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TO3 mounting query

Discussion in 'Electronic Repair' started by N_Cook, Nov 28, 2011.

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  1. N_Cook

    N_Cook Guest

    In typing this I thought of a reason for it but will continue for any one
    else's input. I've never seen it before but seems quite sensible now I have
    a likely treason.
    5 amp rated TO3 mounted through heatsink and pcb. One C terminal just bolt
    and nut , no electrical connection. The other is bolt , solder tag, star
    washer and nut and the solder tag fully swathed in solder ,bolt hole to
    wiring hole , merging with thick pcb trace, but only requiring to pass 5amp.

    I can see that star washer could punch through the thin copper on the pcb so
    this method avoids that potential problem, and would constitute good
    practice assembly
  2. N_Cook

    N_Cook Guest

    Curiouser and curioser. There is a black "washer" between tag and pcb pad.
    Undoing a bolt this was a standard black nylon TO3 mounting insulated washer
    but on the underside. So instead of bolt head, insulator washer, TO3 flange,
    mica, heatsink, pcb it is bolt head, flange , mica, heatsink, pcb, pcb and
    trace, TO3 insulator upside down, solder tag, star washer, nut. Flange is
    same thickness as pcb and nylon insulator is only long enough to half
    penetrate the pcb so what advantage ? or cock-up?
  3. Jamie

    Jamie Guest

    Some times if you were mating with the board copper, they used different
    types of material to prevent erosion.

  4. N_Cook

    N_Cook Guest

    But an insulator ? I reckon cockup , myself included. I should have said
    plain cylinder insulator set solely in the heatsink hole rather than necked
    washer under the head, or there would be no electrical connection to the TO3
    flange .
  5. N_Cook

    N_Cook Guest

    ISTM the main problem with this arrangement of bolting through pcb is
    compression of the polyester/glass of the pcb. Another amp in yesterday had
    a nut bouncing around inside causing mayhem. Just a 3mm nut with star washer
    for holding pcb via standoff spacer to chassis. Or was, until pcb
    compression and vibration. Personally ,in non hot areas, I prefer star
    washer and nut with nail varnish , or a nylock nut. I'm not convinced that
    the radial edges of spring washers is enough to stop vibration induced
    undoing , combined with plastic pcb material
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