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Nico Ras 29th October 2008 06:32 PM

[QUOTE]Originally posted by PMA
Jan, 2000 hours is at maximum operating temperature, i.e. +85C or +105C, according to type used. At some 40-50C, the cap life time is much much longer. [/QUOTE

Hi PMA, you are correct. The term is MTBF (mean-time between failure) and this is a very well researched subject. Both practical and theoretical methods are used in industry, the latter being the least expensive.

The stressing of components are most severe under switch on/switch off conditions than under continuous on or off, components even age while sitting on a shelf. The manufacturing of electronic components and systems in modern factories has excelled tremendously in the past decades to the point that even the typical household television set has an MTBF of several hundred thousand hours.

gain 29th October 2008 08:53 PM

some of you all's are forgetting your basic electronic theory who are talking about capacitors and temperature and failure times.

capacitors do not produce heat. voltage and current are 90 degrees out of phase, thus no power is being produced in the device

therefore, caps operate at ambient temp the whole time. sorry as much as i hate correcting people some needed to be reminded of this fact about capacitive reactance.

PMA 29th October 2008 09:18 PM

It is usually pretty hot inside class A amplifier, it can be roughly 50 - 60 C.

gain 29th October 2008 09:35 PM

Quote:

Originally posted by PMA
It is usually pretty hot inside class A amplifier, it can be roughly 50 - 60 C.

then it is coming from some external source. heatsinks perhaps if class A? trust me or not, but nature wont allow a capacitor to generate any amount of heat under any circumstances. this isn't to say you might have a corrosion on a cap terminal that is creating resistance. now that WILL heat up the cap, but again, the key is the source of the heat is external.


Quote:

Nico Ras
The stressing of components are most severe under switch on/switch off conditions than under continuous on or off

so does this mean that continuous operation extends MTBF? thanks very much for your reply btw.

AndrewT 29th October 2008 09:42 PM

Hi Gain,
capacitors do generate internal heat.
It comes from the ripple current and the ESR.
This heat is the reason for the limitation on ripple current and if ignored will destroy capacitors.

PMA 29th October 2008 09:42 PM

It is not important (external or internal).

I can see you (gain) do not know much about caps. Imagine polyester foil cap, with max allowed dv/dt. In case you exceed dv/dt for longer period of time, the cap self heats (dielectric losses) and may explode. This happened to me when I made longterm test of power amp into dummy load by 10kHz square - the foil polyester cap overheated and exploded.

AndrewT 29th October 2008 09:45 PM

I think PMA's example of failure is due to ripple current heating.
Does anyone agree?
Was that an output Zobel capacitor?

PMA 29th October 2008 09:50 PM

It was due to losses in dielectric, high peak current

Ic = C x dv/dt

and high dv/dt (slope, derivative of voltage)
You must not exceed maximum alowed rate of voltage change.

Huh, I should better disappear, do not want to get involved in such debate.

gain 29th October 2008 10:12 PM

Quote:

Originally posted by PMA

I can see you (gain) do not know much about caps. Imagine polyester foil cap, with max allowed dv/dt. In case you exceed dv/dt for longer period of time, the cap self heats (dielectric losses) and may explode. This happened to me when I made longterm test of power amp into dummy load by 10kHz square - the foil polyester cap overheated and exploded.


this is not a valid counterpoint imho. you were operating the cap way outside of its specified parameters. of course if you hook up a 50V cap to a 500V rail its going to destroy itself.

if you don't believe me, which obviously you don't, just do the math yourself. take cap equation

q = CV = CVsin(wt)

take derivative with respect to time

dq/dt = I = CVwcos(wt)

so V = Vsin(wt) and I = CVwcos(wt)

hence they are 90 degrees out of phase!!! hence no power (heat) can be produced within a cap!


Quote:

Originally posted by PMA
Huh, I should better disappear, do not want to get involved in such debate.

me neither :)

Nico Ras 29th October 2008 10:58 PM

Quote:

Originally posted by gain


so does this mean that continuous operation extends MTBF? thanks very much for your reply btw.

MTBF extends when the system is not powered:o However, MTBF worsens when at turn on turn of conditions. There is a simple light bulb test, leave it off and it will last for an infinite period, zero stress just fatigue. Turn in on and leave it, it will run for its predicted life MTBF. Turn it on and off for a thousand times and the life is reduces exponentially.

MTBF reduces by the number of components likely to fail in a system and would normally boil down to the first most likely component failing.


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