Capacitor Wattage Capability

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?

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 liste by the manufacture

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Miketew

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

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

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

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

N

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.

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.

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----------------------------------------------- Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA

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