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Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
You cant really caluclate an undamped oscillation based on L/R since there is no C. It is true that the exponential decay does not apply to the Inductor. And the exponential behavior would additionally be more or less concerning the Inductor current. The voltage (thats measured) would (if considered exponential decay) would be 3V= 375*exp(-t/(5.4mF*ESR)) -> which gives an ESR of 7Ohnm. Some of the 7Ohm are wrong due to the inductive component, but that leaves a much bigger part than 300mOhm left to discuss. That could be bad.. bad soldering or other stuff that we have no information about.
Sry, thx to Signification, the idea was the right one, the component was the wrong one. Like a blown SCR or a Puck-Style-SCR which has not enough pressure on it
Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44
I just bought an inductance meter on ebay. Should be here within a week or so.
That pulse width has to be wrong. The primary coil is 60mm long and my projectile is 45mm long weighing 41grams. Currently, muzzle velocity of the primary coil is 29m/s (about 2.3% efficient).
There's no possible way I'm getting any kind of a launch on a 200ms pulse width. Ballpark it should be ~4ms to be getting those numbers. I think my scope is set up incorrectly somehow. The shape of the waveform looks plausible, so I'm thinking it has to be a measurement error?
I don't have it in front of me right now, but the settings to the best of my memory were:
AC Coupling on Channel and Trigger Trigger Set to Rising Edge (I tried Falling Edge and got the same waveform.) Probe 1x
I have the scope ground connected to the system ground (system is isolated from mains) and then the probe is on the positive side of the inductor.
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Probe 1x? Usually Oscillscopes cant handle that voltage then. its commong that the input stage has maybe 35V-50V cabability which is then scaled up by te probe to provide higher input voltage capability. If i am right you are saturating your input circuit (and possibly overloading it). If not, then the measurement is correct. A saturated input circuit of your Oscilloscope could also make you see such waveform. Just test this theory by charging up your capacitor and probe the voltage as short as you can (so that you would see a rectangle on your scope. If that rectangle has the exponential decay instead of sharp edges... you know whats going on.
On the other hand: you say you made a multistage CG.. is this the first stage? (30m/s with 40g is cool!)
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
Is the peak at only a few volts!! Is this a x1 reading? IIRC you mentioned 2kA with about 260 milliohms. Amplitude-wise, your reading is as if you are across a section of wire! Where, exactly, are your two probes connected on the (R)LC circuit? Are they effectively across the coil only? One more small thing to try--take a glance at the DC-coupled input--is it still zero? It looks like both scales are about x20 off (V attenuated and t amplified). The waveform is great! I assume this capture is with the projectile loaded? A bit more R and you will have critical damping. You need current readings--after figuring this out.
Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44
DerAlbi wrote ...
Probe 1x? Usually Oscillscopes cant handle that voltage then. its commong that the input stage has maybe 35V-50V cabability which is then scaled up by te probe to provide higher input voltage capability. If i am right you are saturating your input circuit (and possibly overloading it). If not, then the measurement is correct. A saturated input circuit of your Oscilloscope could also make you see such waveform. Just test this theory by charging up your capacitor and probe the voltage as short as you can (so that you would see a rectangle on your scope. If that rectangle has the exponential decay instead of sharp edges... you know whats going on.
On the other hand: you say you made a multistage CG.. is this the first stage? (30m/s with 40g is cool!)
Yes, this is just the first stage. I'm pretty happy with the results so far. I'll have to see if I can get my hands on a 10x probe . . .
Signification wrote ...
Is the peak at only a few volts!! Is this a x1 reading? IIRC you mentioned 2kA with about 260 milliohms. Amplitude-wise, your reading is as if you are across a section of wire! Where, exactly, are your two probes connected on the (R)LC circuit? Are they effectively across the coil only? One more small thing to try--take a glance at the DC-coupled input--is it still zero? It looks like both scales are about x20 off (V attenuated and t amplified). The waveform is great! I assume this capture is with the projectile loaded? A bit more R and you will have critical damping. You need current readings--after figuring this out.
The negative peak is about -3V. Those waveforms were generated with about 80 milliohms of total resistance. What I'm so confused about is that this doesn't follow Barry's RLC simulation in the slightest. So this either means the negative pulse is being damped by the protection diodes and resistors or my inductance is way off.
The probe is directly directly across the coil. I'll see what happens with DC coupling.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
The inductance of the coil would be very helpful, but a few more parameters are needed: You mentioned that the coil was wound with 10AWG wire and was 60mm long is this correct? If not, please correct this. The additional parameters needed are:
1) Number of layers on the coil 2) Number of turns per layer (~20?) 3) The ID (bore) and OD of the coil
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
Why did you ignore the quick test, if your probe is the problem? Since you do get 2.3%eff its seems your design is kind of right. it must be the probe.
Registered Member #2848
Joined: Tue May 04 2010, 05:19AM
Location:
Posts: 44
DerAlbi wrote ...
Why did you ignore the quick test, if your probe is the problem? Since you do get 2.3%eff its seems your design is kind of right. it must be the probe.
That's what I'm thinking . . . but it still doesn't explain why the negative pulse is damped so much - I haven't done the quick test yet because I've been away from the project for a few days - hopefully I can get back to it tonight. I mean I understand that if the input circuit is saturated, it could result in this waveform, but if that were the case, I should not be getting a 30m/s projectile velocity, especially when using a 300milliohm dampening resistance results in a critically damped wave (according to Barry's RLC) and when testing that, my muzzle velocity was a mere 14m/s.
What that means is the circuit is more damped or similarly damped, but with a higher current pulse with a lower dampening resistance, which doesn't follow Barry's RLC. . .
I've attached a screenshots of Barry's Inductor Sim with the primary coil specifications entered.
My dampening resistor is currently 50 milliohms + the 38 milliohm coil resistance. Plugging the simulated inductance, resistance, capacitance and bank voltage into the RLC sim I get the following (see attached). I'm not getting the large oscillations as shown in the simulation which doesn't make sense.
So either my inductance is way off (I've built the coil exactly to spec on the inductor sim (I've ordered an inductance meter)) or something else is going on here. I'm still wondering if my negative voltage protection diodes and resistors are dampening the negative part of the oscillation. The schematics for these is also attached. (Note the resistance and inductance values in the schematics for the dampening resistors are no longer accurate. As I said, stage 1 is at 50 milliohms + 38 milliohms coil resistance and the inductance is ~145uH.)
Registered Member #2906
Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727
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.
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Method for measuring firing coil voltage pulses with an Oscilloscope
