Unless the current travels through the sides of adjacent coils of the spring
by metal to metal contact when it is compressed the linear length of the
spring and thus the electrical resistance ( and heating and ampacity, etc.)
is the same until the spring is stretched to its in-elastic point.

Ray Klein

-----Original Message-----
From: Jack C. Olson [mailto:[log in to unmask]]
Sent: Tuesday, October 07, 2003 9:57 AM
To: [log in to unmask]
Subject: [TN] Max Current Brain Teaser


Does anyone out there enjoy brain teasers?
I was asked an interesting question today.

We need to have current traveling across a spring.
The spring connects a connector pin to a SMT pad.
We don't care how hot the spring gets as long as
it doesn't melt the board.
When the spring is compressed it is only 20 mils
away from the board, but the current may have to
travel around the circumference, so the distance
may be more like 200 mils.
The spring material is berrillium copper and the
thickness is equivalent to AWG26. So even though
a wire of this thickness may be rated 1.7 amps:

What is the maximum current we can push
through this short distance continously?

We have been discussing the fact that even though
the wire in your house may be rated for 10 amps
and it won't get hot even on a 200 foot run, how
much current can you drive through a half-inch
piece? Especially if you don't care if it gets hot?
Nothing I found on Doug Brook's site (temp calc
and fusing current study) seemed to help.
( http://www.ultracad.com )

thanks,
Jack

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