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Forums
4hv.org :: Forums :: Electromagnetic Projectile Accelerators
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Coilgun Questions

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Navrit Bal
Mon Nov 22 2010, 08:07PM
Navrit Bal Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
I will use your 'fancy' method as it seems more reliable. I will have to find the tables later, but for now, I might use my school's equipment which should be better. Is there a particular way to measure capacitors ESR ? Forgive me if its just attaching a multimeter to the 2 terminals cheesey
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Bjørn
Mon Nov 22 2010, 08:38PM
Bjørn Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The fancy way to measure milliohms is to force a known DC current, say 1 amp, and measure the voltage between the coil ends.
Consider ramping the current up and down. Depending on the coil you might end up hurting yourself and the instrument if you just connect/disconnect it instantly.
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Navrit Bal
Mon Nov 22 2010, 08:51PM
Navrit Bal Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
I will follow your suggestion, but hopefully I will have my Link2 diode by then, so with that in antiparallel, it should help at least.
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Artikbot
Tue Nov 23 2010, 09:48AM
Artikbot Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137
Navrit Bal wrote ...

I thought it would be best to use a big wire, then add resistance with resistors as needed. Also, I have 3 450V 1.8mF capacitors and 105 330V 160uF photoflash capacitors (16.8mF overall). Where did the 350V come from?

Oh forgot this part.

No, it isn't a good idea at all to put resistors. They will kill and bury your efficiency, and they will cost a freaking fortune, since you need incredibly high powered power resistors.

The idea is doing the math before buying anything... But I learnt by hitting my head to the walls aswell wink gosh... Burnt a 555 timer, replaced two times an inductor, burnt 5 resistors and 2 pots... All on the same board LOL. They say we aren't born knowing everything tongue

350V is just a round-up from 330V. Don't know really, but the thing is that the pulse will behave mostly the same way, since both capacitance and inductance are the same, and voltage isn't that different.
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Barry
Tue Nov 23 2010, 12:31PM
Barry Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Bjørn wrote ...

The fancy way to measure milliohms is to force a known DC current, say 1 amp, and measure the voltage between the coil ends.
Consider ramping the current up and down. Depending on the coil you might end up hurting yourself and the instrument if you just connect/disconnect it instantly.
A reasonable setup is to use a car battery connected in series with a large 8-ohm (10-watt) resistor and your coil. Measure the voltage across the coil. Either measure the current directly, or measure the voltage across the known resistance to find the current. From the current and the coil voltage you can calculate the coil's d.c. resistance.

Don't have a big 8-ohm resistor? Try a 60w light bulb. Or two or three. You'll have to measure current with an ammeter since the bulb resistance depends on temperature and is unpredictable.

I don't see a need to ramp up current with coils like this. The inductance is pretty small, so the total stored magnetic energy is pretty low: 1/2 L I^2.

Navrit Bal wrote ...

I will use your 'fancy' method as it seems more reliable. I will have to find the tables later, but for now, I might use my school's equipment which should be better. Is there a particular way to measure capacitors ESR ? Forgive me if its just attaching a multimeter to the 2 terminals cheesey
No. cheesey The best we can do without special equipment is read the manufacturer's datasheet.

Oh yeah, I recommend against adding resistors to the high-current path of a coilgun. It kills performance. In fact, I'd say to do everything possible to reduce wiring resistance. Use fat and short wires, keep all connections very tight or better yet soldered.

Cheers, Barry
Centrifuges: They're what separates the men from the boys.
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Neuuubeh
Tue Nov 23 2010, 01:34PM
Neuuubeh Registered Member #3411 Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33
Why do you want a critically dampened circuit anyhow?? Any resistance that you bring into the circuit is eating away your precious current, and magnetic field strength is directly coupled to that same current. Take a look at this wiki diagram, it plots the current in a LCR circuit - Link2

If im allowed to quote Barrys site :

A critically-damped RLC circuit will deliver a current peak that is only 36% of the maximum possible peak from an undamped circuit. This is a very heavy penalty for the convenience of a simple circuit!
64% less than a perfect circuit with 0 resistance. Much weaker field, much weaker force on the armature compared to the ideal circuit..


Artikbot wrote ...

50V is pretty low, so dampening is not as critical (back EMF is nowhere as destructive unless you're talking about several dozens of amsp) as if you use higher voltages.
While the maximal possible voltage generated would seem to be dependant on the rate of change in the current that created the magnetic field, I remember reading some paper about inductors used as power supplies for railguns mentioning the "desirable" feature of inductors to supply whatever voltage is necessary to force the current into the load. You provide a way for the voltage to flow, it wont reach its peak value?
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Artikbot
Tue Nov 23 2010, 04:34PM
Artikbot Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137
Neuuubeh wrote ...

Why do you want a critically dampened circuit anyhow?? Any resistance that you bring into the circuit is eating away your precious current, and magnetic field strength is directly coupled to that same current. Take a look at this wiki diagram, it plots the current in a LCR circuit - Link2

If im allowed to quote Barrys site :

A critically-damped RLC circuit will deliver a current peak that is only 36% of the maximum possible peak from an undamped circuit. This is a very heavy penalty for the convenience of a simple circuit!
64% less than a perfect circuit with 0 resistance. Much weaker field, much weaker force on the armature compared to the ideal circuit..

It's the balance between pulse lenght and pulse power what makes you dampen more or less the pulse.. It's completely useless to have a 2MW pulse if it lasts for a millisecond and you need at least ten for your projectile to acquire the needed acceleration. It also puts a lot less stress on the dampening equipment needed to ensure the integrity of your cap bank.

So it's not always that you need to critically dampen, what you always need is to avoid secondary pulses due to underdampened coils since they can cause suckback.
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Neuuubeh
Tue Nov 23 2010, 05:56PM
Neuuubeh Registered Member #3411 Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33
Ah, but a higher field strength equals more force, equals more acceleration, equals the need for a shorter pulse :).

How about using the diodes to rectify the back pulse into the capacitors? Its what im planing to do (waiting on my scrs tho, just now saw that paypal hasnt even completed the transaction for some reason... more waiting :( )

PS. I am more interested into inductance coilguns however, so suckback doesnt really concern me for my own little experiment
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Navrit Bal
Tue Nov 23 2010, 06:56PM
Navrit Bal Registered Member #3323 Joined: Sun Oct 17 2010, 03:19PM
Location: West Midlands, UK
Posts: 116
I didn't expect so many responses then cheesey

Is this what can be used for DC rectification Link2 ? Would it rectify the voltage? If not, please tell me what would be used.

Is the most desired ζ value 1, I am referring to the diagram linked before.

Which wire gauge would be best for the following capacitor banks: 330V 16.8mF, 450V 1.8mF, 450v 5.4mF ? (just before I waste more money)

I am probably going to buy a 12v 7.2Ah SLA battery for something completely unrelated, so could it be used in the same way which you (Barry) suggested. Although I doubt I will measure the resistance of the coil as it looks like I am going to switch to a higher gauge wire.

Since, I can't find the datasheet for the capacitors I am going to use, I guess I will have to leave that alone.. Could I use the 2mm wire as very fat wiring connecting components? (since I can't think of a good use for it anymore)

Is 'B' usually used for magnetic field strength? Finally, is a large change in inductance the most wanted or a small/large inductance?
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Artikbot
Tue Nov 23 2010, 10:44PM
Artikbot Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137
Recitfying the voltage... From where? It mostly depends on the type of charger. If you're using a flyback core based one, you'll need a full rectifyer otherwise you'll be wasting half of the current (all that's on the negative side of the wave will go trash if you just let the positive side pass).
If it's a boost converter, a simple Schottky rectifier diode after the inductor will do, since it's pulsed DC and all you need to do is avoid current from the HV side go to the LV side of your board (logics part, the HV current would make everything on that side go BAM and so on, not really desirable).

For the huge wire/coil? Save it bro... You'll say "I WANT MOAR POWAH" in less than you think, and then you'll regret not having heavy gauge wire wink

Wire is dependant on the needed coil, and that can be found by using Barry's (thank God Barry is here with us, I can't imagine my life without his sims and his awesomesaucely clear explanations) RLC sim. There you get your inductance, and with it, you can play with his air core inductor sim and get the desired wire gauge.

Just make sure not to get too thin wire or it will instantaneously melt down when you apply a heavy pulse to it smile
Talking from experience... Had two meters of very thin wire (I believe it's 30AWG or so... or better say it WAS lol) lying around and wanted to do an experiment... Wound it around a plastic tube, applied all the energy from my 3.5mF 330V bank... and guess what? It cut the tube in three halves!! Not to mention the wire literally 'vapourised' LOL.
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