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Twospoons
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Perhaps your bimetallic bridge wire could be made by plating one metal on top of the other? Maybe by plating Pt on Al wire. The intimate contact
between the metals should allow the reaction to start much faster, which would be important in this case.
Cu0/Al by plating Cu over Al then oxidising the Cu?
Helicopter: "helico" -> spiral, "pter" -> with wings
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nitro-genes
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| Quote: | quote]Originally posted by Twospoons
Cu0/Al by plating Cu over Al then oxidising the Cu? |
Selectively oxidising the copper while in contact with aluminium is inpossible...But maybe thin aluminiumwire can be dipped in some sort of CuO/resin
mixture to coat the aluminium wire. I have no idea if aluminium is usable as a bridgewire however...
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Boomer
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To put the welder idea to the grave: As jpsmith pointed out you need MEGAwatts, i.e. a thousand welders in parallel. Plus, you need it FAST. Think 3
TW/s or four *billion* horsepower / second rise speed. Cap banks have NANOhenries, a welder has henries inductance. That's a *billion* times more. The
cap discharge produces a shock wave of tens of thousands of PSI, and a deafening bang that makes the ears ring - without the explosive! It powders
adjacent materials by the shock before they have time to even melt.
Of course a welder could heat said Pd/Al wire to its reaction temp, as could a NiCd pack. But for EBWs said metals make no sense. You evaporate and
then explode the wire, and the energy Axt quoted is lattice energy from alloying them. No alloy in a plasma!
And using such a wire in a normal cap would also defeat the purpose. It will not detonate a secondary (other than by cook-off), and putting lead azide
in means you can as well use normal wire. This will go off before the Pd/Al reaction starts (I assume it needs over 300C?).
But we miss the idea of the thread in discussing EBWs. The question whether Cu/Al would work if heated electrically can be rephrased as "Can this
thermite produce a shock to detonate a secondary?" Putting in additional electrical energy would be a waste, since due to the higher mass it would
need even more power than exploding a bridge wire. The mix has to have the potential to det say PETN from its own reaction rate. Anyone tried that?
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quicksilver
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| Quote: | Originally posted by Boomer
To put the welder idea to the grave: As jpsmith pointed out you need MEGAwatts, i.e. a thousand welders in parallel. Plus, you need it FAST. Think 3
TW/s or four *billion* horsepower / second rise speed. Cap banks have NANOhenries, a welder has henries inductance. That's a *billion* times more. The
cap discharge produces a shock wave of tens of thousands of PSI, and a deafening bang that makes the ears ring - without the explosive! It powders
adjacent materials by the shock before they have time to even melt.
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Below is the best I could come up with and - in fact - they use just about the same concept as originally mentioned!
These guys are quoting about 180amps! (for some models)
http://www.risi-usa.com/0products/1ebw/new/new.html
Therefore I thought that the thrust of "the welder" concept was what is in line with commercial models.
It appears that commercial stuff stays within what would be considered "low power" in so far as the discussion thus far.
[Edited on 11-1-2006 by quicksilver]
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jpsmith123
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Actually, my idea is that the "reaction rate" is artificially "accelerated" by the electrical energy input.
IOW, the electrical energy doesn't just "ignite" the thermite, rather, it ignites a certain weight or volume of the mixture essentially
instantaneously.
I'll try to put some numbers in to clarify the idea. I'm just "thinking out loud" and these numbers are really ball park and probably incorrect, so
please correct me where I'm wrong.
Let's say you want the equivalent energy output (heat of detonation) of 0.25 grams of mercury fulminate, i.e., about 0.45 KJ.
The number I have for a stoichiometric CuO/Al mixture (heat of combustion) is about 4.4 KJ/gram, so you need about 0.1 grams.
I don't know what the ignition temperature of CuO/Al is, but for the sake of argument, say it's 600 degrees C. (I suppose you could add a small amount
of another oxidizer to adjust it downward if need be).
So the idea is to try to bring 0.1 g of CuO/Al up to it's ignition temperature, essentially all at once.
Assuming a heat capacity of about 0.6 J/(gram)(degree K)
(a composite number based on 20% Al and 80% CuO), it would take about 36 J of energy to reach 600 degrees C.
As I see it, the rate at which you add the energy isn't that critical, vis-a-vis a conventional EBW, since the thermite mixture is packed into, say, a
narrow paper tube, so electrically, it's all in "series". If the mixture in the tube is uniform, and the negative temperature coefficient of
resistance of the CuO is working in your favor, then maybe the bulk of the material in the tube will heat up approximately uniformly.
Maybe a photoflash cap switched with a mosfet would work in this case. Without some experimenting, I don't know what the electrical load would look
like exactly, but 450uF at 400 V would give you 36 J.
| Quote: | Originally posted by Boomer
"Can this thermite produce a shock to detonate a secondary?" Putting in additional electrical energy would be a waste, since due to the higher mass it
would need even more power than exploding a bridge wire. The mix has to have the potential to det say PETN from its own reaction rate. Anyone tried
that? |
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12AX7
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I wouldn't use a photoflash, and I wouldn't use a FET in series with any sort of discharge, ever. What you want, if anything, is a robust switch
(ignitron for example, Google "quarter shrinker") and, if higher voltage is needed, a transformer.
Tim
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jpsmith123
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Well, the idea is that hopefully you don't need exotic, expensive parts, otherwise you're back to an EBW configuration and much of the proposed
advantage is lost. If you don't like Mosfets for some reason you can probably use a BJT, SCR, relay, etc.
Off hand, I have no idea what the electrical load would look like, and I'm basically just "thinking out loud" here, but something like the attached
circuit may be a reasonable starting point.
Attachment: Circuit.pdf (7kB)
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Twospoons
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A chunky thyristor might do it. I designed a 36J electric fence energiser some years back that used an 85 amp (RMS) thyristor to deliver a 2000A
pulse, 100 microseconds long. The capacitor bank ran at 900V.
A triggered spark gap is another fast, high current switch thats fairly easy to build.
You need to be very careful with that sort of hardware - the caps can retain their charge for a long time. As I found out when I picked up a circuit
board I'd turned off an hour earlier. The experience was .... unpleasant. It could have been fatal.
[Edited on 12-1-2006 by Twospoons]
Helicopter: "helico" -> spiral, "pter" -> with wings
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FPMAGEL
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Most people hear will proalbly know this but for the rest.
Exploding wire has a VOD of 4000m/s, for .5mm wire 40v 150amp will cause 20cm to vapourize, it the current not just the amps.
Theroy: the wire gets current flowing throught it, which creats a Mag feild, as the wire heats up to 1040C(temp of copper) it loses its abilty to
conducted electricty and the Mag feild dies, releaseing the densi plasma.
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12AX7
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SCRs would work, if you get fast units. The problem is getting microsecond edges, not something slow SCRs are apt to do. SCR inverters don't go much
over say, 20kHz.
JP: your schematic is basically the right idea (ignoring the FET as mentioned), but for the transmission line to operate as described, the load has to
be zero ohms. In practice, the wire has resistance, especially due to skin effect at this rate, it could be considerable. Thus, the transmission
line ought to have a reasonable impedance mismatched to the load, if I'm understanding TL theory properly.
An IGBT may also work, certainly fast enough, but keeping it from exploding might be difficult.
An ignitron isn't an exotic piece of equipment, obviously you didn't Google "quarter shrinker".  It's two bolts with air inbetween and an ionizing electrode. Put voltage between the bolts near the breakdown
voltage, spark the ignitor and it discharges with minimal impedance. Such devices are used to switch 10kJ or so in microseconds, so one would
certainly be suitable.
Tim
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12AX7
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| Quote: | Originally posted by FPMAGEL
Most people hear will proalbly know this but for the rest.
Exploding wire has a VOD of 4000m/s, for .5mm wire 40v 150amp will cause 20cm to vapourize, it the current not just the amps. |
I find this suspect. Do you have a reference?
| Quote: | | Theroy: the wire gets current flowing throught it, which creats a Mag feild, as the wire heats up to 1040C(temp of copper) it loses its abilty to
conducted electricty and the Mag feild dies, releaseing the densi plasma. |
Well, you'd be suprised to know I can pass large amounts of electricity through molten copper.
At this rate of change, the wire does not act as a fuse. As I recall, the function is such that, in the first microsecond, current only flows in the
outer ten-thousandth of an inch of the conductor (skin effect). So much current is sent down the wire that the layer which is conducting is instantly
joule heated to vapor. This transformation occurs in microseconds, so nothing has moved physically, and it tends to create a shockwave. I suppose
you would estimate VOD based on energy, risetime and dimensions.
I don't know if these EBW use this effect, the wires may be too thin for layer burning and instead the whole wire conducts at that risetime.
Tim
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Boomer
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Right, it conducts completely. Skin depth for 1 MHz is 66µm, for 10 MHz 21µm. Wire dia is 38µ (commercial units).
I know that website (RISI), thats where I got the ballpark number of 200 Amps. They do state the delay time for 400-500 Amps though, and that is the
recommended minimal current for reliable operation. I only need 1 kA min and 1.5 kA for reliability because my bridge is too thick.
The 4000 m/s are BS. Velocity of *detonation* for a gold plasma? The current keeps flowing even after the wire *evaporates* (not only melts). "it the
current not just the amps...and the Mag feild dies, releaseing (sic) the densi (sic) plasma" says it all. FPMAGEL are you gonna delete your post
*again* after we proved it makes no sense?
I get the idea of speeding up the thermite by bringing it up to reacton temp electrically. The question remains whether the subsequent reaction will
detonate a secondary. Even the most sensitive need kilobars, most need tens of kilobars, in a sharp-rising pulse.
Twoospoons I remember that fence. Are you possibly on E+W under another name?
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jpsmith123
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| Quote: |
SCRs would work, if you get fast units. The problem is getting microsecond edges, not something slow SCRs are apt to do. SCR inverters don't go much
over say, 20kHz.
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AFAIK, the relevant parameter would be turn-on di/dt. A fast inverter SCR might be capable of 1000 A/usec, which is probably plenty fast enough. As I
recall, the limiting factor for inverter duty is the relatively slow turn off, which is obviously irrelevant here.
| Quote: |
JP: your schematic is basically the right idea (ignoring the FET as mentioned),
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Ok, at the risk of going off topic, I'll bite; explain to me the fundamental problem with MOSFETs. (BTW, the MOSFET in this circuit is strictly
representational at this point, primarily since I have no clue what the resistance of the CuO/Al mixture (when used as described) would be. Nor do I
know exactly how fast the energy would need to be added.
| Quote: |
but for the transmission line to operate as described, the load has to be zero ohms.
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Huh? If the load were zero ohms, basic physics dictates that I could not deposit energy into it. A transmission line in this case is merely a
convenient way of transmitting power from a source to remote load.
| Quote: |
In practice, the wire has resistance, especially due to skin effect at this rate, it could be considerable. Thus, the transmission line ought to have
a reasonable impedance mismatched to the load, if I'm understanding TL theory properly.
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Apparently you have it backwards. Generally, the idea is to match the transmission line to the load reasonably well.
| Quote: |
An IGBT may also work, certainly fast enough, but keeping it from exploding might be difficult.
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LOL!
| Quote: |
An ignitron isn't an exotic piece of equipment, obviously you didn't Google "quarter shrinker".
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I don't need to "Google 'quarter shrinker'" to know what an ignitron is. Sorry but compared to a MOSFET or SCR I can get from stock at Allied or
Digikey for probably less than fifty bucks, an ignitron is exotic and expensive.
| Quote: |
It's two bolts with air inbetween and an ionizing electrode. Put voltage between the bolts near the breakdown voltage, spark the ignitor and it
discharges with minimal impedance. Such devices are used to switch 10kJ or so in microseconds, so one would certainly be suitable.
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You've just described a triggered spark gap, not an ignitron.
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12AX7
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| Quote: | Originally posted by jpsmith123
AFAIK, the relevant parameter would be turn-on di/dt. A fast inverter SCR might be capable of 1000 A/usec, which is probably plenty fast enough. As I
recall, the limiting factor for inverter duty is the relatively slow turn off, which is obviously irrelevant here.
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Ah yes, that's right. I remember reading quarter shrinkers get slower results with SCRs than with a spark gap, so there's bound to be some cost of
speed at any rate. 1kA/us is certainly fast enough here.
| Quote: | | Quote: |
JP: your schematic is basically the right idea (ignoring the FET as mentioned),
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Ok, at the risk of going off topic, I'll bite; explain to me the fundamental problem with MOSFETs. (BTW, the MOSFET in this circuit is strictly
representational at this point, primarily since I have no clue what the resistance of the CuO/Al mixture (when used as described) would be. Nor do I
know exactly how fast the energy would need to be added. |
Right. Well the thing is, the MOSFET looks roughly like a resistor when on. A FET of the required voltage is going to be around half an ohm Rds(on),
which means peak current can only possibly be, say, 400 / 0.5 = 1kA, if any part of the chip can even handle that (I wonder if the silicon even has
that many charge carriers available or something?). The silicon chip and bonding wires will have I^2t ratings comparable to the EBW wire itself,
making an ugly situation!
You can of course use a bunch in parallel, but then...you need a whole bunch...
IGBTs look like a few-volt diode drop when on (I have a quad of IRG4PC50UD that run up to 200A peak for all of 3Vce!). I don't know what kind of peak
capacity you can get out of one, and I'm not about to test either ;-)
A good stocky, fast SCR is probably the way to go, especially since this is one-shot.
| Quote: |
| Quote: |
but for the transmission line to operate as described, the load has to be zero ohms.
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Huh? If the load were zero ohms, basic physics dictates that I could not deposit energy into it. A transmission line in this case is merely a
convenient way of transmitting power from a source to remote load. |
Well yeah, but I mean for developing current through the load- obviously it won't dissipate power, but looking at the I^2R loss, you want to maximize
I.
A power match would be best, so you meet V^2/R and I^2R (that is, current and voltage are both proportioned to the capabilities of the driver), but if
the idea is to have the transmission line bounce current around a bit, you at least aren't going to be able to match line to load.
| Quote: | | Apparently you have it backwards. Generally, the idea is to match the transmission line to the load reasonably well. |
Yeah, that's kind of what I'm wondering. Not like it's a very well-behaved load, though...
Tim
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Goldstein
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LANL describes atleast two primaryless hot wire detonators based on both DDT under some confinement and by using a flyer plate propelled by
deflagration.
There are a number of documents with experimental results on the LANL DVDs, under titles such as:
"The deflagration-to-detonation transition in PETN and HMX"
"All secondary explosive hot-wire devices"
"All-secondary explosive flying-plate detonators"
"Nonprimary-explosive, hot-wire detonator"
Also patented under US3,978,791 and US4,317,412.
Another patent worth looking at is US3,726,217 describing another clever principle for initiation of a PETN or HMX column.
And for those interested in reactive bridges. I'm sure there is more on the topic both in patent and academic databases.
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jpsmith123
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| Quote: |
Right. Well the thing is, the MOSFET looks roughly like a resistor when on. A FET of the required voltage is going to be around half an ohm Rds(on),
which means peak current can only possibly be, say, 400 / 0.5 = 1kA, if any part of the chip can even handle that (I wonder if the silicon even has
that many charge carriers available or something?). The silicon chip and bonding wires will have I^2t ratings comparable to the EBW wire itself,
making an ugly situation!
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Well, there's going to be some kind of on-state loss with any switching device you choose. MOSFETs aren't unique in that regard. The essence of the
design process is making various tradeoffs based on the applicable parameters and constraints. Obviously, for the sake of reasonable efficiency and
reliability, you would generally need to make sure that R(load) >> RDS(on). Anyway, IIRC, you can get a 500V MOSFET with RDS(on) less than 0.1
ohms for under $20.00.
In the case at hand, once you have some idea of what the load looks like and how fast you need to put the energy in, you can nail down the design.
| Quote: |
Well yeah, but I mean for developing current through the load- obviously it won't dissipate power, but looking at the I^2R loss, you want to maximize
I.
A power match would be best, so you meet V^2/R and I^2R (that is, current and voltage are both proportioned to the capabilities of the driver), but if
the idea is to have the transmission line bounce current around a bit, you at least aren't going to be able to match line to load.
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Other than to accomodate the current I need to handle to meet my ostensible design goals of maximum power into the load at minimal cost and
inconvenience, I don't care about current...I certainly may not want to "maximize" it. Generally, I want to maximize the power and minimize the cost;
ipso facto - I don't want it to "bounce around", either. Don't forget, this transmission line might be rather long and will have losses.
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Twospoons
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| Quote: | Originally posted by Boomer
Twoospoons I remember that fence. Are you possibly on E+W under another name? |
Yes, that was me. I changed names to dissociate myself from E&W - I don't go there anymore.
MOSFET conduction loss is I^2 R. Since I is very big, I^2 R will be huge.
IGBT conduction loss is I x Vsat. Trouble is if you try to poke too much current through one they come out of saturation and the power loss
skyrockets - followed shortly by the IGBT!
Thyristors are nice because they handle massive peak currents - generally 10 to 20 times their rated contiuous current. The datasheet will usually
tell you the I^2 T rating of the device - which is rather handy. The key to getting the buggers to turn on fast is delivering a really big gate
current pulse - this can be done with a smaller thyristor.
Nothing is as robust as a spark gap, however.
[Edited on 13-1-2006 by Twospoons]
Helicopter: "helico" -> spiral, "pter" -> with wings
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jpsmith123
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Thanks for that great link. I knew we were on the right track. Particularly interesting to me is the idea of the integrated shaped charge.
A similar idea I had was to put an EBW on "rails". IOW, in order to try to take advantage of energy that might otherwise be lost, especially with a
slower energy input; i.e., making 0.5LI^2 energy more effective by electromagnetically driving the plasma.
I
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FPMAGEL
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"says it all. FPMAGEL are you gonna delete your post *again* after we proved it makes no sense?"
PROVED it makes no sense, well first off ,how about you prove it. And the replies from you two, a pinch of salt will have more sway over me.
"I find this suspect. Do you have a reference?"
Its from a book, a friend of mine in laser research told me.
"Well, you'd be suprised to know I can pass large amounts of electricity through molten copper."
Stop nit picking.You should try to get a containg Mag feild(liner field) around molten copper wire.
"instantly joule heated to vapor."
And the Mfeild dies, realseing that dense plasma.
[Edited on 13-1-2006 by FPMAGEL]
[Edited on 13-1-2006 by FPMAGEL]
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jpsmith123
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In the course of looking to see what mixtures might work best, I came across some interesting parers:
http://www.me.umn.edu/~ap/aerosolgel.pdf
http://www.menet.umn.edu/~ap/KMnO4.pdf
| Quote: |
MOSFET conduction loss is I^2 R. Since I is very big, I^2 R will be huge.
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Hopefully for the thermite idea as I've stated it, the load would be measured in ohms and the minimum power required would be no more than a few tens
of KW, in which case a MOSFET would work fine, IMHO.
[Edited on 13-1-2006 by jpsmith123]
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Twospoons
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A MOSFET and a thyristor of similar rating will be about the same price, but I'd put money on the thyristor being more rugged. Its also much easier
to get high voltage thyristors, should the need arise. You'll hunt for a long time for a 1200V MOSFET with the current rating you will need.
Just looked in the RS catalogue:
1200V 8A fet = $86,
1200V 55A thyristor = $17
And that thyristor will handle a 500A pulse, while the fet is only rated at 32A pulsed.
[Edited on 13-1-2006 by Twospoons]
Helicopter: "helico" -> spiral, "pter" -> with wings
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jpsmith123
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Perhaps we should start another thread entitled: "MOSFET vs SCR for as yet completely undefined loads".
As I've endeavored to point out, ad nauseum, my use of the MOSFET in that circuit was representational (and reflected my optimism that you don't need
a massive amount of power); in practice, determine what you need to do and use the appropriate device of your choice.
I responded to 12AX7 because I disagreed with his unqualified assertion that you never "ever" use a MOSFET for such purposes. That's really the extent
of my interest in this "side issue" at this point.
(BTW, if you look in Digikey you can find a 500V, 50 amp (and probably at least 3 or 4 times that value for a non-repetitive pulse) MOSFET for $8.75.
I would hope this would suffice, but as I said, I have no idea).
[Edited on 13-1-2006 by jpsmith123]
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jpsmith123
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Another Interesting Patent
In US Patent 6227116, it is claimed that fuel rich thermite mixtures can cause a charge of PETN to detonate. I haven't read the whole patent yet, but
here's the abstract and an example from the text:
Abstract
A detonator comprising a shell with a secondary explosive base charge, igniting means and an intermediate pyrotechnical train, said train comprising a
novel ignition composition with a specific redox-pair of a metal fuel and a metal oxide oxidant, said fuel being present in excess to the amount of
stoichiometrically being required to reduce the metal oxide, the ignition composition being able to ignite said secondary explosive into a convective
deflagrating state to reliably detonate the same. Use of said novel ignition composition for the ignition of secondary explosives in general.
"Into 20 initiating elements in the form of aluminium tubes, each having a length of 20 mm and an internal diameter of 3 mm and an outside diameter of
6 mm, an ignition charge consisting of 20% by weight of Ti+80% by weight of Bi.sub.2 O.sub.3 was pressed to a column height of 5 mm. Adjacent thereto
a column of PETN with a density of 1.3 g/cm.sup.3 was pressed.
In the same way 20 initiating elements were manufactured with the exception that the ignition charge (i.e. 20% of Ti+80% of Bi.sub.2 O.sub.3) also
contained 8% by weight of Fe.sub.2 O.sub.3 as an additive.
This experiment showed that all 40 detonators containing said initiating elements worked excellently with a qualitative detonation of the base
charge".
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franklyn
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Exploding bridgewires are a rather mature technology at this point. It is
afterall how the implosion assembly of A-bonbs has always been triggered.
The firing of automotive sparkplugs is less high tech and except for the smaller
energy required essentially the same.
Attached below are two pages excerpted from Explosive Trains
of the Army Material Command
Exploding Wires is the definitive reference on the technology ( just click submit at this page ) ->
http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?Search_Arg=expl...
Coaxial or instead low inductance stranded cable keep impedance low. See ->
http://www.litz-wire.com/applications.html
Low inductance capacitors are also required ->
http://www.sbelectronics.com/pdf/PPC2005%20Presentation.pdf
Of further interest see my post here about a conductive explosive _
http://www.sciencemadness.org/talk/viewthread.php?tid=851#pi...
At this thread ->
http://www.sciencemadness.org/talk/viewthread.php?tid=851#pi...
.
[Edited on 5-7-2006 by franklyn]
Attachment: Explosive Trains AMCP.pdf (888kB)
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