Electrolytic capacitors at low temperature - Page 1
Forum section: electronics / projects

author: Simon
author_rank: Unknown
timestamp: July 01, 2014, 09:05:37 am
content: I'm looking to design a circuit that needs to work down to -46C (oh how many times have I said that). So once I'm over hurdles of semiconductor devices that won't go under -40C my next concern is capacitors.I think technology has changed much recently for capacitors so I'm not sure if what I find online is still relevant. So far from what I gather aluminium electrolytics are infact the best performing for low temperatures and even ceramics may not be a good idea.Looking at parts on Farnell the datasheets seem to just say that they work from -55 to +105 with no information about characteristic changes over temperature.

author: Psi
author_rank: Super Contributor
timestamp: July 01, 2014, 09:09:23 am
content: Can you add a little heating element to keep the case internal temp above -20.That would make it a lot easier to find components.

author: wraper
author_rank: Unknown
timestamp: July 01, 2014, 09:29:35 am
content: Most noticeable is significant ESR increase with lowering the temperature.

author: Simon
author_rank: Unknown
timestamp: July 01, 2014, 09:30:10 am
content: I'm going to try and do that, what I am trying to understand at the moment is what my options are and what happens at extremes.

author: rob77
author_rank: Super Contributor
timestamp: July 01, 2014, 09:39:13 am
content: i would definitely go for a hermetically sealed (because of moisture) case with thermal insulation and a heating element inside. and as i'm a bit paranoid, i would hermetically seal the case at 20celsius in a room with dry air. or even more paranoid and install 2 valves on the case and flush it with nitrogen

author: Rerouter
author_rank: Super Contributor
timestamp: July 01, 2014, 09:49:36 am
content: went off and had a read for you, Vishay state that most of there -40 rated capacitors can be used all the way down to -65 (i would assume the electrolyte solidifies there) but that the ESR dramatically increases towards the low end, but as an added bonus the leakage current drops dramatically, also found the below link with a crude calculation for the ESR vs Temp,Furthur reading drags back up the old lifetime calcs, the lower temperature should mean the caps last multiple human lifetimes, but equally digs in that below some point between -20 and -150 the capacity of the cap will drop (to about 10% capacity at -150C for one example)http://www.edn.com/design/components-and-packaging/4327019/ESR-calculations-for-electrolytic-capacitors-at-lower-temperaturesthere is a perk with the ESR increase, that if the signal is AC, it will cause self heating which will decrease it a little (this actually implies capacitors have a crude form of load balancing :/)

author: lewis
author_rank: Frequent Contributor
timestamp: July 01, 2014, 10:25:49 am
content: I had a feeling you were going to ask about caps!Use OS-CON solid electrolytic caps. Normal electrolytics increase in ESR significantly below -20 or so, but OS-CONs remain relatively flat. There isn't much info out there, but I found this:http://www.element14.com/community/groups/panasonicindustria/blog/2014/01/23/technical-advantages-of-panasonic-polymer-capacitors-1-stability-over-temperature-changesSearch Farnell for OS-CON.Capacitance reduces too at low temperatures, by as much as 40% at -40C. If you do need to use a wet electrolytic (maybe you need a high capacitance or high voltage), parallel two of them to ensure you get the capacitance you expect.One thing to watch out for with os-cons is the low ESR! It can be too low - they can cause instability in some circuits. Treat them like an MLCC, and test test test.Or, use X7R ceramics.

author: Psi
author_rank: Super Contributor
timestamp: July 01, 2014, 10:31:16 am
content: +1 for OS-CONThey make some awesome caps.

author: Simon
author_rank: Unknown
timestamp: July 02, 2014, 07:51:18 am
content: I'll go for os-con caps, looks like digi-key is now my friend, farnell have 2 only and they are being "run out of stock", I have a similar problem for atmel chips, farnell don't even include temperature range in the parametric search which is pants and high temperature grade models of chips are also harder to come by.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 02, 2014, 09:45:34 am
content: Polymer (not just os-cons, everyone makes them)Tantalum (assuming current is limited so as to avoid China Syndrome)There are also Ta-polys available, don't know much about them. Presumably they don't burn up as dramatically as others.Most ceramics, I think, freeze out at low temperatures?Tim

author: Simon
author_rank: Unknown
timestamp: July 02, 2014, 09:53:53 am
content: ok, well farnell don't have all that I want anyway so I'll have to get some stuff from digikey or even mouser, both outstrip both RS and Farnell for range and stock availability and lead times.

author: David Hess
author_rank: Super Contributor
timestamp: July 02, 2014, 12:47:08 pm
content: In the past when working at low temperatures my solution has been to use solid tantalum capacitors but now I would consider solid polymer electrolytics as well.

author: SeanB
author_rank: Super Contributor
timestamp: July 02, 2014, 06:40:50 pm
content: Use wet slug tantalum in a glass seal package. They are rated for the full aerospace temp range, and will work well at -55C. Otherwise polyester caps for low values, and watch out for ceramics that are very bad at low temperature.