# How to calculate current to melt wire?

• posted

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

• posted

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

• posted

It's an I-squared t hing I believe.

• posted

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.

• posted

"Michael"

** 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

• posted

Here are some numbers:

Best regards, Spehro Pefhany

```--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com```
• posted

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

Tim

-- Deep Fryer: A very philosophical monk. Website @

• posted

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

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

• posted

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

• posted

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@Hovnanian.com
------------------------------------------------------------------```
• posted

Mostly urban legend, according to:

Cheers, John

• posted

Except it's not. I contracted (*) at 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      |```
• posted

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

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

• posted

[snip]

OK ;-)

...Jim Thompson

```--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

The TV show "Mythbusters" duplicated the experiment using a compressed air cannon and assorted airplanes. They discovered that since 70% of the chicken is liquid water, it doesn't matter whether it's frozen or thawed. That much incompressible mass, launched at high speeds, is going to do roughly the same amount of damage. However, when retested, they determined that the frozen chicken will go through more plates of glass, allegedly giving some weight to the legend:

Never mind that airplane windows are not made of glass and their apparent ignorance of brissance (shattering shock effects).

Drivel: I'm tempted to do my own testing, using the neighbors noisy rooster. Roosters are only suppose to announce the dawn and shut up. Not this neurotic rooster. It goes 24 hours, non-stop.

```--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com```

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