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

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?

N_Cook wrote:

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?


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

Jamie

Jamie t wrote in message
...
N_Cook wrote:

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?


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

Jamie




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 .

In article , "N_Cook"
wrote:

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?

I think the star washer used alone is not a good idea. True, it may dig
in and make a good electrical contact, but thermal cycling may cause it
to loosen. There should also (or instead of) be a split washer in the
stack, too.

Isaac

isw wrote in message
]...
In article , "N_Cook"
wrote:

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?

I think the star washer used alone is not a good idea. True, it may dig
in and make a good electrical contact, but thermal cycling may cause it
to loosen. There should also (or instead of) be a split washer in the
stack, too.

Isaac

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