dissipation. But capacitors, no: as long as
the (C) and (V) ratings are identical, they are interchangeable (generally;
specific application aside).

So, why is it not important to consider power dissipation and, in turn, (I)
when choosing a capacitor?

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.

 

[Eeyore]

Stephanie wrote:

Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

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

 

[John Larkin]

On Sun, 14 Oct 2007 16:38:15 GMT, Stephanie <nada@notavail.net> wrote:

Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

What got me thinking about this is that I looked up a capacitor (via (C) and
(V)) to replace a 20-year-old design and it's tiny compared to the original.
Then I got to thinking that, were it a resistor, I'd presume that the larger
one was rated for greater power dissipation. But capacitors, no: as long as
the (C) and (V) ratings are identical, they are interchangeable (generally;
specific application aside).

So, why is it not important to consider power dissipation and, in turn, (I)
when choosing a capacitor?

Thanks.
Stef

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

 

[Michael A. Terrell]

Eeyore wrote:

Stephanie wrote:

Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

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).

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.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Jamie]

Michael A. Terrell wrote:

Eeyore wrote:

Stephanie wrote:


Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

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).



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.


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.



--
"I'm never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

 

[ehsjr]

Stephanie wrote:

Resistors are rated by 2 parameters: ohms and (W).

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

 

[John Larkin]

On Mon, 15 Oct 2007 02:58:38 GMT, ehsjr <ehsjr@bellatlantic.net>
wrote:

Stephanie wrote:
Resistors are rated by 2 parameters: ohms and (W).

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

 

[Phil Allison]

"Stephanie"

Why is a capacitor not rated for power dissipation?

** 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.

What got me thinking about this is that I looked up a capacitor (via (C)
and
(V)) to replace a 20-year-old design and it's tiny compared to the
original.

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

Then I got to thinking that, were it a resistor, I'd presume that the
larger
one was rated for greater power dissipation. But capacitors, no: as long
as
the (C) and (V) ratings are identical, they are interchangeable
(generally;
specific application aside).

** Not true at all.

So, why is it not important to consider power dissipation and, in turn,
(I)
when choosing a capacitor?

** 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

 

[Eeyore]

"Michael A. Terrell" wrote:

Eeyore wrote:
Stephanie wrote:

Resistors are rated by 2 parameters: ohms and (W). Capacitors are rated in
(C) and (V).

It's taught to make sure that (W) doesn't exceed the resistor's power rating,
which takes (I) into calculation, but capacitors are only chosen by their
primary parameters, (C) and (V).

Why is a capacitor not rated for power dissipation?

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).

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.

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

 

[Eeyore]

ehsjr wrote:

Stephanie wrote:
Resistors are rated by 2 parameters: ohms and (W).

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.

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

Graham