Well, in practice, 80% sounds about right. Not for any reason than that's how most people seem to pick 'em. I think the temperature rule is aimed more at the higher temperatures. Above I talked about falling temperatures. That was more about equipment running in a cold environment, or powering things up right off the UPS truck in the winter. Not a good thing to do in a marginal design case.
IF they offer no warranty, they could just as well not derate as suggested by the cap manufacturer and let their device fail relatively sooner than later.
Not a good way to make repeat customers, that is for sure.
This life-temperature dependence actually impacts how you should derate the voltage on the capacitor.
In practice, aluminum electrolytic capacitors typically are used at about 80% of their rated voltage.
I don't believe either of these statements are true..
You are certainly entitled to an opinion. I would suggest you take it up with Cornell Dubilier and ask them to change their capacitor usage guide.
from https://www.cde.com/resources/technical-papers/AEappGuide.pdf
Estimating Lifetime for Capacitors without an Online Calculator
We offer online calculators for many of our capacitor series such as our screw-terminal, snapmount and flatpack capacitors. For our capacitor series without a calculator, you will find on its datasheet a “load life rating” with an ambient test temperature and duration at the rated ripple current, which is tabulated for each capacitor within that series. To estimate the minimum lifetime, you may select a capacitor whose tabulated ripple current is at least equal to your application’s ripple current, and apply the “doubles every 10 °C” rule between the load life test ambient temperature and your application’s ambient temperature. If your DC voltage is derated, then you may multiply the lifetime by the voltage multiplier Mv given in equation (6). This will be an estimate of the minimum expected life
...
Operating Lifetime Model
Onset of wear-out is determined mainly by the capacitor’s average operating temperature. Operating voltage has some effect. For capacitors operating at moderate temperatures the operating life doubles for each 10 °C that operating temperature is reduced. Our online lifetime calculators available at http://www.cde.com/technical-support/life-temperature-calculators our best estimates of the useful lifetime for many of our more popular capacitor series. The expected operating lifetime is approximately
Lop = Mv × Lb × 2((Tm – Tc)/10[°C]) (5)
The above equation is the simple, classic “doubles every 10 °C” rule used for many decades, and Mv is a DC voltage deratingmultiplier equal to
Mv = 4.3-3.3 VA / VR (6)
Thus for example when the DC voltage is derated by 10%, this multiplier is equal to Mv = 1.33. As in the failure rate equation discussed in the previous section, VA is the applied DC voltage,
VR is the rated DC voltage, Tc is the core temperature, Tm is the maximum rated core temperature and Lb is the base lifetime.
Well, in practice, 80% sounds about right. Not for any reason than that's how most people seem to pick 'em. I think the temperature rule is aimed more at the higher temperatures. Above I talked about falling temperatures. That was more about equipment running in a cold environment, or powering things up right off the UPS truck in the winter. Not a good thing to do in a marginal design case.
The US Navy requires derating Aluminum electrolytic caps by 70% as of 2007.
https://www.navsea.navy.mil/Home/Warfare-Centers/NSWC-Crane/Resources/SD-18/Products/Capacitors/Derating/
You are certainly entitled to an opinion. I would suggest you take it up with Cornell Dubilier and ask them to change their capacitor usage guide.
Sadly, I don't have a good source of information off-hand. Shelf-life aging and low-ripple aging under DC bias are much, much less discussed in appnotes, because expected lifetime will normally be very long, and manufacturers don't want to give guarantees that long.
I apologize if you take offence to my statements.
Perhaps some meaning is lost in translation. I am not a scientist that researches and designs capacitors. I freely admit I may be wrong. I like so many other people only follow what we have been instructed by manufacturers.
I stated my understanding of the general rules of derating capacitors used by a number of sources and was told that it was not to be believed. No reason given.
I then quoted 'A' manufacturers directives on derating. If that is bad, I am again sorry.
I work in a field where things change. Rules that were true before change. The internet exacerbates this problem by retaining information that is used for refence effectively forever. When old material on the internet has wrong information, it makes people like me think and say the wrong things.
If I am not permitted to use references to illustrate my understanding of general principals of implementation without being accused of "an argument from authority" because I cited my sources, I don't know what to say.
Where a capacitor is used at lower than the rated voltage, the lifetime may not be adversely affected, which means that the effect of the applying voltage is negligibly small, while the effect of the ambient temperature and heat generation due to ripple current is significant.
However, for capacitors of larger size and higher rated voltage contain a larger volume of electrolyte, difference in applying voltages can affect degradation of the oxide layer, other than the diffusion of electrolyte. Therefore, for screw mount terminal type capacitors with the rated voltage of 350Vdc or higher, the lifetime estimation includes the effect of applying a lower voltage than the rated voltage (derating voltage).
There is a cool presentation that I need to dig out somewhere, sorry for the blunt "I don't believe that's true" without further explanation. But the gist of it is, most manufacturers' aging calculators don't include the voltage stress in the equation whatsoever, it's not even a variable. That tells us that the voltage stress on a elcap is at the very least a minor stressor, if not totally negligible compared to thermal stress.
OP's application is close to "DC biased shelf life" case, though. Voltage and ambient temperature are the two factors (ripple current isn't since it's near zero; and core temperature simply becomes equal to ambient temperature).
OP's application is close to "DC biased shelf life" case, though. Voltage and ambient temperature are the two factors (ripple current isn't since it's near zero; and core temperature simply becomes equal to ambient temperature).
We're assuming this, but without confirmation. Suppose whatever is going on with their device results in an occasional 15V spike from inductive kick of some sort?
We're assuming this, but without confirmation. Suppose whatever is going on with their device results in an occasional 15V spike from inductive kick of some sort?
It's misleading because it might lead people into believing that reducing DC bias will extend lifetime, which isn't true in 99% of cases in the field, but reducing the temps absolutely, and dramatically would.
It's misleading because it might lead people into believing that reducing DC bias will extend lifetime, which isn't true in 99% of cases in the field, but reducing the temps absolutely, and dramatically would.
There is going to be a relationship between lifetime and both DC bias and temperature, but it isn't going to be a simple one that applies at all values. For example, I don't think lowering the temperature from 1000C to 970C will increase the capacitor life 8X, nor will raising it from -25C to -15C reduce it by half. It's all very complex and all the rules are just gross oversimplifications that make it possible to make some decisions.
I would point out that failure rate and lifetime, while related to an extent, can also be looked at as different issues. Derating addresses more than simple models, it also addresses manufacturing variances in both the component and the device in which it is installed, as well as abuse, user error and other random or unknown issues.
Long ago, in the '60s, Sprague was very free with technical data, unlike today where everything is a state secret. Technology has changed, though the general trends are probably still true. I have several of the old Sprague papers on my site, all of which are worth reading, but you might look at page 16 of this one- http://www.conradhoffman.com/papers_lib/Sprague_62_7.pdf One of the other papers talks about running caps at low voltages and says there's no issue right down to zero volts, or at least not enough to worry about.
Long ago, in the '60s, Sprague was very free with technical data, unlike today where everything is a state secret. Technology has changed, though the general trends are probably still true. I have several of the old Sprague papers on my site, all of which are worth reading, but you might look at page 16 of this one- http://www.conradhoffman.com/papers_lib/Sprague_62_7.pdf One of the other papers talks about running caps at low voltages and says there's no issue right down to zero volts, or at least not enough to worry about.
I was taught the adage that real engineers do the math, psychics and contractors make guesses. Rules of thumb are fine for making estimates, but they are no substitute for the math.
If a bridge has over engineered specs nobody notices, if it is under engineered people could die.
I was taught the adage that real engineers do the math, psychics and contractors make guesses. Rules of thumb are fine for making estimates, but they are no substitute for the math.
Well math doesn't do you much good unless you have accurate data and models. A manufacturers specification doesn't give you the underlying information and the way those specs are determined can vary. At least a well-considered rule of thumb can add in some empirical data--the wisdom of experience.QuoteIf a bridge has over engineered specs nobody notices, if it is under engineered people could die.
Whether the bridge collapses or not is a matter of the actual materials and construction used, not their listed specifications. Headline or advertised specifications are especially unhelpful and misleading in many cases. I suppose modern expectations would be that specs would be exaggerated best-case maximums, but not always. How much weight can a 1-ton pickup truck haul?