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Unread 15-11-2007, 07:55
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Al Skierkiewicz Al Skierkiewicz is offline
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AKA: Big Al WFFA 2005
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Re: Silicon Controlled Rectifier

Mitch,
I am just getting around to rereading some of this post and your other question about bleeder reisistors. Bleeders are most often used to "bleed" off the charge on a high voltage cap when the power is removed. High quality and computer grade caps can store lethal voltages for quite a while. The bleeder then slowly discharges the cap to prevent these voltages from causing other problems in your design or hurting someone. The value is chosen so that the cap is discharged in a reasonable length of time but does not provide too great a load on the power supply. My old ham power supply, HP23B, uses 2 100k 2 watt resistors across a 125 microfarad cap in the 850 volt supply. In that configuration the resistors also help stabilize the power supply by providing a permanent load. Since the supply is used in the transmitter, load current doesn't reeally appear until you transmit without the resistors. (BTW the 850V supply is a voltage doubler)
In one of your schematics, there is a resistor in series with the cap. That resistor limits the "inrush" current to the cap. A capacitor that is discharged appears as if it is shorted when voltage is first applied. By using a series resistor you limit the current to a point that will not blow the primary fuse but still charge the cap. The digital TV transmitter I work on uses a bank of such resistors on a three phase high voltage power supply. There is a timed delay relay that shorts out the resistors after the caps in supply are partially charged. This transmitter uses a 35,000 volt power supply at 2 amps and the two main filter caps (in parallel) are 15" x 6" x 30" each and live inside an explosion proof enclosure with the transformer and rectifiers.
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Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
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Storming the Tower since 1996.