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350v & 3760uF coilgun. Are back emf diodes a must?

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LAN_403

Gratitude

Wed Nov 19 2008, 01:05AM

Registered Member #1702 Joined: Wed Sept 17 2008, 10:14PM
Location: Ukraine, Kiev
Posts: 33

I think Barry is right If the remaining negative voltage is more than 10% of the positive starting voltage, then you need the diode, other wise just enjoy

Backyard Skunkworks

Wed Nov 19 2008, 02:23AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

A good thing to keep in mind is that electrolytic capacitors will undergo reverse breakdown at around 1.7V. After reverse breakdown occurs, any current flow will chemically damage the capacitors, the more current flow, the more damage.

As a general rule, I'd always reccomend using (a) diode(s) in a coilgun of any signicant power (such as yours). This will really help if you want to get thousands of shots out of your caps.

rp181

Wed Nov 19 2008, 03:20AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Regarding reverse breakdown, this can be repaired by slowly (over a few hours) charging the capacitor. Sooner or later though, the electrolytic solution will run out.

KLH

Thu Nov 20 2008, 07:13PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
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Posts: 137

Casey wrote...
the projectile flies about 5 feet at 90v, but doesn't leave the barrel with 140v. thats with the protector diodes

One note on protection diodes and what is going on in the inductor...

When an inductor is hooked up to a high-impedance load, its energy dissipates faster and at a higher voltage into the load because it behaves as a current source. With a diode in reverse-parallel with the main firing coil, this causes the coil to discharge into a low-impedance load (the diode). This means that the magnetic field is sustained for a lot longer, and this can cause suckback of the projectile. So the protection diodes on your 140V test are probably causing suckback.

In order to fix this, you can put a snubber resistor in series with the protection diode. To calculate the value of the resistor, use Ohm's law (V=I*R, where I is stored inductor current and R is the snubber resistance) to find the maximum resistance that will not exceed the breakdown voltage of your main power switch.

I highly recommend a snubber resistor for all spike protection circuits because the only thing that will bring the inductor's current down to zero is if the current is dissipated as heat, and I certainly wouldn't want all of that current dissipating in a diode.

Note that any back-emf that comes out of the inductor is wasted energy that was not transferred into the kinetic energy of the projectile. Therefore less back-emf before the protection diodes are added means a higher efficiency.

buzzer

Thu Nov 27 2008, 10:17PM

Registered Member #680 Joined: Fri May 04 2007, 03:20PM
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Posts: 15

"ringing, not back EMF" said Barry. Can you explain the difference between ringing and back EMF?

rp181

Thu Nov 27 2008, 10:36PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

Ringing is voltage oscillations. This happens with too much inductance and too little damping resistance. EMF is when the magnetic field collpases, this will always happen with an inductor, you just manage the energy differently.

Barry

Fri Nov 28 2008, 04:26PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

"Ringing" is what happens in an under-damped RLC circuit. When the total loop resistance R is less than a certain value (the critical point) then the circuit response will begin to oscillate to some degree. With progressively lower total resistance the ringing continues longer. Look here for comparison graphs.

Ringing depends on all three values of R, L and C. It is not just "too much inductance" as rp181 suggests. If resistance R < 2 sqrt(L/C) then it will ring. With any more resistance than this there will be no overshoot and no negative voltage on the capacitor.

"Back EMF" is a generated voltage in electric motors due to armature motion. In normal electric motors the back EMF is what you get when you operate it as a generator. In coilguns, as the tail end of a projectile passes through the coil it can interact with the magnetic field to generate current and therefore cause a voltage spike.

Make sense? I searched but didn't find a really good graph of a coilgun's inductor voltage, showing a small spike on the tail end of the operating cycle.

Back EMF in coilguns, by the way, is related to the DSE (dreaded suckback effect). If the magnetic field remains on too long, the projectile motion is slowed. While this is happening the kinetic energy converts back into additional coil current, causing a small voltage spike. It is not a big voltage but visible on an oscilloscope.

Cheers, Barry

KLH

Wed Dec 03 2008, 06:23PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Back EMF applies to anything that involves a collapsing magnetic field, and it behaves like a current source. This EMF from an inductor results whenever an applied voltage is removed. This is what causes ringing, which damages the capacitors through reverse charging.

In this case, the ringing is irrelevant; the back EMF (if strong enough) will destroy the capacitor regardless if it rings down or not.

Backyard Skunkworks

Wed Dec 03 2008, 07:01PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

killah573 wrote ...

Back EMF applies to anything that involves a collapsing magnetic field, and it behaves like a current source. This EMF from an inductor results whenever an applied voltage is removed. This is what causes ringing, which damages the capacitors through reverse charging.

In this case, the ringing is irrelevant; the back EMF (if strong enough) will destroy the capacitor regardless if it rings down or not.

I don't think the back emf is generally strong enough to destroy a capacitor in one shot; normally the reverse charging just breaks down part of the electrolyte. Once this happens enough, the capacitor fails during chargining/discharging/charge storage.

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