magnetics design -- 60mJ energy impedance matching

Jon Kirwan · 2009-05-16T21:17:01+00:00

I'm considering a low power design for a rocket launcher. It will usean MSP430 and a CR2032 as the power source for everything. (I need totest this, but I'm hoping to pull up to 5mA, for about 100,000 pulses,to reach a necessary 60mJ charge on a 1.5uF cap.) I'll adjust the OFFtimes accordingly to reduce the time-to-charge to a minimum (shorterand shorter as charge is added. ON time for each pulse is fixed bythe 3V, the inductor, and the peak current I can reach.I want this to be absolutely safe at the rocket end. In other words,no energy stored there. A circuit will be clipped to the ignitor, butno source of energy present at that end -- nothing that can evenremotely place things at risk as an absolute guarantee is importantuntil the remote end is wired up 100' away. I had considered crazyideas such as using a flash tube with 300V across it and requiring the4-5kV trigger to come from the remote end, but that still means theenergy is present at the remote end and that if there is a short ofsome kind it's possible for an accident to take place. So scratchthat idea.The ignitors have an all-fire requirement of the delivery of 0.5W in50ms to what amounts to a .68 ohm resistor. This is 25mJ in 50ms. Thewire will be 30 gauge with 100' out and 100' back, so about 20 ohms orso in the wire. Finer wire would be a convenience, but the ohms go upfast. 40 gauge is an ohm/ft, so that would be 10 times theresistance.So it appears its important to keep the current __low__ going out.I've found that 300V across a 1.5uF cap does the job nicely, directlyconnected. (I don't want to waste energy, either, which is a part ofwhy I'd prefer something on the 1.5uF side and not something larger.)However, with 20 ohms of 100' out and back 30 gauge wire, only about2mJ gets to the squib and nothing happens, at all. (The rest goesinto the wire. Not good.)Impedance matching is suggested. So I started looking at transformersI might design and make (toroid windings.) Not being very...

May 16, 2009 80 Replies

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Jasen Betts 17 years ago

someone suggested a capacitor and sidac(or other thyristor) at the rocket end, but that was no good for some reason...

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magnetics design -- 60mJ energy impedance matching

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