[pure-silver] Re: Speedotron 2401A


----- Original Message ----- From: "Ralph W. Lambrecht" <info@xxxxxxxxxxxxxxxx>
To: "PureSilverNew" <pure-silver@xxxxxxxxxxxxx>
Sent: Monday, October 02, 2006 2:47 AM
Subject: [pure-silver] Re: Speedotron 2401A


Jim

The Speedotron units do this as a matter of operation. When you turn the
unit off, it fires all lights. Speedotron suggests to turn the power to full
before turning it off to discharge the capacitors. Why do you think they
suggest that? Or did I misunderstand something?





Regards



Ralph W. Lambrecht

http://www.darkroomagic.com


Virtually any high voltage power supply will have a filter leak resistor across the capacitors to insure they do not stay charged when the thing is off. This is a safety factor.
Electrolytic capacitors have a curious construction: while the capacitor has two aluminum plates (Tantalums work the same way) the effective dielectric is a layer of Aluminum oxide formed on the positive plate. The effective spacing of the capacitor is the thickness of this layer. Because the capacitance of a capacitor is inversely proportional to the spacing the capacatance of electrolytics can be very high. The layer of electrolyte between the plates serves as the other plate and to heal or reform the oxide layer. The second Aluminum plate serves mainly as a contact to the electrolyte, it does not serve directly as the negative plate. Because reversing the polarity of the voltage across the capacitor will destroy the electrolyte these capacitors are polarized. Reverse voltage will cause them to heat and eventually explode.
The oxide layer can recombine eventually and must be re-formed. This is done by applying a slowly increasing voltage to the capacitor. Electrolytics can also dry out. Most have a safety valve to allow escape of the electrolyte in case of reversal of polarity without causing the capacitor to explode. These valves are often the way the electrolyte dries out.
BTW underrating electrolytics does not extend their lives, because of the constant reformation of the oxide layer a 600 volt capacitor used on 300 volts will eventualy become a 300 volt capacitor.
Electrolytics have the advantage of small size for their capacitance. The vices are high leakage (that is high series and parallel resistance) and poor dielectric characteristics. They are used for power supply filtering and for some types of energy storage. There are other, much better, capacitors, but they are usually much larger for a given capacitance.
Old practice for high voltage is to use oil-filled paper dielectric capacitors. These have very much lower leakage than electrolytics and are not sensitive to polarity, however, they are very bulky.
Capacitors used in strobe units have a special construction to withstand the constant cycling.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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