35V Electrolytic cap on 5V rail (SMPS output)

[Flyback]

Hi,
Ive just been "told off" for using a 35v electrolytic cap on a 5V rail......output of 24 TO 5v, 200mA Buck converter.
Surely this is OK?
Said El caps dont "form" till get at least 20% of their rated voltage on them.......i can find no such corroboration on the internet, after a wide search.

 

[For The Popcorn]

Surely you have access to a variable power supply, and can get a similar cap to the one you plan to use? A few minutes of experimenting should prove or disprove the theory?

 

[Flyback]

Sorry, my live in landlord wont allow any electronics activity...not even soldering......i can do the test when i go to present to the customer in a few weeks time....but i should have it all finished for then...at least, a basically working prototype.

 

[For The Popcorn]

I guess that includes using alligator clips and reading a voltmeter!

That's certainly a place most members of this forum would never live.

 

[Flyback]

The reference about el caps at low voltage is in the babani book "ic 555 projects" page 11.
this was first published in 1978., but reprinted 2006.

..Babani says Electrolytic caps of too high voltage rating cannot be used as the timing cap in a 555 timer, because they are "not capacitors when operated too far below their rated voltage"

..and the Babani book range was class.....still is....there must be something in this.......web search on the subject just reveals shedloads of conversations...with nobody really knowing about it.

Its definite that El caps dont last as long when operated well below rated voltage.

 

[AnalogKid]

True, but they also do not last long when operated at or near their rated voltage. The consensus rule of thumb among a dozen MIL companies I danced with back in the day is that an aluminum electrolytic should be rated for a hard minimum of 2x above and a soft maximum of 4x above the peak voltage it is exposed to in a circuit. That's the voltage across the cap itself, not the circuit's operating voltage(s).

ak

 

[Flyback]

Thanks....its interesting, because virtually every boost PFC stage runs to a 390VDC output....and they all use 450V rated caps. Its also been said that 'lytics are like batteries, kind of, and like to have their full rated voltage on them all the time.
Indeed needing leakage current correction when they havent been run at rated volts for a while.

Ceram and Tant fully agree...derate the volts

 

[danadak]

https://chemi-con.com/wp-content/uploads/2021/04/Precautions-and-Guidelines-for-Aluminum-Electrolytic-Capacitors.pdf

https://www.coilgun.info/theorycapacitors/AEappGUIDE.pdf

Aluminum Electrolytic | Capacitors | Vishay

Aluminum Electrolytic, Capacitors manufactured by Vishay, a global leader for semiconductors and passive electronic components.

www.vishay.com

https://www.cde.com/resources/technical-papers/AEappGuide.pdf

The above might be helpful.


Regards, Dana.

 

[AnalogKid]

virtually every boost PFC stage runs to a 390VDC output....and they all use 450V rated caps.

Guilty, but to get the same energy storage margined for 800 V takes *4* of those fat boys. We did it so often that I whipped up a standard disclaimer form for our customers to sign.

Life is choice.

ak

 

[Flyback]

Thanks Danadak, i must admit ive perused those docs over the years, but they dont tell what is the maximum voltage rating allowable for eg a cap on a 5V rail etc.
I guess these manufacturers dont have to spec it, so why bother.......just when misers like me have to buy ten 50V caps and so want to use some on a 5v rail in a prototype....instead of me having to buy say a 6v3 el cap on top of the ten 50v ones i already had to buy.

I once worked at a LED place, and the extremely experienced electronics manager there used to say that a elec'ic capacitor had to have at least 10% (and prefereably more) of its rated voltage on it to properly be a capacitor of anywhere near of the stated capacitance.

Along with that, its common knowledge that you cant use lytics with 555 timer circuits to get long delays.,....this is seconded by Babani as above.
....____
Incidentally, i used to work in a cap discharger place...for xenon flash lamps....we used to charge 615uF from say 60v to 340V in one second then flash........the caps werent much over 400V rated if at all........i certainly dont remember them being well voltage de-rated........i cant remember now, but you end up getting either 10% more, or 10% less (i cant remember which, but i think its more charge in there than you'd think......almost like you got something [an extra 10%] for nothing.) charge in the capacitor than the calculation tells you for such discharge duty.....we monitored the current going in and did the integral with time to get charge gone in...........the boss thought i did the calcs wrong.....so we both did them and we got the same....then we found an explanation of the phenomina on the web......i posted about it on a forum at the time...about end 2016, early 2017, but cant find the posts now.

 

[danadak]

I use ATTINY85 to get delays, triggers, intelligence. Easy to spin a program,
I use block programming mostly with these, even though I have done many
languages since 70's, including C. Its quick and fast for simple stuff. The timing
is much more accurate, unlike a 555 dinosaur. +/- 1% with simple user cal
procedure.

An example of a flex timer application -

https://www.electro-tech-online.com/articles/pulse-sequence-generator-smart-timer.938/

Here is an example of that it took to make a V to PW converter -

mBlock, converts your blocks to Arduino code, and programs the board/part.



Regards, Dana.