??? 08/14/04 00:15 Read: times |
#75904 - I love electrolytics! Responding to: ???'s previous message |
Well, if the ESR was so high such that the capacitor would heat up as a result, then it wouldn't be a very good capacitor, now, would it?
It's a well known fact, that an electrolytic capacitor can enormeously heat up, when relevant AC currents flow. That's the price you have to pay, when using electrolytics. But: There's no other capacitor technology providing so much capacitance per volume, and this at such a low price! Also, there are enough applications where there's absolutely no alternative to the use of an electrolytic capacitor! You have to live with the well known disadvantages, whether you like it or not. Or, from a different angle, in order to get enough current through that ESR such that heating actually takes place, you'd very likely exceed (by a wide margin) the capacitor's working voltage. It's the AC current which makes heat up the electrolytic! In a properly designed circuit overheating of electrolytic as result of exceeding of rated voltage isn't an issue, anyway. If an electrolytic capacitor must be used, then ambient temperature of application is the key factor. If ambient temperature is below 40°C and no relevant AC current is flowing, then electrolytic rated for +85°C can be used. But if ambient temperature is much higher, as observed in TVs, monitors, tube amplifiers, etc. electrolytic capacitor should be rated for 105°C ambient temperature, even if no relevant AC current is flowing. If this additionally is the case, then electrolytic capacitor should be rated for even higher ambient temperatures! I know this may sound ridiculous to some people. But have a look at datasheet of a typical electrolytic capacitor and you will find, that for a standard type service life is only about 1000 hours at 85°C!!! So, when using the wrong electrolytic, this can result in total desaster after rather short period of time... The most relevant failure mode of an electrolytic capacitor is the evaporation of electrolytic material via lead through openings of terminals (electrodes). An electrolytic capacitor is sealed by a gasket made of special rubber or synthetic material. Some decades ago, only poor materials were available which quickly became hard and brittle at higher temperatures. As consequence the sealing became more and more leaky very quickly, electrolytic material evaporated and capacitance significantly decreased. When you put an electrolytic capacitor out of a tv after let's say ten years, a tv which was prone to become overheated because of bad design, you will very probably notice a tremendeous decrease of capacitance. I have observed this many many times. But the failing of electrolytic capacitor then is not an argument against the use of electrolytics, but only an argument to choose a properly rated one for this task! If your electrolytic capacitor is sitting in a hot tv, then choose a type that is rated for +105°C. An electrolytic capacitor which is rated for higher ambient temperatures than +85°C has better sealings and better electrolytic material. But this costs a bit, of course. Unfortunately, in very many applications electrolytic capacitors are wrongly choosen, regarding to rated ambient temperature. Even the most expensivest tube amplifiers, which tend to become abnormeously hot inside come with cheap standard electrolytics, which are 'programmed' to fail after some years. It seems to be a 'well proven' design practice to choose the poorest and cheapest electrolytics ever avbailable on market... About the effect of coating of housing on cooling: A thin coating which sits directly on housing of electrolytic, means without providing an air gap, does not relevantly worsen cooling properties. If coating is thin enough, cooling performance mainly depends on area and colour, but not so much on used material. Situation would look different, if you have the strange idea to mount a heat sink onto housing of electrolytic. Then, metallic surface would be advantageous... Keep in mind, that in many cases overheating of capacitor is not caused by self heating, for instance caused by relevant AC currents, but by components in close neighbourhood, which dissipate heat. Means, where just the ambient temperature is too high. The only remedy in such a case would be to reduce ambient temperature somehow, or to choose an electrolytic which accepts higher ambient temperatures. Remove of coating wouldn't help then, anyway! Finally, I want to agree with Steve: In my applications, where I always choose adequate electrolytics, even if they are a bit more expensive from time to time, I have never observed any failed electrolytic capacitor. Even 20 year old capacitors do their job without any problem. Never had to remove one. But keep in mind, that this means, that in certain situations rather expensive SIKOREL types had to be choosen. But in a professional environment costs of parts do not necessarily play a role, if the costs of an eventual failure are taken into consideration! What does it cost, when a factory stands still for some hours??? Kai |