Particularly interested in 0.5mm dia tinned copper wire and current pulses of half cycle duration - same conditions as for non repetitive surges in power diodes and SCRs.
I figure it is about 500 amps peak - but are there any tables that cover this?
... Phil
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
** That formula breaks down to I being proportional to the sq.rt of 1/t - suggesting even the largest fuse will melt in a matter of weeks or months at tiny current. But for time intervals of up to a few seconds - it seems plausible enough.
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?
-- John Devereux
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"
** Yeah - and most likely get a:" 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.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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.
-- Regards, Joerg http://www.analogconsultants.com/
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
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
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.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Here's a real thriller:
We're scrounging for some software to flip it over.
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
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.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
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.
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)
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
-- ?? 100% natural
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