> The bleeder resistor network is (or now, was) composed of four
> pieces, 100K ohms, 50 watt vitreous enamel OHMITE L50J100K
> less than 12 months old, less than 1 hour operation. The resistors
> were megged with a DMM before assembly into the plant.
>
> When examined today, one piece was infinite resistance.
> A second piece is now 29.2 meg. The third resistor is 110K,
> and the fourth, get this: 53.2 ohms.
I use 100 watt resistors in my more robust power supplies, 4 @ 50k
ohms.
1. More dissipation area
2. Wound with larger wire
3. The form (body) thickness is thicker and a larger diameter
4. I never mount a spring clip at chassis ground potential; I mount
the bleeders on a plastic board
>
> It sounds to me like there was something of a domino effect.
> However, at the actual HV level of 3804 volts, is nine and
> one-half mils. This is thirty six watts. Fifty watt resistors
> should have handled it, especially mounted with spring clips
> in an airflow. If one resistor shorted to 53.2 ohms, the
> network then was then 12.6 mils. This would have brought
> the dissipation to 48.2 watts. There's a hefty airflow, remember,
> so that may or may not have been the trigger for the
> other devices to fail.
>
> Comments???
Sounds to me like many turns shorted together on the 53.2 ohm unit,
and big-time overload on the other three.
This could have been caused by an arc from a resistor body to the
spring clip.
Been there, done that, got the "T" Shirt!!
(((73)))
Phil, K5PC
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|