Aluminium Electrolytic Capacitor in low temperature

Discussion in 'General Electronics Chat' started by bug13, Sep 10, 2013.

  1. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    Hi guys

    Does someone has experienced on Aluminium Electrolytic Capacitors in low temperature, like -20 degree C or so.

    I have tried three random caps in my junk box, here is what I got (measure at 100Hz):
    Room temperature:
    220uF - ESR 0.7 ohm
    330uF - ESR 0.5 ohm
    1000uF - ESR 0.4 ohm
    After two hours in my fridge at about -18 degree C:
    220uF - ESR 1.4 ohm
    330uF - ESR 0.7 ohm
    1000uF - ESR 1 ohm

    Capacitance doesn't drop much.

    Is there anything else I should worried about at low temperature?
    Is my simple test seasonal correct? or Am I missing something important here?

    Thanks guys
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,151
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    From the wiki:

    " Additionally, low temperature is a problem for most aluminum capacitors: for most types, capacitance falls off rapidly below room temperature while dissipation factor can be ten times higher at −25 °C than at 25 °C. Most limitations can be traced to the electrolyte. At high temperature, the water can be lost to evaporation, and the capacitor (especially the small sizes) may leak outright. At low temperatures, the conductance of the salts declines, raising the ESR, and the increase in the electrolyte's surface tension can cause reduced contact with the dielectric. The conductance of electrolytes generally has a very high temperature coefficient, +2%/°C is typical, depending on size. The electrolyte, particularly if degraded, is implicated in various reliability issues as well."

    In other words, look for ones specifically designed and rated to operate at your temperatures, if you want a reliable design.
     
  3. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    I read that part too, but my simple test doesn't look as bad as it sounds, have I done something wrong in my test?

    I mean the capacitance is still within specs 10%-20%, the ESR is Ok, more like double compares in room temperature.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Yes, and if that is your limit low temp, perhaps you will get away with that brand of capacitor. But it's hard to know how close you might be to a cliff. What if the electrolyte freezes at 2°C below your test temperature?

    If you're launching a satellite, you'd do more research. If it is for a device you need to keep you alive, ditto. If it's not life or death, maybe you can take some prudent risk. It's up to you.

    But I think a little more digging might get you the data you need.
     
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  5. The Electrician

    AAC Fanatic!

    Oct 9, 2007
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    Have a look at this:

    http://www.cde.com/catalogs/AEappGUIDE.pdf

    On page 7, you'll find this:

    The capacitance varies with temperature. This variation itself is
    dependent to a small extent on the rated voltage and capacitor
    size. Capacitance increases less than 5% from 25 ºC to the high-
    temperature limit. For devices rated –40 ºC capacitance declines
    up to 20% at –40 ºC for low-voltage units and up to 40% for high-
    voltage units. Most of the decline is between –20 ºC and –40 ºC.
    For devices rated –55 ºC capacitance typically declines less than
    10% at –40 ºC and less than 20% at –55 ºC.

    And this:

    The dissipation factor decreases with increasing temperature.
    DF declines about 50% from 25 ºC to the high-temperature limit,
    but increases more than 10 fold at the low-temperature limit.
    The DF of the better devices rated –55 ºC increases less than 5
    times at –40 ºC.
     
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  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The ESR goes up exponentially at colder temps. It looks like a 1/f curve if you know what I mean.
     
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  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    The only feasible elec caps for cold temps are Tantalums. That's what we had to use on MIL designs.
     
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  8. richard.cs

    Member

    Mar 3, 2012
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    In a power application capacitors with high ESR at low temperatures will self-heat to a nice equlibrium point. So long as the circuit can tolerate the higher ESR (higher but not as high as might have been expected) stuff just generally works.

    For a real solution to the problem high-capacitance ceramics are getting very good now.
     
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  9. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    You mean MIL as military designs? So you work for military? that's cool!!
     
  10. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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  11. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I didn't work for the military, our power supply design house did some designs that had to meet MIL standards. The units were used in missiles or aircraft so they had to be made to the highest reliability standards.

    That's where the big $$$$$ is.
     
  12. bug13

    Thread Starter Well-Known Member

    Feb 13, 2012
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    lol, sorry, what I really want to say is you have design/involved in the design of MIL specs stuff, that's very cool! :D
     
  13. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Sounds a lot cooler than it was......

    doing a MIL standard design means you are severely restricted on what components you can use, you have to follow the MIL standard spec, they require supervised testing, and there is a ton of documentation that has to be generated. The only cool part was the $$$ my bosses put in their pocket, the crappy part was ll the work..... and I was the flunky doing it all.

    I actually hated doing MIL designs worse than root canal.
     
  14. wayneh

    Expert

    Sep 9, 2010
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    My brother once worked on insulation foam that ultimately rode on the space shuttle. Sounds cool, but what a huge pain it was. I think management often salivates at the prices they can get for such stuff, but never fully account for the increased costs of dealing with such a project. They don't have to do the work, therefore they rarely appreciate how difficult it can be.
     
  15. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    People would be really angry if they knew what a rip off it is: our company contracted to deliver ten small power supplies to the China Lake weapons research facility that powered HARM missiles under test, and we charged $2000 each for them..... not that much, but ou company soaked them about $500,000 for the "documentation" and development costs. When it's taxpayer money, they just write the check.....



    Funny. MIL contracts automatically "update" to the latest revision of the spec. One of our contracts called out for the units to be fungus tested, the method was called out in MIL STD 810C. So, the next time they ordered some more, the new standard was MIL STD 810D. Slight change? Not exactly. The new fungus testing was so complicated that the only lab in the world certified to do it was a US military chemical warfare lab....... and they had a backlog of several years work and were charging about $300,000 for doing it (probably 10X what we were being paid to do the job).

    But that was Joe Varozza..... dangle the money and it's all he could see. Then find out later what a mess he had gotten us into.
     
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