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Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
Well, I'm convinced your idea will work, but I'll try my idea first this friday or saturday and if it doesn't work, I'll try your idea. if my idea works, I'll design a PCB for my idea, if it doesn't work, I'll try your idea and if it works, I'll use your circuit.
Edit: Yan, I think you made a small mistake in your schematic. The last enable pin should also be connected to a phototransistor, because else the last coil won't switch of if the projectile is longer than the barrel.
Yes, you right. What was the MOSFET drivers and IGBT you was going to use initially, anyway? Because if it all powered from LiPo battery under 100V, then MOSFET is a better choice and any MOSFET driver is an acceptable solution.
Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
no, it´s powered by 400V 16.8mF cap bank. IXYK140N90C3 was my choice for the IGBTs and IXDN614PI was my choice for the gate driver and I'm using uzzors' zvs cap charger, just so you know. I think I'll first make 4 stages and later on, I might make some more and also increase cap bank size. I'm planning to use your coil winding technique btw.
Well, with MCP14E9 your switching time will be around 100ns, with IXDN614 your IGBT will swing in 10ns. Yes, the last one is better in general, but your gun does not switches millions times per second, so you will not feel any difference with any of those drivers. But the schematic is so much simpler with just one IC, isn't it? Consider switching takes 100ns, happens at current 800A and 800V voltage at collector (worst IMpossible case). The amount of energy dissipated will be: E=U*I*t=64mJ -per shot per IGBT. Is it really so enormous to care about?.. The amount of heat dissipated on collector-emitter drop during the ON period will be much higher than that anyway.
Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
I don't really care about simplicity. I would like it better if my gun has a circuit inside that's partially designed by me. If I can get my idea working, I'll use my idea; If I can't get my Idea working, but yours will work, I'll use your idea. I know those small differences in numbers don't matter and using a single IC and a lot less components is easier, but I just think it's nice to be able to say I made a contribution to the designing of the circuit I'm using in my coil gun. Also, it's not that much harder to build up my circuit, but I'll first have to see if it works or not.
Totally agree. I understand and deeply respect that. Howether, to make anything by yourself you need to learn a lot so to put parts not in a random fashion or according to intuition. F.e. in your schematic the collectors go to the opamp's inputs, but there are no ballast resistors to pull the collectors down. You also need to think over all cases (combinations of lighted/dimmed gates) and model the voltage potentials in all possible cases. If any result in equal voltages at + and - of the opamp - the output is undefined and may even start oscillating quickly destructing the IGBT.
Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
you mean like a big resistor in parallel with the 50Ohm resistor and the fototrasistor? If so, I don't see why that would be necessary. Also, Isn't the resistor over the output and the + of the comparator supposed to prevent oscillation of the output and don't the resistance differences of the resistors connected to the inputs of the comparators and the control point and the 50Ohm connected to the collector prevent the same voltage at both input pins?
No, I mean the collector of the transistor - it can only pull up, and without the ballast resistor to pull that point down it will just stay there. For some reason you think that + and - of the opamp pose any actions - they are not. Inputs of the opamp is like a probes of a multimeter - they measure whatever you give them. They neither pull measuring point up nor down. If you have collector of the pnp transistor pulling the input up, then be sure to add resistor to automatically pull points down when transistor goes off.
Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
Hey Yan, I've got the schematic (kind of) working. At least, with direct connections to the ground instead of fototransistors. I'm using my final schematic, but with a 180Ohm resistor instead of a 50Ohm connected to the collector (or in this case, a wire going to the ground). Tomorrow I'll test my schematic with the fototransistors. For now, I got around 4.5V at the output when the first wire isn't connected to ground anymore and the second is. all other configurations give less than half a volt at the output. I've tried it with 3 stages for now, tomorrow I'll test 3 stages again, but with light barriers. I haven't tried your idea, with MCP14E3's, but I will definitely try it, even if I can get my circuit working with fototransistors etc.
Oh, surprising. Well, whatever works better for you. Just test the circuit thoroughly - if any driver jams in ON state, IGBT will go in smokes, you know.
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Multi-stage coil gun, switching the stages.
