[Amps] FYI: Steel Screws

[wc6w at juno.com]

  This observation may be of some use to anyone in the group working with high current.

  I recently retreived a Y2K vintage Astron RS-50A Power Supply (13.5V @ 50A) from the time closet and I thought I'd check the insides for spiders and such before I powered it up.

  There was only one spider web but, I noticed that the screws on the main (100,000uF) electrolytic capacitor were severly corroded, i.e. rusted.  I had to use a pair of channel locks to unscrew one of them. 

  These (10-32) screws pass through two large wires (on both the positive & negative terminals) then the trace side of the control circuit board (which is also supported by means of these screws) and finally into the capacitor termimals.

  Upon closer examination, the insulation of the wires at the crimp terminals was heat damaged, the plastic encased the postive terminal of the filter cap. was destroyed and the (94V-0) circuit board was well toasted adjacent to the screw holes.  Some portion of the volatiles involved had boiled off and were deposited on the inside of the cover an inch above.  

  Apparently, the ripple current running through these steel screws was sufficient to cause all of this high temperature damage.  Note: The supply uses 1/4" diameter brass screws (with steel nuts...) for output terminals.

  I made a couple of small (.062 thick) aluminum strips to connect the circuit board about an inch away from its original position and reconnected the capacitor with the wire crimp terminals directly on top of the capacitor terminals so, the ripple current no longer needs to pass entirely through the screws.

  A final note, just guessing from the holes in the circuit board, it appears that it may be employed in other (lower current) models where the wires and capacitor are terminated directly to the PCB. 

73 & Good evening,
  Marv WC6W









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