How to calculate current to melt wire?

[Michael]

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

 

[Tom Bruhns]

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

Of course it depends a lot on how the wire gets rid of heat; if it's
in air and the air is still it will be considerably less current than
if there's high velocity cool air flowing over it. OTOH, the power
goes as the square of current--and in fact as something a bit higher
than that because the resistance of the wire goes up with temperature,
too--so the rated fusing current won't be too far off from what you'll
actually get. Google "fusing current" and you'll get quickly to
tables and papers--including one that addresses fusing current of PC
traces. Obviously, fusing current for a thin, wide rectangular
conductor of given cross-section will be different than for a round
wire of the same material in an otherwise similar environment.

Cheers,
Tom

 

[[email protected]]

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

It's an I-squared t hing I believe.
http://www.circuitprotection.ca/fuseology.html

 

[Phil Allison]

"Michael"

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?

** According to you, the pin did not melt.

What likely happened is that a DC arc formed between the pin and whatever
had been shorting it to ground - just as that short was broken. Heat
generated by the arc then melted through part of the pin.

The heat production of a DC arc is given by V times I - a much larger
value than I squared R, when R is only a fraction of a milliohm.



...... Phil

 

[Tim Williams]

Ballpark 100-200A. I recall discussion here some time ago concerning those
MOSFETs that claim something like 120A peak drain current.....

Tim

 

[Eeyore]

Additionally, is there a rough calculation for how to find
out how much current would actually melt a wire in open air?

In short, no.

Graham

 

[Rich Grise]

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.

You're shooting birds out of a cannon? With HE for propellant?

Wouldn't you want something more like cordite or ordinary BP, so
that you shoot whole birds rather than bird molecules? ;-)

(in college once, we blew up a hamster with an M-80; we found
one piece, and the rest was just gone.)

Would it be possible to get videos? ;-)

Thanks,
Rich

 

[John KD5YI]

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."

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

Cheers,
John

 

[John KD5YI]

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 | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

America: Land of the Free, Because of the Brave

Yes, Jim, I know birds are actually fired into aircraft windshields and
engines. The part about the British being so stupid as to not thaw the bird
first is the legend.

John

 

[neon]

open air ? when is wet it will never blow the current will be bypassed. my guess that pin has a bad joint or a pass to somewhere else from there. what goes out of one pin must have come in from another pin a bad joint will have resistance causing IR drop causing heat at the pin.