I recently replaced a couple of electrolytics in a flat screen TV for a cus tomer. The caps were in the power supply and were of course rated for 105 d egrees C. So this brought to mind a question. Could this possibly be an ope rating temperature? Or is it a storage temperature? Or perhaps it's an int ernal temperature? It would seem like it would have to be a very high frequ ency component to ever cause an electrolytic to ever approach anything like this. Could one of these parts rated as such actually get this hot and rem ain operational? Would this actually be within prudent design parameters fo r the device? In theory if the caps are not actually operating at even 85 d egrees C then why wouldn't you be able to use a lower rated temperature cap for that application?
It would seem to me that if a piece of equipment were designed to run a cap acitor that hot or even at 85 degrees C for whatever reason then in my mind that would certainly constitute a very poor design. I have been repairing TV's for many years and the only capacitors I've ever seen get too hot to touch were bad ones. Could someone please explain this rating to me? Thanks , Lenny
As the owner of an electronic assembly service, I can help you with this. It has nothing to do with design and everything to do with price and availability. The manufacturer may be using the cap in another product and got a good price for buying 50,000 of them. Or, the lower temperature cap may have had a long lead time, so, with engineering approval, the purchaser ordered these so the production line was not shut down.
We fight engineering all the time when they design a product with several different sizes of SMT resistors, or several capacitors of the same value, but different tolerances. This adds quite a bit to the manufacturing cost because each component has to be ordered and inventoried and used in a separate feeder on the pick-and-place machine.
When engineered for manufacturing efficiency, a single sized resistor could do the job for all circuits needing that value and wattage. A single capacitor with a low tolerance will work in all the other circuits and cost the same in quantity and need only a single item order, inventory and p-p feeder.
It is unlikely that a cap in a TV set will reach 105C and fail.
What is more likely is that it will reach 50C, especially if there is dust blocking the vents and an 105C capacitor has a much better chance of surviving operating at 50C than a capacitor rated 85C.
There may also be an assumption by the design department that some capacitor manufacturers lie, and an 105C capacitor is really an 85C or
50C capacitor.
If you are reading this it is likely that you would say "if they lie about the temperature rating, why would I buy from them?", while an engineer in China who has to buy from a specific vendor would just accept it and specify higher temp parts to compensate.
Or they just specify the capacitance and size of the cap and the manufacturer supplies what they make with the 105C rating having absolutely no significance at all.
Geoff.
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It's called 'Derating', or not operating a component right at it's limits. The more of a margin, the longer it lasts. That 85°C, 105°C or
125°C rating is how hot you can run it for its rated useful life. That can range from 500 to 25000+ hours, depending on what you want to spend. Input capacitors in the power supply have high ripple current, which generates heat. The more heat they have to deal with, the shorter their lives.
Digikey offers 74,859 different types & brands of electrolytics for a reason. :)
Pinching pennies reduces reliability, like the several years of crap computer motherboards that were built with substandard crapacitors.
No, No, and no. They're the absolute maximum operating temperature, at which the maximum safe applied voltage hits zero.
The 85/105C temperatures are actually the "knee" on the derating curve. Aluminum electrolytics are near the bottom. See Fig 13:
The temperature also has a big effect on the life of an electrolytic.
Wrong. A capacitor only draws current when the voltage across the leads changes. The capacitor only dissipated power, and converts it to heat, when the voltage changes. Pure DC across a capacitor does nothing to produce heat. The AC voltage change might be nothing more than a few millivolts of ripple in a switching power supply, but since the current in such supplies is huge, the heat dissipated is quite large. Ignoring frequency dependent effects, the power dissipated is: Power = Ripple_voltage^2 / ESR or Power = Ripple_current^2 * ESR where ESR = equivalent series resistance. For example, the CPU filter caps are typically 1000uF/6.3V electrolytics (0.12 ohms ESR). With a current probe, I can usually see at least 4A average ripple current on the CPU power leads. While trying to keep the voltage constant over such large current variations, each cap would smoke: P = 4^2 * 0.12 = 2 watts each. There are typically about 10 caps in the string, dissipating a total of 20 watts. That's gonna get hot.
Sure. I've burnt my fingers on electrolytics and the machine continues to run. I just replaced an ATX power supply on a server that was running 24x7. Every cap in the PS had the top blown out by boiling electrolyte. Yet, the only indication of trouble was that the speedtest.net performance was erratic. When I finally turned off the power to look inside, things cooled down. Then it wouldn't turn on again, so the problem was obvious.
No. However, there are many design parameters. The one that should be a concern is the expected lifetime. See the formula at the top of: One can easily calculate when the capacitor is going to being giving problems. If you're designed for a 1 year warranty and a 5 year target life, it makes no sense to use more expensive parts where the caps might last 30+ years, when you can save a few pennies making it blow up just after the warranty expires. (Hint: I always recap with better or higher voltage caps).
Look at Fig 13 in the URL I cited. At 85C, the maximum working voltage is zero.
Well, the math is easy enough. What's the highest ambient temperature you plan to operate the LCD monitor? Looking at various random spec sheets, it looks like 85C is the common maximum for commodity monitors. With an 85C capacitor, that means that you have absolutely no way to make this work, because you can't put any voltage across the cap. So, we go to 105C caps, which now allows you to self-heat the capacitors with: 105 - 85 = 20C rise in temperature. That should work, but only if you also take into consideration voltage deration and expected lifetime.
Generally true. You can take an IR thermometer, thermistor probe, thermocouple, or if you have money, an FLIR thermal imager, and measure the case temp. Note that the inside temperature is much hotter. That works, until you try it with CPU filter caps, which are heated via radiation by the CPU and by the fan via convection.
See Fig 13 again.
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Jeff Liebermann jeffl@cruzio.com
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They were NOT built with substandard capacitors. They were built with BAD capacitors. At the time a Japanese company famous for their capacatiors had a Chinese engineer who was studying their production methods.
Not trusting him, they allowed him to learn everything EXCEPT a preservative in the electrolyte.
He left the company, went back to Tiawan and helped start a capacitor company. This company offered significant discounts (I've heard 50%) over their Japanese competitors so all of the Tiawanese computer companies started buying from them.
Their products worked flawlessly for about 6 months and then started to leak and fail. By that time there were millions of computers in the field.
It took several years before the last of these capactitors were used in production, some companies made consumer goods with 90 day warranties and were willing to take their chances with capactiors that lasted about
6 months of constant use.
They also found their way into 2005 vintage Apple computers, so it must have been more pervasive than people thought.
I ran into a conflict in 2002, with a vendor who had supplied 14 computers that all failed at about the same time. I wanted them to come in over a weekend and replace them all, they wanted me to ship them one a week until they were all fixed. Since the vendor was a friend of the CEO's brother in law, you can guess who won.
Geoff.
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Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379
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Oops. I goofed. I was looking at storage temperatures. Maximum operating temp is about 40C. Might as well do it right. See:
Temp at 70% Temp at 40% Temp at 20% of rated voltage of rated voltage of rated voltage
105C rated 75C 85C 95C 85C rated 55C 65C 75C
An 85C rated cap, with 70% of the rated voltage applied, can run up to
65C with a self-heating rise of: 65C - 40C = 15C The relationship between watts dissipated internally and temp rise for electrolytics is different for each package. For small Al caps, I use about 5C/watt. So, this cap can dissipate internally: 15C / 5C/watt = 3 watts Now, if we replace that with a 105C rated cap, with 70% of the rated voltage applied, it can run up to 85C with a self heating rise of: 85C - 40C = 25C This cap can now burn internally: 25C / 5C/watt = 5 watts In short, the 105C cap can handle about 60% more ripple current for the same internal temperature rise and: (5 / 3)^0.5 = 1.29 or about 29% more ripple voltage than the 85C cap.
There's an example of the math here:
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Jeff Liebermann jeffl@cruzio.com
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They were still substandard, but for the all too well know industrial espionage. The companies stuffing the motherboards bought them because they were the cheapest crap they could find.
** Its the actual temp of the cap itself - so it depends on the ambient temp and any heat dissipation in the cap due to ripple current. The figure gives the maximum allowable temp for a rated life in thousands of hours.
See any electro data sheet for the details.
** The electros in many valve amp get very hot, especially if sited near the output valves. Marshall guitar amps are a classic example - I have measured the surface temp on the large can electros at 85C.
The ripple current in many SMPS is enough to heat electros significantly - that is why you see fans in them.
** Complete crap.
85C, 105C and 125C are max usable temperatures at the rated DC voltage.
** What about leakage ?
** What about leakage ??
2mA of leakage times 500 volts = 1 watt.
Dickhead radio ham.
** ESR is not a fixed number - it varies dramatically with temperature.
The ESR measured at room temp is typically 5 to 10 times higher than when the cap at say 80C. Check this out with any electro, an ESR meter and hot air any time you like - electrolytes become way more conductive when HOT.
This makes nonsense of your calculation.
** Complete crap.
All electros are speced for full voltage at max rated temp.
But at max temp, the rated life is typically only a few thousand hours - before the electrolyte vanishes.
Do you see where it says "absolute maximum rating"? Notice the recommended operating voltage at the absolute maximum rating point for all 3 caps. It's zero because it's not recommended running the capacitor at the absolute maximum rating. That also includes semiconductors, automobiles, and thermionic valves, all of which to disgusting things when operating at their absolute maximum rating.
What about it? At the typical computer and solid state LCD monitor voltages, its negligible. It might a problem in hi voltage caps, as in your tube stuff, solar power inverters, or in electric vehicles, but not low voltage computah and LCD panels.
Show me where I can find 500 volts in an LCD monitor or TV? Incidentally, todays monitors and TV's use LED backlighting, not CCFL tubes.
In your world, nothing happens without drama.
Ummm... yeah. See: Look at Fig 2 and: At 25C the ESR of their electrolytic is unity (at 100KHz). When the temp climbs to 85C, it's 0.3 or 0.4 (as best as I can read the graphs). That makes the room temp value about 3 or 4 times the ESR at
85C, not your 5 to 10 times value. The internal dissipation change between room temp and 85C is 1/3. My 2 watts of smoke becomes 0.67 watts.
The problem is that nobody runs electrolytics at 85C. Even with 40C maximum ambient for the monitor, I doubt if anything gets hotter than maybe 55C. It might be hotter in a computer, where the caps are wrapped around a hot CPU, but those are usually polymer caps, not electrolytics.
My guess(tm) is that designers take advantage of the drop in ESR, and run the ripple current even higher, thus negating any alleged benefits to running the caps hot.
The capacitance of an electrolytic increases about 5% from 25C to 85C, which would reduce the ripple voltage by about the same percentage. That also helps keep the cap cool, but the effect is not large compared to the change in ESR with temperature. (Hint: Always design for the worst case, which always seems to happen at inconvenient times).
Nothing is ever specified simultaneously at maximum voltage and maximum temperature. If you're ever tried to run a semiconductor at more than one of the maximum ratings at the same time, you will likely have a pile of smoking silicon. Same with an electrolytic capacitor. All that the max rating really mean is that you can possibly hit *ONE* of those ratings, and not destroy the part.
For fun, and when it cools down somewhat, I'll make some boiling water (for tea) and drop in an electrolytic while measuring the ESR with my Bob Parker ESR meter. It should be interesting to see if practice follows theory.
Yep. That's because the rated lifetime of a capacitor is specified at the rated maximum temperature. At 85C, you'll get about 2000 hrs. Drop the temp 10C, and the expected life will double.
You can get a fair idea of how it works if you ignore ripple current heating for now: Plugging in: Rated life = 2000 hrs Rated max voltage = 6.3 VDC Operating voltage = 5.0 VDC Max temp rating = 85 C Ambient = 40 C Projected Life => 57,000 hrs = 2,375 days = 6.5 years which is about what I'm seeing with commodity capacitors in ATX power supplies. If I add 10C to the ambient for self heating by ripple current (which was somehow left out of the calculator), I get: Projected Life => 28,511 hrs = 1,188 days = 3.3 years which is close to what I see with computers running in a burn-in room.
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Jeff Liebermann jeffl@cruzio.com
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Ok, a matter of semantics. When I think of substandard I think of someone selling parts that are not manufactured to spec, for example, a 50C (yes, I know no one sells them as such) cap labeled 85C.
Or a 33mf capacitor that is really 25mf.
These really were up to standards, they had the correct capacitance and were properly temperature rated. The failure was due to them being unable to age, which may be considered a manufacturing error, or a design flaw, planned obsolecence, or outright fraud.
I guess the standard they failed to perform to was MTBF, but was it specified?
Is there a standard for capacitors? Or is that something you compute based upon temperature rating, expected operating temperature etc, and there is no standard at all, beyond your calculations?
Geoff.
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Geoffrey S. Mendelson, N3OWJ/4X1GM/KBUH7245/KBUW5379
It's Spring here in Jerusalem!!!
Only a fool would buy caps without a rated lifetime. In most places, engineering has to approve components before they can be placed in the system. An 'Item Master' is generated, which has all the specifications, and should include a datasheet that was in effect at the time the part was evaluated. Any company that ignores the approval process is crooked, or incompetent.
The first step is being placed on the 'AVL', or 'Approved Vendors Lists'. Then each and every component has to be tested & approved. The testing will depend on the level the company builds to. I approved some Hitachi and Bourns parts to replace EOL Motorola & Beckman ten turn pots with an unacceptable failure rate of over 1% due to a manufacturing flaw caused by their using undersized 'O-rings'. The Bourns had a zero fail rate for the thousands we used, per year. I also found and added some 'Capstore' RAM to replace the lithium battery powered 2K * 8 we used. They were not allowed aboard the shuttle or space station, and early EEPROM was too flaky for the application. The maximum write cycles would have given too short of a useful lifetime and it's expensive to service something in orbit.
One of the most difficult parametrs to evaluate is life expectancy. It's trivial to test a lot of caps and determine if they meet capacity, ESR, and leakage specs. Note that even high end (Panasonic) caps have a rated life expectancy of less than 10,000 hours at rated temperature, surge, etc. That's less than 14 months. Calculting a probable life expectancy at the much lower temperature and surge they nrmally operate at would be difficult. Still, it is obvious that some brands (Capxon, Elite Lelon) consistently fail much earlier than others.
While Phil may actually know what he is talking about - the way he presents the information - not to mention the personal attacks, places him in the category and until his manners improve he should simply be ignored.
I don't read his posts, while I do read yours...
John :-#)#
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Before I got an ESR meter I used to see how hot the electrolytics got while it was running, the aluminium can isn't isolated so touching them can be dangerous - the first line check was to see which ones could melt the end of a glue stick.
I'd take it to be the maximum recomended operating temperature - exceeding that significantly reduces its life.
Apart from radiated heat from other components (like power resistors) the main cause of heating is power dissipated in the apparent internal series resistance; ESR, ripple current - especially high frequency in a SMPSU constantly alternates between charging and discharging the "ideal" capasitor, the current going in and out develops a voltage across the ESR - AxV=W.
It depends on the composition of the glue stick. About 80C is typical, although it can anywhere from 66C to 147C depending on composition.
Good test, but lousy accuracy unless you "calibrate" your glue stick. I just use an IR thermometer, or if I want better accuracy, a thermocouple or thermistor thermometer.
So, has anyone measured the temperature of the capacitors in a tube type amplifier?
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Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
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