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

Yeah, "forming" caps... how important is this process, and isn't some
"forming" done for us at the factory?

I know it's whining, but: IMO electrolytics are almost always a weak
link in equipment. If you were to build a device you wanted to last a
hundred years, you could probably do it with the resistors, diodes,
transistors, I.C.s, inductors, crystals etc. our industry uses all the
time. But you have to worry about electrolytics-- they just "go bad".
This, to me, is a sorry shame. I realize I have no data to substantiate
my claim... its just my personal observation. Has anyone got data to
prove/disprove that electrolytics are as good as the rest of the
component chain?

Ripple current also has a bearing on size too and is sometimes listed by the manufacturer

On Tue, 29 Mar 2005 12:59:05 -0330, "Terry"
wrote:


"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.

No, the capacitance is not a function of applied voltage.


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.

That is not the result I would expect.

On 29 Mar 2005 09:36:54 -0800, "Glynn R."
wrote:

Yeah, "forming" caps... how important is this process, and isn't some
"forming" done for us at the factory?

I know it's whining, but: IMO electrolytics are almost always a weak
link in equipment. If you were to build a device you wanted to last a
hundred years, you could probably do it with the resistors, diodes,
transistors, I.C.s, inductors, crystals etc. our industry uses all the
time. But you have to worry about electrolytics-- they just "go bad".
This, to me, is a sorry shame. I realize I have no data to substantiate
my claim... its just my personal observation. Has anyone got data to
prove/disprove that electrolytics are as good as the rest of the
component chain?

Older electrolytics were certainly unreliable components. That is no
longer the case assuming good electrical and thermal design. Poor
thermal design is responsible for the failure of many electrolytics as
elevated temperatures significantly degrade their reliability.

Designers must adhere to all of the limitations specified by the
manufacturer to achieve a solid reliable design.

John

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.

"Glynn R." bravely wrote to "All" (29 Mar 05 09:36:54)
--- on the heady topic of " Capacitor Wattage Capability"

GR From: "Glynn R."
GR Xref: aeinews sci.electronics.repair:44431

GR Yeah, "forming" caps... how important is this process, and isn't some
GR "forming" done for us at the factory?

GR I know it's whining, but: IMO electrolytics are almost always a weak
GR link in equipment. If you were to build a device you wanted to last a
GR hundred years, you could probably do it with the resistors, diodes,
GR transistors, I.C.s, inductors, crystals etc. our industry uses all
GR the time. But you have to worry about electrolytics-- they just "go
GR bad". This, to me, is a sorry shame. I realize I have no data to
GR substantiate my claim... its just my personal observation. Has anyone
GR got data to prove/disprove that electrolytics are as good as the rest
GR of the component chain?

If the electrolytic capacitor is built and used in a manner for which
it was designed, then it will last many decades. Circumstantial proof
is from operating equipment I own which dates back 40 years or more.
As long as the seal remains intact the electrolytic capacitor is
self-healing once the dielectric layer has been formed. Indeed, every
time power is applied to the equipment an electrolytic capacitor
undergoes this self-healing process. The worst possible use one can
put an electrolytic capacitor through is with zero volts applied.
Thus using an electrolytic capacitor at a much lower than its design
voltage is accordingly something to avoid. Naturally high heat is
another bad environmental ingredient to shorten life expectancy.
The capacitance varies only very slightly with voltage.

A*s*i*m*o*v

.... Bad Beer Rots Our Young Guts But Vodka Goes Well.

In article ,
John Bachman wrote:

On Tue, 29 Mar 2005 12:59:05 -0330, "Terry"
wrote:


"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.

No, the capacitance is not a function of applied voltage.

For aluminum electrolytics, it most certainly is. Operating such a cap
well *below* rated voltage can cause a loss of capacitance over time.

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.

That is not the result I would expect.

For a capacitor, the "voltage rating" is the voltage the cap is
*guaranteed* to withstand, continuously, over the entire temperature
range specified, and for the entire design lifetime of the device.
Naturally, in order to accomplish that, *almost all* of the devices
manufactured will actually be able to handle stress well in excess of
the "rated" value, at more moderate temperatures, and/or for shorter
times.

But designers apply components beyond their ratings at their peril --
that may be the way to a courtroom.

Isaac

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.

"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

Terry wrote:

"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


normally , there i a Temp rating on the cap.
calculate the ESR (effective Series R) and thus calculate
the wattage verses current.. to get your heating factor.
for those that don't denote the temp, i don't know.

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
-----------------------------------------------