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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: Re: black gates in parallel posted by Robert Karl Stonjek on December 23, 2001 at 12:05:17:
Robert S. Of course YOU are talking about ripple current. I have mentioned to you previously how to measure it exactly, and not approximately. When you or Steve throw around numbers of 100 amps or so, it can only happen during the turn on surge. However, you have brought forth an 'abnormal' example of a cap with a very low ripple current rating, that could get into trouble.
This got me thinking about it, but until now, I have never had a problem with a cap overheating from ripple current alone, in my decades of amp design experience. I looked through my Cornell Dubilier cap catalog, and found out a few things. First, the current rating of the cap is primarily dependent on the heat dissipation ability of the cap itself. As the current increases, the residual ESR creates heat, and it has to be dissipated. Normally this is not a problem, because most caps in power amps are larger in voltage rating than 25V and are therefore physically larger. There are also special caps designed for very high dissipation, if it is necessary. Some even have heatsinks.
Normal power amps of the class A/B variety have relatively low quiescent current, compared to the rating of the transformer, so the ripple current developed in the cap to replace the current drain from the amplifier is usually reasonable. However, if you were making a flashlamp driver or an arc welder, the drain from the supply would be much greater and could overheat a marginal cap. A class A amp with high quiescent current (4+ A ) like the original Mark Levinson ML2, and with 25V caps could present a problem.
Of course, there would also be a great deal of voltage ripple with only a 10,000uF cap, and usually one would increase the cap value and this would increase the current ripple rating of the cap.
So, without being very specific, I wouldn't worry too much about cap overheating from just putting it into a typical power supply.
For the record, your thermistor is probably the same as a 'Surgistor' which is probably a trade name.
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Follow Ups
- Re: black gates in parallel - john curl 12:56:05 12/23/01 (0)
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