Cu wire fusing amps ?

Google for "copper wire i2t". The first hit gives a PDF which contains the formula

((I/A)^2)t = 0.0297 log ((T2 + 234)/(T1 + 234))

I = short circuit current (A) A = conductor area (circular mils) t = duration T1 = max op. temp T2 = max s/c temp

Sorry about the non-SI units.

Allan

With 10mS and 20 thou diameter, the result is about 600A - for a half sine wave, the peak is about double so circa 1000A.

The interesting conclusion is this: a simple 0.5mm tinned copper wire fuse will very likely survive direct connection across a 10A, 240V domestic supply for a full half cycle.

Cos that supply very likely cannot deliver more than about 600 amps peak into a short.

OTOH, a C curve, 16 amp rated, thermal magnetic breaker will trip at 180 amps in under 2 milliseconds.

... Phil

AIUI the concept of I^2.t ignores thermal conduction losses. I^2.R.t is the energy you are putting into the wire, it is assumed this all goes into raising its temperature I think. Obviously this fails for intervals long enough to allow heat to be conducted away.

Seems to also assume constant resistance for the wire even though it gets ~white hot?

Yeah I assume it's for the case when the heat has no time to leave the piece of copper. You might try some 'first principle' type calculation.

So resistance R ~ rho*L/A (rho is resistivity, L is length and A is area) (of course rho will change with Temp (T) and that will make it harder.)

Then energy dumped into the copper I^2*R*t will be heat capacity (HC) * change in temperature (t is the time.) HC = constant * volume = C*A*L

So delta T = E/HC = I^2 * R * t / (C*A*L) = rho/C *t*I^2/A^2

So as you say, at least the units make sense. To do better you'd have to put in some estimate for how the resistivity changes with Temp. Assuming some linear relation might be OK. (rho ~ T(in kelvin)) I'd expect the heat capacity to be roughly constant at room temp and above. (I'll scribble some more.)

George H.

"George Hairoil"

" solution that only works for a spherical chicken, in a vacuum "

... Phil

We are currently engaged in RDP, the Resistor Destruction Project. The question is when resistors die, at high pulsed power dissipations, like hundreds of times rated power for hundreds of microseconds. I^2T is probably a good model for metallic conductors, but thickfilms seem to have other failure mechanisms, more like a structural change or electromigration sort of thing over many pulses.

Pushing thickfilm far past datasheet limits is risky. They might some day cook up another pot of conductive material, it veers a bit towards lower specific resistance, trim cuts get deeper ... *PHUT*

Wirewound may be better here. Ayrton-Perry style if it has to be low inductance.

My first lesson in electro-migration happened around age 16. A loud bang, large chunks of a capacitor migrated themselves into the plaster of a wall, with gusto. Some metallic parts rained down from there and hissed out on the carpet. My parents were not enthused.

Fun! A bit off topic, (but then Phil A. doesn't want any spherical cows :^) I was wondering last night if I put a piece of heat shrink (or maybe several layers) around a 1/4 watt through hole resistor if I could raise it's maximum DC power.. *NOT* it's pulsed power rating.

George H.

There's a somewhat unintuitive fact about small copper tubes--up to some thickness, adding insulation makes the heat loss go _up_, because the extra surface area more than compensates for the temperature drop in the insulation. So it might work.

Cheers

Phil Hobbs

One might add a blob of epoxy on top of a surface mount resistor to add short-time-constant thermal mass to the resistive film. A fast pulse will dump the heat into the film, and it has to diffuse into the alumina. The epoxy adds a heat path in the other direction.

Of course, the best resistor is a bulk conductor, like solid manganin or nichrome. Hard to vaporize, too.

Lots of people confuse thermal _capacity_ and thermal _resistance_.

When designing regulators for the TOW missile in the early '70's I used _steel_ TO3's instead of Aluminum... the thermal capacity was enough higher that I needed no heat sink path... the target was reached before thermal limits ;-) ...Jim Thompson

Here's a real thriller:

We're scrounging for some software to flip it over.

Bloody cool. You should really publish a paper about your test results. Maybe on your company website. That would be a real contribution.

?-)

Rsense_Flir.MPG

I don't know what you did to that file but all my browsers and extensions and MPG viewers found it incompatible.

?-/

My Sony camera took it, and I copied it to Dropbox.

veral layers) around a 1/4 watt through hole resistor if I could raise it's maximum DC power.. *NOT* it's pulsed power rating.

That's what I was thinking. It's a very trivial issue. We give people an led with wires and 1k ohm ser ies resistor attached. The resistor and solder joints get covered with hea t shrink. I was wondering about the max voltage till the resistor fries. (The led can handle 40mA.) Maybe if I get any time I'll test and see.

George H.

No problem with mplayer:

mplayer -vf flip=yes Rsense_Flir.MPG

Flips it, or mirrors it, but won't do both at the same time.

There may be that kind of info in the Fire Protection Handbook by the NFPA; less informative sources is the National Electrical Code (NEC).

Strange it's MPEG level one, nothing fancy.

OTOH all your players may be relying on the same codec collection.

do both like this: mplayer -vf mirror=yes,flip=yes Rsense_Flir.MPG

or fix the recoriding using avconv (ffmpeg):

avconv -i Rsense_Flir.MPG -vf hflip,vflip Rsense_rightside.mpg

or mencoder (mplayer):

mencoder -o Rsense_rightside.mpg -oac copy -ovc copy -vf mirror=yes,flip=yes Rsense_Flir.MPG

however MPEG1 video is pretty weak, and PCM audio is worse, we can do better.

mencoder -o compressed.mpg -oac mp3lame -ovc x264 -x264encopts bitrate=360 -vf mirror=yes,flip=yes Rsense_Flir.MPG