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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
2bytes wrote ...
In fact liquid metals can dissolve most metals.
i Have always wanted to build an AIM-9 Sidewinder missle with 40 lbs of mercury as the warhead, just to piss off the russians.
im going to class tomarrow, ill stop buy theh chemistry lab prepareres room and ask about low melting point metal and wetting, do your rails have to be copper ? aluminum, gold and silver are probably prohibitively expensive or prone to liquidification.
EDIT: have you been using the car battery? i think from a new battery you can get dangerously high currents, much more than it takes to get starter motor to turn.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys
Yes, one of my concerns is that gallium based liquid metal may alloy with the surface of copper and remain solid and unmeltable at practical temperatures - or even worse, it may embrittle copper like it would aluminum.
Gallium-indium-tin would be perfect, though too expensive to ship. I'm most likely going to use french's metal - need to find a way to limit the release of cadmium though...
EDIT: have you been using the car battery? i think from a new battery you can get dangerously high currents, much more than it takes to get starter motor to turn.
Well, yes, and this battery was fairly new, though I'm not convinced I can get the currents I require with a reasonable number of batteries. The battery is rated to drop half it's voltage at cold cranking current, which yields about 8 miliohms of internal resistance. Even if the "hot cranking" situation is 2x better it's all still way too much resistance. I need less than a miliohm over entire current path!
Has anyone actually measured short circuit currents of some of the best lead acid batteries? That'd make some very valuable data for me.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Marko wrote ...
Has anyone actually measured short circuit currents of some of the best lead acid batteries? That'd make some very valuable data for me.
i have 2 brand new large group size batteries, how can i do this test for you? i have a USB Oscope good for 2MHz, and bolt stud SCR's a low value resistor for high power would be needed too. i coulduse tham single, parallel or series.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
It's not the easiest test to do. You need a high current shunt and some way of breaking the circuit after you fire the SCR. A big fuse could be used to break the circuit. The fuse may interfere with the measurement, though. You could also simply have a plug plug or big switch which you pull manually a fraction of second you fire the scr... or just use the switch/plug alone without the scr. This would be the simplest and most accurate method, but I wouldn't really call it safe and good for the battery.. your contacts may weld or some other horror may happen.
I'd probably simply press two big pieces of copper braid together as a crude switch, which can be pulled apart quickly. To my understanding of physics, a few seconds of short circuit should not damage the battery by thermal means, but I may be wrong, no idea about possible negative battery health consequences!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Patrick wrote ...
what if i usea cap bank to simulate a ultra low resistance, high current temporary load. i have a bunch of 3300uF 20 volt caps that could be banked.
or better yet i could get 000 awg welding cable and then 10 awg wire and make the resistance shunt do the melting/switching.
A few 3300uF caps would have internal resistance much higher than the battery itself. The idea may work if you could find farads worth of caps wth sub-miliohm esr, not something very practical.
I'd probably just make some sort of a switch that can be quickly connected and disconnected manually (no scr's) and use a DSO to capture the shunt voltage O_O
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Permanent Magnet Rail Gun (PMRG)
