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Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
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Posts: 44
I tried searching (albeit not very hard) on this forum for an answer to this question but didn't find one, so I'm sorry if I'm repeating an old topic.
I'm further along into building my 3 stage coil gun - I have the coils built and the optocouplers in place and working for triggering the 2nd and third coils.
I'm in the process of tuning the coils by varying the inductance and dampening resistances to get an optimal pulse through each coil. I'm using another set of optocouplers and an arduino with some added logic to measure the projectile speed.
When I designed my coils, cap bank, projectile, and dampening resistors, I heavily relied on barry's RLC sim and inductor sim, as well has his other documentation. I found according to his sim, I would need a dampening resistor on the order of 300 milliohms for the primary coil for example. Through testing, I've found increasing /decreasing the inductance of the coil has done very little to change the speed of the projectile, but decreasing the dampening resistance to 50 milliohms resulted in a 100% increase in projectile velocity.
I'm confused, because when I plug those values into his RLC sim, this results in a massive negative current spike and the RLC curve is not damped at all. This should result in a much slower velocity. . . .
I'm not sure what this means. It could be that my coil's inductance is far off from what his inductor sim specified, but I've built them exactly to spec. I don't have a method for measuring inductance though.
So ultimately I'm looking for a way to measure and visualize the pulse through the coil(s) in the real world. Obviously I can't just hook this up to a scope (or can I?). . . are you guys aware of any methods.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
When I designed my coils, cap bank, projectile, and dampening resistors, I heavily relied on barry's RLC sim and inductor sim, as well has his other documentation. I found according to his sim, I would need a dampening resistor on the order of 300 milliohms for the primary coil for example. Through testing, I've found increasing /decreasing the inductance of the coil has done very little to change the speed of the projectile, but decreasing the dampening resistance to 50 milliohms resulted in a 100% increase in projectile velocity.
are you including the resistance of the coil in the total damping resistance?
Obviously I can't just hook this up to a scope (or can I?)
That's the way it's normally done, it depends what you mean by 'just'. Just make sure that your scope probe and scope input have adequate voltage rating, and also make sure that you understand which terminals are ground, and how you are going to handle implicit connections between grounds. That means either grounding CG and scope, floating one or the other (safely), or using a differential probe. Most 2 channel scopes will allow you to display A-B, to get an effective differential probe. Almost all scopes and probes will be fast enough for CG waveforms, even home-built probes if you don't have adequate shop bought ones.
Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
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Posts: 44
Yep, I took into account the resistance of the coils. I'm using 10awg magnet wire and the first coil is comprised of about 11.75m which is a resistance of ~38milliohms. The ~300milliohm dampening resistor includes the 38milliohm coil resistance.
My capacitor bank voltage is 520v and the max input to my scope is 400V. I guess I always assumed that since I'm dealing with a current pulse in the 2KA range I'd want to keep that away from my scope . . . but since a scope is essentially a voltmeter over time, that doesn't matter. I feel like an idiot lol.
To compensate for the voltage issue, I guess I would need a 10X probe?
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
aarpcard wrote ...
I'm confused, because when I plug those values into his RLC sim, this results in a massive negative current spike and the RLC curve is not damped at all. This should result in a much slower velocity. . . .
How did you determine your L? I agree 100% with Dr.Slack (which shortens my reply). How about a simple series high-current shunt to a scope? Since this is a pulse the shunt won't have to be rated nearly to your current, but isolate it. Decreasing R can quickly bring on oscillations and a faster pulse, however, the pulse is also increased in amplitude/energy--perhaps your projectile escaped before any current-drop could significantly suck it back. Do you have reverse-voltage protection?
Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
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Posts: 44
I do have reverse voltage protection in the form of a properly rated HV diode and small value bleed resistor in parallel (with reverse polarity) across each capacitor. I'm using this configuration because having the diodes in series with the capacitor seems to result in a much longer pulse width. This way any reverse voltage should drain across the diode before it charges the capacitor.
. . . . I wonder if the bleed resistor is dampening the reverse pulse enough to make it negligible?
I'm determining my L based solely on Barry's inductor sim - based on the the dimensions of my coils and length and gauge of wire used. I don't have access to an induction meter.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
Are you using an IGBT switching system? I would look into putting a reverse diode/speedup resistor (in series) across the coil. I don't know about a diode in series with any components. What is your total capacitance @ 525v?
Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
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Posts: 44
So I attempted to measure the pulse with the scope. I only charged the bank to 375V as my scope has an input limit of 400V. Here's the waveform. I don't understand what's going on here.
The main pulse width is about 200ms, which is obviously wrong. And the inverted part of the wave has magnitude of ~-3V. This doesn't seem right either. My only guess is that I'm capturing the tail end of the wave and not the initial oscillation, but I can't get it to trigger on anything else, so should I assume this is correct? Especially since the overall shape looks to be spot on?
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
DerAlbi wrote ...
Do the math. You oscilloscope is completely saturatet. your voltage scale is totally wrong
Just assume exponential decay... 3V = 375V*exp(-R/L*200ms) Now extract the LR-Timeconstant L/R and think about how short your pulse effectively is.
Thats like discharging a battery with a LED. it just wont stop emitting light. that doesnt mean that the battery is still full at the end.
btw: voltage is not really a usefull measure. Current is important.
@DerAlbi: I 'did the math'. From 3V = 375V*exp(-R/L*200ms) I did extract the L/R time constant. It is 42ms. Then using 300milliohms for R, I was able to get an inductance value for the inductor of: L=12mH. I get an UNDAMPED RLC circuit time constant of 124 seconds! Now taking into account the addition of a series resistance: R=300milliohms, The circuit time constant becomes 32 seconds! Still Absurd!
@aarpcard: Can you describe the details of your measuring apparatus and scope settings/hookup. I do agree that you should be looking at current.
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Method for measuring firing coil voltage pulses with an Oscilloscope
