The coveted 105deg C rated caps. Why is it 105 and not 100? - Page 1
Forum section: electronics / general

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: May 02, 2013, 05:28:17 am
content: Just wondering why 105 degree rated capacitors aren't instead simply 100 degrees. Nice round number, wouldn't you think?Why one oh five?

author: kizzap
author_rank: Unknown
timestamp: May 02, 2013, 06:53:01 am
content: so you can design to say 100 degrees and have 5 degrees of a safety margin?more likely design to say 50 degrees and have the FoS of greater then 2...-kizzap

author: Wytnucls
author_rank: Super Contributor
timestamp: May 02, 2013, 07:15:41 am
content: Why 65, 85, 95, 105, 125, 130, 135, 150?There doesn't seem to be any logic behind the temperature progression.I suspect temperature limits were selected by manufacturers according to their early product behavior and these eventually became the standards.

author: peterthenovice
author_rank: Regular Contributor
timestamp: May 02, 2013, 11:08:45 pm
content: why not make a 132.232428378273973/c rated cap

author: c4757p
author_rank: Super Contributor
timestamp: May 02, 2013, 11:30:18 pm
content: I don't really know, but can I take a guess? 85° was already established as a fairly sensible "higher than normal operating temperatures" rating, and 105° was chosen to be a multiple of 10 higher because of the "half life expectancy per 10°" rule of thumb.

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: May 03, 2013, 12:22:44 am
content: It has to do with room temperature being 25 degrees C and their capacitance change over temperature rating.http://electrochem.cwru.edu/encycl/misc/c04-appguide.pdf"Capacitance Temperature characteristicsThe capacitance varies with temperature. This variationitself is dependent to a small extent on the rated voltageand capacitor size. Capacitance increases less than 5%from 25 °C to the high-temperature limit. "Operating Temperature RangeThe Operating Temperature Range is the temperature rangeover which the part will function, when electrified, withinthe limits given in the specification. It is the range of ambient temperatures for which the capacitor has been designedto operate continuously. Largely the formation voltage setsthe high-temperature limit. Higher formation voltages permithigher operating temperatures but reduce the capacitance.The low-temperature limit is set largely by the cold resistivity of the electrolyte. The higher cold resistivity increases the capacitor’s ESR 10 to 100 fold and reduces the available capacitance.Typical temperature ranges are –20 °C to 55 °C, –25 °C to85 °C, –40 °C to 85 °C, –55 °C to 85 °C, –40 °C to 105 °C,–55 °C to 105 °C and –55 °C to 125 °Chttp://www.digikey.com/Web%20Export/Supplier%20Content/Elna_604/PDF/elna-reliability-of-aluminum-electrolytic-caps.pdf?redirected=1

author: amspire
author_rank: Super Contributor
timestamp: May 03, 2013, 12:29:57 am
content: My guess is that 105 degC comes from the maximum practical working temperature of the electrolyte used.The 85 deg C rating choice is obvious - that is a standard temperature rating for consumer grade electronics and so if you are using IC's rated to 85 deg C, it makes sense to use 85 deg C capacitors.The electrolyte used in 85 DegC electrolytics boils at between 120degC and 135 degC. The electrolyte used in 105 degC caps boils between 180 degC and 200 deg C. To get a reliable wet aluminum electrolytic up to 125degC operating temperature, you need an electrolyte with a boiling point of over 204 degC.Apparently above 105 degC, there are a lot of other issues relating to chemical stability and you need a much higher degree or purity in the aluminum foil and the electrolyte. It is possible to make 125 degC wet aluminum electrolytics, but it is not easy, and there has been active research in the last decade into this issue. The electrolytes in wet aluminium electrolytic capacitors consist of salts dissolved in a non-aqueous solvent and different capacitor companies have their own magic recipes. There is a big difference in conductivity and voltage rating of various electrolytes, so I am sure that the electrolytes available for a 85 degC cap can give much better capacitance per square cm then a 105 degC electrolyte.I hadn't really thought about it, but the electrolyte boiling point is dependent on pressure, and so to retain the temperature performance, the capacitor has to keep its internal pressure up. The maximum elevation you can use a wet electrolyte capacitor is up to about 80,000 feet and above that, the seals or the case may fail.Richard.

author: c4757p
author_rank: Super Contributor
timestamp: May 03, 2013, 12:31:43 am
content: Quote from: amspire on May 03, 2013, 12:29:57 amHuh. Never thought about that. Of course, I guess I considered 80,000 feet to be well above the "just whack any old component in there" range and almost into the "custom components made of unicorn farts and pixie dust" range...

author: nukie
author_rank: Frequent Contributor
timestamp: May 03, 2013, 12:57:36 am
content: The 80k feet elevation is new to me, very interesting.

author: amspire
author_rank: Super Contributor
timestamp: May 03, 2013, 02:27:15 am
content: Quote from: nukie on May 03, 2013, 12:57:36 amI saw that information on page 11 of this PDF from Cornell Dublier. It is a really good primer on electrolytic capacitors by a company that knows how to make great quality caps:http://www.digikey.com.au/Web%20Export/Supplier%20Content/CDE_338/PDF/CDE_AEappGuide.pdf?

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: May 03, 2013, 03:01:44 am
content: Knowing what to look for sure helps web searches doesn't it.