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Method for measuring firing coil voltage pulses with an Oscilloscope

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aarpcard

Thu Mar 12 2015, 10:52PM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
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Posts: 44

DerAlbi wrote ...

Wow. you again ignored the quicktest. What can i say. If the measurement is wrong, where is the point to dicuss the negative 3V.... also you coupled AC. if the timescale is right thats totally enough to offset your decoupling capacitor (low cutoff-frequency is 5Hz mostly)
You try to analyze a measurement where you dont know if its right. Please explain the point of this.

And you really avoid perfectly to figure out, if your oscilloscope is overloaded and how if handles such an overload. maybe is like a multimeter at mains voltage during ohm-measurement... it doesnt blow, but for some time after the overload all measurements are way off.
please stop ignoring the basic question here... is the 3V you are discussing actually relevant?
Remember your Capacitors are a diode in reverse polarity. putting any kind of resistors in front of your reverse polarity protection diodes is just... well lets say.. the caps will degrade REALLY fast.

I haven't had the opportunity to do the quick test yet - I fully plan on doing it.

The scale on the scope is definitely off - whether it's being saturated or something else, I'm not sure until I do the quick test. However, considering the point below, the general shape of the wave looks like it could be correct.

However, as I've been saying, the reduction in dampening resistance should result in the wave shown in Barry's RLC which would result in significant projectile suckback and a slower projectile when compared to the critically damped circuit with a resistance of ~300 milliohms. What I'm confused about is that the circuit as it is (with the lower resistance) is functioning 100+% better than the critically damped circuit. This suggests the wave is indeed critically damped which didn't make sense to me which is why I wanted to measure it - and when I measure it, it's close to being critically damped (although yes, I need to sort out the scope).

My main confusion is why does the general shape of the wave on the scope match what I'm getting for velocity numbers, but doesn't match Barry's RLC sim. And even if the scope is wrong due to how I'm measuring it, and the simulated RLC wave is close to what I'm actually getting, then the velocity of the projectile should be much lower than the 14m/s I was receiving with the original 300 milliohm critically damped wave. Mainly, a less optimal pulse should not be performing better (and 100% better that that) than an optimal pulse especially when the coil and projectile were designed for the optimal pulse.

And that's why I started looking at the negative voltage protection circuitry as that could be dampening the negative oscillation.

Obviously I need to measure the inductance and properly test the scope, but I won't be able to do that for a little while.

Signification

Tue Mar 17 2015, 06:17PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

@aarpcard,
How do you configure the barrel of your multistage coilgun. For a single stage, I can see the coil wound in a coil-winder which can handle that length, but, I assume, a multiple stage coil would require windings whose total length is not all done in the on a very long shaft in a coil-winder. Do you wind multiple coils, one at a time, on a short solid shaft and then, with the coil removed from the winder, slide the coils over a single section of tubing long enough to hold all three (or more) coils? Here, I see the winding shaft's diameter equal to the OD of the tubing (barrel).

IOW, If you wind each coil separately, how to you assemble the series of coils on a single barrel?

johnf

Tue Mar 17 2015, 07:06PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

i THINK YOU WILL HAVE TO MEASURE THE INDUCTANCE THE AIR CORED COIL IS WRONG
You have a core in it (your bullet) this will increase your inductance by several orders of magnitude

Signification

Tue Mar 17 2015, 08:31PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

johnf, (assuming you were responding to my last msg in this thread) I am pretty sure we are aware of the varying inductance "L" as the bullet transverses the barrel. I, for one, am working on a mathematical relationship between the total "L" of the coil as a function of projectile position in the barrel--given the appropriate parameters, such as permeability of a bullet of particular size, and coil details.

But, at present, I was curious of other people's methods of placing multiple coils on a (metal or non-metal) long barrel.

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