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Supercapacitor coilgun

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LAN_403

pisoiu

Mon May 22 2017, 08:12AM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
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Posts: 14

For the above design, I think this is it, I will not modify it further. Even if the capacitor can withstand higher current pulses, increasing current through coil would mean thickening the wire, but that comes with reduction of number of windings (to keep the same length), which is not desirable. Increasing the length creates other mechanical problems.

klugesmith

Tue May 23 2017, 02:40AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Nice work there, Pisoiu.

Before joining this party, I want to find a missing factor in an independent analysis of your coil. Mind checking to see where this goes off track?

Coil ID was given as 6 mm. If the OD is 11 mm, then a single winding of 34 turns would use 908 mm of wire, for R of about 3.24 mÎ© and L of about 800 nH (by Wheeler's formula, the same as Barry's inductor sim). Putting the same mass of wire into 17 double turns, R and L come down to 0.81 mÎ© and 200 nH. Resulting current with 2 V across the coil would be almost 2500 amps!

Pisoiu, somewhere you mentioned wire length of 70 cm. That's not enough to form 34 turns, so it must be the length of double wire to make 17 turns. Together, the two strands would use 140 cm of wire. Coil ID and OD would be about 10.6 and 15.6 mm. Now R = 1.25 mÎ©. Current from 2.0 volts would be 1600 amperes, just like you said. 27,200 ampere-turns around the plastic tube. As long as you keep the wire at 20 Â°C.

My spreadsheet gives that coil an inductance of 470 nH, which is much higher value than other values seen in this thread. Brings L/R to almost 0.4 milliseconds, if we don't count the ESR of the supercapacitor. How much is that, by the way?

pisoiu

Tue May 23 2017, 07:03AM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
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Posts: 14

Thanks for helping with the calculations, I don't mind at all. I'm not very good at those, mainly because I have some gaps in the theory, here and there. I usually replace those gaps with intuition. Sometimes it works, sometimes not, but I like smoke shows and fireworks anyway :)
Coil ID is most certainly 6mm because I used a 6mm aluminium rod to build the coil, then I extracted it out and replaced with that plastic tube which has also 6mm diameter. Also, the copper wire is 2.5mm OD.
But the wire length is very approximate, because I initially prepared 2 wires, aprox. 1m length, then winded those on the aluminium rod, then cut the excess off. But I was not wise enough to exactly measure the length before winding it, so I cannot say for sure how much wire is inside. What I can say is that the coil length is very close to the capacitor length, which is 108.5mm according to datasheet.
The DC ESR for this capacitor is 270 microohms and the combined rdson of mosfets is 137.5 microohms typical. But rdson is measured at 100A Id, so, I cannot say for sure what is the rdson at 1000 amps.

klugesmith

Wed May 24 2017, 08:38AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Thanks for clarifying the dimensions. From geometric calculations I'm still getting about 810 microohms for the coil (two 454-mm wires in parallel). That doesn't match your posted voltage and current values, even if we accounted for the change in cross-section and resistivity from tightly coiling the wire.

You've told us the resistance of your supercapacitor and MOSFETs with high precision. Why not do the same for the coil resistance?

You could weigh the coil and look up ohms per kilogram (or per pound) in a standard wire table.
That depends on being sure of the wire gauge. 2.5 mm OD is too small to be 10 AWG, especially if measured over the insulation. And it's not close to any standard square-mm conductor size.

The best way to get the coil resistance is to measure it. Can you run a steady current through the coil, perhaps about 10 amperes AC or DC, and measure the voltage drop? You can get measurements 4 times larger by temporarily separating your bifilar winding at one end, then driving the strands in series instead of parallel. Don't forget that copper resistance goes up 10% for each 25 degrees C.

Der Albi wrote: >> supercapacitor's energy density is 10x - 100x higher than aluminium caps (lets say 50x) according to wikipedia.

Yes, but supercapacitors have lower power density. You need seconds to get all that energy out, compared to milliseconds with the aluminum electrolytics. And microseconds with high voltage (multi-kV) caps.

1495616017 2099 FT1630 Energy Power Density

pisoiu

Wed May 24 2017, 08:56AM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
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Posts: 14

I told you parameters for capacitor and for transistors with high precision because it's in the datasheet. The coil was built on the rush and unfortunately I was not paying enough attention to its details. But I will try to measure it as soon as I have some time. I have means to run high currents through it, even if with not great precision (it's a 10A benchtop power supply) and I can also measure voltage with precision of 4 decimals, 2 volts full scale. Will get back with data when it's available.

klugesmith

Wed May 24 2017, 09:11AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Sounds good. I forgot to mention that it's easy to protect your MOSFETs from overvoltage when they turn off. Just put a suitable pulse-rated diode in parallel with the coil, as you would with a relay coil. It makes the current decay more slowly, but that's insignificant in this case, because L/R is so short.

pisoiu

Wed May 24 2017, 09:36AM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
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Posts: 14

Thanks, I'll keep that in mind. However I have some doubts, mainly because the physical connection is long considering the coil dimensions and I'll have to put some wires(read inductance) on the diode leads. And if I remember correctly, that pulse looked extremely steep on the scope, under 20ns. It may be even steeper, as my scope is not a very high speed one, and those 20ns may not be the real signal, rather the limitation of the scope...

pisoiu

Thu May 25 2017, 08:31AM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
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Posts: 14

I did the measurement, with a current of 4.67A, the voltage at coil terminals is 5.22mV, results R=1.117 miliohms. With this value, the current through coil, at 2V voltage, is around 1700A.

LE: I'm very sorry, I was mistaking about diameter of the coil. The ID of the coil is 8mm, not 6. The ID of that plastic tube is 6mm actually. There was a time when I was younger and my memory was better....

klugesmith

Fri May 26 2017, 01:27AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Oh good. With your new coil diameter, my resistance model is only 10% different
from your new, measured resistance value (using method suggested by Der Albi).
Close enough to say there's no mystery.

Where did you get your coil inductance numbers, given as 10 to 100 nH? Wheeler's formula is now giving me around 250 nH.

Those voltage spikes at MOSFET turning-off time indicate a circuit dominated by inductance, at the time scale of switch transitions. Just as it's dominated by resistance, at the timescale of the projectile-launching pulse.

You could get the inductance value by measuring the height and width of a braking-voltage spike.
For example, suppose FET drain to source voltage is 40 V for 5 microseconds, as the coil current ramps from its final on-state value to zero, and the other end of coil and supercapacitor remain at +2 volts.
That would be (40-2)*5 = 190 volt-microseconds on the coil.
Same as the product of coil L and the on-state current, in ampere-microhenrys. e.g. 1000 A * 0.19 uH, or 1700 A * 0.11 uH.

We don't need to ignore the resistive aspect of the circuit. The I*R drop from the coil's own wire resistance helps to stop the currrent. It's about 2 V during most of the 0.01-second ON pulse. Let's say 1 V, on average, during the on-to-off ramp. So it contributed 5 volt-microseconds of stopping power, bringing the total to 195. This "refinement" would increase the bottom-line inductance result by about 2.5% in this example.

Suppose you deliberately made the MOSFET gate voltage slew more slowly, enough that the "braking" voltage amplitude did not exceed the "breaking" V_DS specification on the data sheet. I think that would be less abusive to the transistor, even though the total energy is the same. Maybe a power MOSFET expert will speak up here. Isn't this a place for a snubber network?

I stand by my earlier recommendation of a plain clamp diode. It would be nice to find one with an impulse current rating (I^2*t, given in A^2*s). Not counting inductance of the diode circuuit, the braking voltage (between coil terminals) is then set by Vf(diode), on the order of 1 volt. When helped by the coil's IR drop, which we said averages about 1 volt, it might take about 100 us to bring the current to zero.

pisoiu

Fri May 26 2017, 07:21AM

Registered Member #61593 Joined: Mon May 01 2017, 07:55PM
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Posts: 14

Coil inductance numbers were mostly to indicate range, rather than an exact value. They were taken from some calculator from net, don't remember which one. If your calculation is 250nH, then it must be close to reality.
I don't know if I can measure true ramp of that spike. I have several scopes, but the fastest have 100mhz bandwidth. In this case I'm not sure if the scope shows the true ramp of the spike.
Thanks for analysis, it will sure help if I decide to proceed to next design. But this is uncertain due to lack of time.

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