On Sun, 2007-06-17 at 21:18 -0400, Randall Nortman wrote: > How do aluminum caps fail -- open or closed? Does it buy me anything > to have redundancy -- several different capacitors, maybe some > aluminum and some tantalum? I will have a low-ESR ceramic or two, but > it will be relatively small, meant to deal with the high frequency > switching of the regulator, not the 120Hz ripple.
Due to aging / high temperature, they go high resistance. The equivalent series resistance of the caps will climb until they basically can't support the ripple current demanded from them. They get hotter, more electrolyte dies / evaporates etc.. Symptoms include excessive ripple leaking through the supply, destabilised control loops which can oscillate, and possibly even blow up the switching elements. > Yes, and that's why I'm going with a switcher, but I have noticed that > looking at the efficiency curves, as a general rule you will do better > with a lower differential between input and output. The effect is > much more subtle than what you get with linear regulators; > appropriately enough, the efficiency of those is pretty much linear > with the dropout voltage. In theory, the efficiency of an ideal > switcher would not depend on dropout voltage, I understand. If > anybody happens to have a supplier for an ideal switcher, please > forward me the datasheet. I believe you can get them from Unobtainium Inc. I forget the web address. They can also supply you with suitable capacitors ;). If you have a 3-Phase supply by any chance, the capacitance you need is less. Just a thought, but if its going in domestic attics, then I guess you're on 1-Phase. At the working temperature you mention, cooling the semiconductors will be a real pain - good luck with the designs! -- Peter Clifton Electrical Engineering Division, Engineering Department, University of Cambridge, 9, JJ Thomson Avenue, Cambridge CB3 0FA Tel: +44 (0)7729 980173 - (No signal in the lab!) _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
