nobody@Kodak.COM (Barry Ornitz) (03/07/90)
In article <2701@uwm.edu> #SBDORS%WMMVS.BITNET@VTVM1.CC.VT.EDU writes: > The secret is capacitance. Lots of it. With good quality >electrolytics that have very low series resistance. Photoflash >caps, or barring that, switching supply caps. Photoflash capacitors are rated for moderately high current peaks that occur infrequently. Their equivalent series resistance is usually quite high compared to power supply electrolytics. About the only thing going for them is their size - and perhaps price. Switching supply capacitors are rated for high ripple current and have very low ESR's. They are generally available only in smaller capacitance sizes [no one farad caps]. A good choice would be to parallel a few ultra-low inductance switchmode capacitors with several high value computer grade electrolytics. NEVER use photoflash capacitors as input filter capacitors in power supplies; they cannot take the ripple current. >High current regulators will indeed help the caps charge up faster, >so the higher supply current you can get, the better. But spend >your money on capacitors and then get the regulation... Motorola >makes some nice TO-3 packaged regulators that you can probably get >free samples of. But watch out for the fact that many regulator circuits are unstable with large capacitive loads. Also many three terminal regulators need special protection against reverse polarity with capacitive loads. Most data sheets dicuss how protective diodes should be added. One possibility to avoid instability in the regulator circuit is to use small inductors to isolate the large capacitive loads from the regulator. Now the problem is finding high current chokes! While on the subject, excessive input filter capacitance leads to very high ratios of peak to average current in the power transformer and rectifiers. The high peak currents, occuring in very short pulses, can lead to hash and buzz magnetically coupling into low-level stages. A properly sized inductor input filter will bring the peak to average current ratio way down. Remember that transformers DO saturate. >Does anyone out there still use ballast tubes and gas regulators? Yes, in precision laboratory power supplies. In some cases, I have kept the 6SA7/6080 pass tubes but added solid state regulators to drive the grids. Why not use high voltage MOSFET's? Well, tubes are more forgiving! ;-) Barry ----------------- | ___ ________ | Dr. Barry L. Ornitz WA4VZQ | | / / | | UUCP: ....rutgers!rochester!kodak!ornitz | | / / | | Eastman Kodak Company | |< < K O D A K| | Eastman Chemical Company Research Laboratories | | \ \ | | P. O. Box 1972, Building 167B | |__\ \________| | Kingsport, TN 37662 615/229-4904 | | INTERNET: ornitz@kodak.com -----------------