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From: Winfield Hill
Subject: Re: 12kV 8kW metal-evaporation power supply
Date: 17 Nov 2002 09:23:54 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.10
> Winfield Hill wrote ...
>> A friend calls up and says his 12kV 8kW metal-evaporation power
>> supply is on the skids. Seems the designers used six 250k 50W
>> power resistors in series for two tasks: HV bleeder and the
>> voltage-feedback servo path. A resistors goes, so the HV supply
>> soars and becomes erratic. It's sad the decisions that engineers
>> sometimes make. I have a nice foot-long 60 meg-ohm glass >25kV
>> resistor that looks ideal for the task. Not to mention some nice
>> 3-inch long parts. I also suggested an interlock set to operate
>> if inadequate current is present in the bleeder chain. BTW, the
>> company has since gone out of business.
> I was going to suggest that the 1.5Meg chain would be better
> distributed amongst a large optimised mesh of series/parr lower
> wattage resistors (making the assumption that 1.5Meg was a
> maximum value for safety discharge purposes.
> Then I saw the posters name!.
> What the hell. In for a penny, in for a pound ;-)
Hey, I didn't design it, I simply reported on it. :)
The 50W resistors are apparently classic Ohmite molded Vitreous
Enamel types with the hollow center. Although each 50W resistor
only dissipates 19 watts, the designer's assumption that they can
each handle 2kV seems faulty. Breaking the chain up into 18 100k
or 36 50k resistors, etc., would certainly be a good idea.
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