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 « on: June 24, 2012, 04:05:36 am » Bigger Smaller Reset

Hi,

This might be a VERY stupid question, but I was wondering about capacitor voltage, specifically supercapacitors. Here's what got me started.

I am designing a circuit that involves an RTC chip (BQ32000) with a built in supercap trickle-charging feature. The chip itself runs off 3.3v, but requires a minimum of 2.0v on Vbackup in order to be able to switch over.

I have a few 5.5v supercaps ranging from 0.22F to 1F. The example circuit on the datasheet calls for a .22F capacitor, but doesn't specify a voltage, so I wanted to know if it's OK to use the supercap I have for the circuit even though it is 5.5V.

As far as I understand it, the voltage rating of a standard capacitor is its maximum charged voltage, but the actual voltage across a charged cap is determined by the voltage charging it. Is this correct, and does it work the same way for supercapacitors?

Thanks
 « Last Edit: June 24, 2012, 04:17:59 am by boredat20 » Logged

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 « Reply #1 on: June 24, 2012, 05:07:17 am » Bigger Smaller Reset

You are right with normal capacitors.  The voltage rating is the voltage which, if you exceed it, they run the risk of exploding and filling the room with a horrible acrid smell, and showering the place with electrolyte (note: you have to exceed it by quite a lot to make this happen, but it is fun when you do - normally they just vent and stop working right).

Supercaps work just the same way, but I would imagine the bang might be bigger - maybe I should try it some time

You can operate at any voltage up to the maximum, and that is the voltage they give you. (n.b.: that is my personal interpretation of the Wikipedia page on supercaps.)
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 « Reply #2 on: June 24, 2012, 05:15:54 am » Bigger Smaller Reset

Got it. Well, I'm not anticipating any capacitor explosions but I can't say I'm not curious now...

Thanks for clearing that up for me!
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 « Reply #3 on: June 24, 2012, 05:37:54 am » Bigger Smaller Reset

Your 5V5 max SuperCap will work fine @ 3V3 and for longer than it would @ 5V.
Electrolytic's exploding is never so much fun...  as when the Da*n thing goes off right next to your ear because you grabbed the wrong part and put that 5 V cap "By" accident (oversight) om the 12V supply line and you are close to the board sniffing for the smelly part... They do stink just before they explode ans that is usually when the boss comes onto the room and smells it too.
The thing most people forget is the boss might have been there, "done that" before and is more concerned with how you handle the problem as why the problem occurred. This the mark of a good engineer, the ability to take responsibility for the design and make it work or rework it until it works according to spec under all circumstances.
Sometimes the spec or circuitry must be changed and when that happens all the above still holds true.

Doc
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 « Reply #4 on: June 24, 2012, 05:44:02 am » Bigger Smaller Reset

When I was a kid a mate and I made a short film using his dad's VHS video camera of blowing up model trains using large electrolytic capacitors connected backwards to large voltages

It was fun...
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 « Reply #5 on: June 24, 2012, 04:00:18 pm » Bigger Smaller Reset

Most impressive brag... I used to get surplus (back in the late 50's early 60's, 1559 - 1960) 100000 uF (.1F) caps free and I didn't bother connecting them backwards, I used a 100 foot/33 meter mains power extension cord and a 500W projector lamp as a ballast resistor... I had a DPST switch salvaged form an old projector and that worked very well... Never had a dud, would blow a nice hole in the ground and there was aluminum foil and junk all over the place.

Doc
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 « Reply #6 on: June 24, 2012, 05:01:49 pm » Bigger Smaller Reset

I must have led a sheltered life........
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 « Reply #7 on: June 24, 2012, 06:08:53 pm » Bigger Smaller Reset

Oh Yeah, I forgot... most of my neighbors hated me, I had a 500 W Rotary Spark Transmitter in my garage... Got hunted down by 3 different "Ham Radio Operators" before my stepdad did for my transmitter... with a BIG hammer... made lots of small useless stuff and the worst thing was that not only did I have to clean up the mess, I had to make another spark gap transmitter I found out that you can't connect  a lot of neon sign transformers in serial... over about 5 they blow up... the insulation breaks down and REALLY makes a mess...

Doc
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 « Reply #8 on: June 24, 2012, 06:19:53 pm » Bigger Smaller Reset

When I was a kid a mate and I made a short film using his dad's VHS video camera of blowing up model trains using large electrolytic capacitors connected backwards to large voltages

It was fun...

Submitted without comment: http://youtu.be/EoWMF3VkI6U
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 « Reply #9 on: June 24, 2012, 07:35:54 pm » Bigger Smaller Reset

Yeah we can tell... I never stopped experimenting and wound up an engineer... You?

Doc

BTW, I am an old retired man trying to share 45 years of work in the Electronics Business...
The last 20 as the design engineer for a small company in Costa Mesa Ca.
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 « Reply #10 on: June 25, 2012, 09:40:05 am » Bigger Smaller Reset

Supercapacitors are unusual in all having a low voltage specification - usually capacitors can be made higher-voltage by increasing the thickness of the insulating layer - you get higher voltage tolerance (but lower capacitance for the same volume).

In super capacitors the "insulating layer" is conceptually one molecule thick (its more complex than this really) and so can't be made thicker...  There's a limit to the voltage across a molecule before it electrolyses (gets chemically split).  For larger voltages supercapacitors have to be stacked in series and then you need some way to balance out the voltages (as the capacities will vary somewhat from device to device).  Perhaps the simplest scheme is a zener diode for each individual capacitor to prevent over-voltage?

I like to think of super capacitors as somewhere between a normal capacitor and a rechargeable battery - they have aspects of both.
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 « Reply #11 on: June 25, 2012, 01:43:57 pm » Bigger Smaller Reset