Re: How to calculate current to melt wire?

On Sat, 24 May 2008 13:46:52 GMT, "John KD5YI"
<groups1_dot_jocjo@xxxxxxxx> wrote:

"Paul Hovnanian P.E." <paul@xxxxxxxxxxxxx> wrote in message
news:483764C3.28C3B102@xxxxxxxxxxxxxxxx
John Larkin wrote:

On Thu, 22 May 2008 20:52:36 -0500, "Mook Johnson" <mook@xxxxxxxx>
wrote:


"Michael" <nleahcim@xxxxxxxxx> wrote in message
news:12285dfc-2e08-4c96-ac0f-fe0b3c212aa8@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hi there - I found an interesting failure on a PCB today. A large part
of one of the leads of a TO-220 was simply gone. That pin was carrying
a fairly high voltage and got shorted to ground.

Does anybody know how much current it would take to blow a TO-220 pin
like this? Additionally, is there a rough calculation for how to find
out how much current would actually melt a wire in open air?

Thanks!

-Michael

Its not just the DC current but the instintanious current that can fuse
a
wirelink. A surprisingly small high voltage capacitance discharged
across a
link of wire can blow it as well.


http://www.teledynerisi.com/products/0products_2fs_page51.asp

This uses a 1 uF cap to fire "non-primary" (ie, damned hard to fire)
explosives like

http://www.teledynerisi.com/products/0products_1ebw_page27.asp

This is a "bridge wire" detonator, just an exploding wire buried in
HE.

One of my customers is planning to use these gadgets to launch 16
starlings, in 4 groups of 4, at a couple hundred millisecond
intervals, into a running jet engine.

John

Reminds me of the story about the 'chicken cannon' used to test
airframes. Bird carcasses were loaded into this pneumatic cannon and
fired at cockpit windows, wing leading edges, etc. to certify them for
bird strike survival.

A British airframe manufacturer was provided detailed plans by a US
company to build and operate said cannon to test their assemblies. Upon
firing their first chicken into a known sound cockpit window, they were
horrified when the window was shattered and surrounding fuselage bent.
They sent detailed test procedures, results, photographs, etc. back to
the American company, seeking advice.

The Americans responded with a one sentence suggestion: "Thaw the
chicken."

--
Paul Hovnanian mailto:Paul@xxxxxxxxxxxxx
------------------------------------------------------------------
I have a truly marvellous sig which this margin is too narrow to
contain.


Mostly urban legend, according to: http://www.snopes.com/science/cannon.asp

Cheers,
John

Except it's not. I contracted (*) at Goodyear Aerospace...

http://en.wikipedia.org/wiki/Goodyear_Aerospace

in the early 70's.

At that time they fired LIVE chickens at aircraft windshields.

Of course ASPCA got all hot and bothered when they heard about it. So
we then had to use dressed processed (dead) chickens. These WERE
stored frozen, and thawed just prior to loading into the "cannon".

It's conceivable that some failed to thaw before firing ;-)

IIRC, I believe the windshield material was LEXAN. I put some of it
into the gaslight at my old acreage, because the juvenile heathens
kept breaking out the regular glass.

(*) Hybrid circuits for side-looking radar.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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