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Author Topic: Electrolytic Capacitor Temp. Ratings  (Read 8900 times)

Posts: 40

« on: January 26, 2012, 12:05:21 PM »

Eating lunch today with a coworker and fellow ham we got onto the subject of an amplifier I recently built.  I explained what I used for the capacitor bank on the HV and why I chose certain values of caps etc etc.  My fellow ham then asked a question I had never been posed and he asked what the temp rating of the caps were (these were electrolytics in series).

I had to stop for a moment and think and I couldn't answer. Honestly I never care a bit, I look at things like terminals, size, voltage and uf... but never temp ratings. Upon looking at pics of the supply it turned out I had used 105c which was to his approval. He told me never to use 50c as they would surely die a violent death and he went on to explain B+ ripple and duty cycle and dielectric strength... blah blah...

Anyone ever been confronted with this question? Capacitors suitability in an amplifier's HV supply based around the temp?
I don't think it will ever be 50c inside the supply.

Posts: 12469

« Reply #1 on: January 26, 2012, 01:56:20 PM »

Well the caps themselves will generate a little heat so it would not have to be 50c in chassis for them to exceed 50c. It could be 10 to 15C cooler in PS. Also though remember 50c is not the point they will fail, only what they are rated at minimum. It could be a lot higher.

Ham since 1969....  Old School 20wpm REAL Extra Class..

Posts: 2721

« Reply #2 on: January 26, 2012, 01:57:24 PM »

I can't say I have ever seen a 50 degree C electrolytic capacitor rated for HV PS use in the 450v range.  The "typical" degree rating on these are either 85 or 105 degree C.  Your friend may have been talking about 85 degree rating, not 50.  Indeed, strickly speaking 50 degree is too low.  Many commercial amps use 85 degree rated caps and seem to be just fine.  105 degree caps add a bit more margin but many companies do not use them.

Posts: 531

« Reply #3 on: January 26, 2012, 06:30:34 PM »

One thing to remember about those temperature ratings is that electrolytic caps are specified for a rated life at a certain temperature and rated ripple current (Usually just 2,000 Hrs), the service life rises RAPIDLY as the ratio of capacitor temperature to rated temperature falls, so a 105 degree part will have a much longer service life then a 85 degree part, even if both are in a 50 degree C environment.

Nobody (with any sense) designs to run electrolytic caps at anything like rated temperature because the life at that point is so short.

73, Dan.

Posts: 4969

« Reply #4 on: January 26, 2012, 07:39:21 PM »

The cheapest ones I have seen are 85 C. I have never seen 50 C. I would use 105 C in a tube amp.

Posts: 212

« Reply #5 on: January 27, 2012, 07:02:17 AM »

The cheapest ones I have seen are 85 C. I have never seen 50 C. I would use 105 C in a tube amp.

##  IF the  ambient air temp surrounding the cap  is say 20-25 deg C...AND  ripple current is low.... the cap will last a LOOOONG time.   Ripple current is just  dc plate current  x 2.56  per all the cap maker's  tech notes.   they also have detailed maths on their tech notes for commercial applications.  Like motor drives, power supplies etc.   Some of the example used are... caps must last at least 14 years... running full bore  24/7.  Ripple must be no more than XXX %.  Expected ambient temp is XXX  degs C.   they take it through the  various choices... and a lot of times.... to get long life... it's wiser to use a cap with a higher ripple current rating... cause THAT'S  what's  gonna heat em up inside. 

##  beware, the 105 deg C rated ones also have a higher internal temp..and they allow for higher spot temps inside the 105 deg C caps.   Those caps used inside the compact lights sold these days ?  They go dead cuz the 105 deg C cap is cooked in em.   The latest version  uses a 125 deg C rated cap.  The heat inside the base can't get out..esp when plugged into the ceiling..all the heat rises..and is trapped inside the base.     They are  not vented either.

###  Just a bit of air from a small ,low cfm fan in any amp power supply does wonders.  So does NOT running them at their max V ratings.  For commercial applications, caps are run at 75% of their ratings.  For aerospace, it's 50%.   Get a cap with a BIG  ripple current rating. They will last the longest.   A lot of linear amps had caps go bad.  They lacked enough bypass caps..and RF will kill em very fast.   A  1000 pf bypass cap at base of a plate choke is not good enough.  You need  2-3  4700pf caps.   u need 2-3 more of em inside the HV supply as well.  RF can easily jump past the plate choke..via the tank coils..and end up on the HV lead that feeds the base of the plate choke.

Later... Jim  VE7RF

Posts: 7718

« Reply #6 on: January 28, 2012, 06:29:27 AM »

The temperature rating of an electrolytic capacitor is the temperature that a specific capacitor life is valid for. And life is defined as a certain degradation in capacitor performance, such as increase in ESR and decrease in capacitance.

For example, a capacitor spec might state a 2000 hour life at 105 deg C for a 20% increase in ESR. As a rule-of-thumb for every 10 deg C drop in temperature the life doubles. 95 deg gives 4000 hours, 85 deg gives 8000 hours, 75 deg gives 16000 hours, 65 deg gives 32000 hours, 55 deg  gives 64000 hours, and 45 deg gives 128000 hours or 15 years.

Now, if the application will operate well with a greater degradation in ESR effective life is extended even further. For amateur HV power supplies, given the amount of operation a typical ham will give it, a 105 deg C cap should outlast the ham.

The wearout mechanism for aluminum electrolytic capacitors is evaporation of electrolyte through the end seals; that is where the leads exit the capacitor.


Posts: 846

« Reply #7 on: January 28, 2012, 12:51:58 PM »

Ok then, I always thought (there I go thinking again) that the temp rating was the temp that the dielectric was rated at and exceeding that would cause a meltdown. Like wire they have ratings of 85C and 105C also and if you exceed the rating of either one the dielectric will melt resulting in a short or worse a fire. Take boats for instance if you use 85C wire the Coast Guard will pull any license or permit if any is found on your boat (commercial & charter boats) and they look the reason is that engine rooms can at times reach close to 85C. But that's just what I thought it was the dielectric melting point.

Roland AH6RR
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