I've never seen wattage ratings on a capacitor--voltage yes. Is it
simply .5CV^2 and that's it? I can design a circuit for a particular
capacitance--but how is the physical size of the Cap determined--I've
seen big capacitors with same MF's as little caps? What am I missing?


Thanks

"cnctut" wrote in message
oups.com...
I've never seen wattage ratings on a capacitor--voltage yes. Is it
simply .5CV^2 and that's it? I can design a circuit for a particular
capacitance--but how is the physical size of the Cap determined--I've
seen big capacitors with same MF's as little caps? What am I missing?


Thanks


There's no such thing as a wattage rating for a capacitor, just voltage and
capacitance. Some newer ones are smaller than older ones, some brands are
different, non-polar ones are much larger and venting temperature or ESR can
also have an effect on size.

James--thanks!

Tut

Hey-- I've kinda wondered this too! In a circuit, I might choose a 16v
in a circuit I expect to have 5 volts running through. I have been told
I could put a 63v there and it works just the same. BUT, would it last
better? Would the extra expense do anything at all for reliability?

On 29 Mar 2005 02:51:37 -0800, "Glynn R."
wrote:

Hey-- I've kinda wondered this too! In a circuit, I might choose a 16v
in a circuit I expect to have 5 volts running through. I have been told
I could put a 63v there and it works just the same. BUT, would it last
better? Would the extra expense do anything at all for reliability?

Reliability is generally improved by "derating" but only up to a
limit. Derating means operating the capacitor at a voltage less than
its rating, ie, a 25 volt capacitor operated at 15 volts is derated
15/25 = 60%. In other words, it is being operated at 60% of its rated
voltage.

Capacitor voltage ratings can be derated 25% - 100% depending on the
type but going beyond that is a waste of money and space except in the
most demanding applications.

John

"John Bachman" wrote in message
...
On 29 Mar 2005 02:51:37 -0800, "Glynn R."
wrote:

Hey-- I've kinda wondered this too! In a circuit, I might choose a 16v
in a circuit I expect to have 5 volts running through. I have been told
I could put a 63v there and it works just the same. BUT, would it last
better? Would the extra expense do anything at all for reliability?

Reliability is generally improved by "derating" but only up to a
limit. Derating means operating the capacitor at a voltage less than
its rating, ie, a 25 volt capacitor operated at 15 volts is derated
15/25 = 60%. In other words, it is being operated at 60% of its rated
voltage.

Capacitor voltage ratings can be derated 25% - 100% depending on the
type but going beyond that is a waste of money and space except in the
most demanding applications.

John

Are there any other considerations; for electrolytic caps?
What I'm getting at is that an electrolytic capacitor rated at say 16
microfarads at say 450 volts DC, if operated at a very low voltage, lets
assume, say 25 volts for sake of argument, may not have the stated capacity
of 16 mfd. because of the manner of the electrochemical action of the
capacitor.
Or am I off track with this idea? Note this ? is only in reference to
electrolytic caps.

I have operated 2 mfd. 'paper' caps rated for 50v DC at 350v DC (with
ripple) knowing that they were tested to 2000v static DC when manufactured!
Also have operated similar 0.5 mfd. cap at 115v RMS (60 HZ.) in a fan
circuit with only one failure in 30+ years.

TIA

On 29 Mar 2005 02:51:37 -0800, "Glynn R."
put finger to keyboard and composed:

Hey-- I've kinda wondered this too! In a circuit, I might choose a 16v
in a circuit I expect to have 5 volts running through. I have been told
I could put a 63v there and it works just the same. BUT, would it last
better? Would the extra expense do anything at all for reliability?

Looking closely at the legend on Bob Parker's ESR meter, a higher
voltage rating for a given capacitance equates to a lower ESR ... most
of the time. The 1uF and 2.2uF caps show the opposite trend, and the
100V and 250V caps also buck the trend on occasion. I've heard that
higher voltage electrolytic caps may also have higher leakage
currents.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

On Wed, 30 Mar 2005 16:51:04 +1000 Franc Zabkar
wrote:

I've heard that
higher voltage electrolytic caps may also have higher leakage
currents.

I suspect that's true only if you measure each cap at its rated
voltage. I believe that if you measure each cap at the same voltage,
while staying within both ratings, the higher voltage electrolytic
will show less leakage.

-
-----------------------------------------------
Jim Adney
Madison, WI 53711 USA
-----------------------------------------------

On 28 Mar 2005 19:29:43 -0800, "cnctut"
put finger to keyboard and composed:

I've never seen wattage ratings on a capacitor--voltage yes. Is it
simply .5CV^2 and that's it?

That's the energy stored by a capacitor. An ideal capacitor dissipates
no power, so the concept of wattage makes no sense.

I can design a circuit for a particular
capacitance--but how is the physical size of the Cap determined--I've
seen big capacitors with same MF's as little caps? What am I missing?

The size of a capacitor is determined at least partly by its voltage
rating. AFAIK, larger voltages require a greater distance between the
plates to avert dielectric breakdown. All other things being equal, to
maintain the same capacitance at this greater distance also requires a
larger plate area.

I believe the other important factor determining the size of a cap is
the type of dielectric used.

Thanks

Another important consideration for caps is their equivalent series
resistance (ESR) and their ripple current rating. The ripple current
causes ohmic heating of the ESR, which then causes the temperature of
the electrolyte to rise. AFAIK, every 10 degC rise in operating
temperature reduces the life of the cap by half.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.

"cnctut" wrote in message
oups.com...
I've never seen wattage ratings on a capacitor--voltage yes. Is it
simply .5CV^2 and that's it? I can design a circuit for a particular
capacitance--but how is the physical size of the Cap determined--I've
seen big capacitors with same MF's as little caps? What am I missing?

The size is determined by the capacitance, the voltage, the ripple current
rating and the technology used.

N