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ben5017

Thu Dec 13 2012, 04:30PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
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Posts: 156

If my wire can blow between layers at 540V, how do some of you guys play with a couple thousand volts? Do you get wire with a special HV insulation?

Yandersen

Thu Dec 13 2012, 09:44PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

"...about the wire. Hardly it will blow inbetween the layers, but keep ends wrapped with additional insulation and away from each other."
I play with 780V. No accidents since I started to insulate iron washers holding the coil from both ends. Used just 1 layer of paper glued to the washer by epoxy.

ben5017

Tue Dec 18 2012, 05:46PM

Registered Member #3315 Joined: Thu Oct 14 2010, 04:23PM
Location:
Posts: 156

Yan, I miss understood your previous post.

Update: I got the zeners set up on both the accelerator stages. It seemed to give a slight increase in velocity, since the sensor is now right up against the coils while minimizing suckback. I have no way (as of yet) to measure the velocity so I have no hard data to provide. However I will put together some data and share it showing various different configurations and average results for each once I do.

Yandersen

Tue Dec 18 2012, 10:13PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

I just highlighted th word "ends" which means you have to pay special attention only on insulation of wire ends sticking out of the coil bobbin, as it is 540V inbetween them.

You can roughly estimate the difference in kinetic energy by shooting into a modelling clay by the deepness of penetration.

If you are ready to be more precise, you can mount two optic gates on a piece of pipe, sensors distanced by, say, 10cm from each other, and check the time it takes for projectile to pass from one sensor to the next via an oscilloscope connecting probes to the optic sensors.

MrFlatox

Sun Jan 20 2013, 09:37AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Hi !

I am also planning to build a LiPO powered coilgun, so I've naturally read this thread. You are talking about putting a zener diode in series of the damper diode to hasten the current drop in the coil, and thus preventing any suckback effects. As I plan to use mosfet to switch main current, I must choose a zener diode with voltage rating just a little below my maximum drain to source voltage ok ? I tried to simulate this , and I got that :

Graph shows the voltage that the mosfet will see. For this simulation, I chose a 51V zener diode. My question is : Why do I get 130V peak voltage with a 51V zener diode ? Is my schematic wrong ?

Thanks in advance

Ash Small

Sun Jan 20 2013, 11:25AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

MrFlatox wrote ...

Graph shows the voltage that the mosfet will see. For this simulation, I chose a 51V zener diode. My question is : Why do I get 130V peak voltage with a 51V zener diode ? Is my schematic wrong ?

Thanks in advance

Try putting a capacitor accross the zener. If the zener is taking a few uS to 'kick in', this should stop the voltage rising until the zener starts 'doing it's job'.

I can't be certain I'm right, but it's worth adding a cap to the simulation to see. Maybe ~500uF, but I'm guessing here. Try a few different values.

EDIT: It may be better to try a capacitor accross the MOSFET instead.

Yandersen

Sun Jan 20 2013, 11:42AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Please, don't listen to the suggestion above.
Voltage is higher because of the coil's resistance and zener parasitic resistance - both add voltage drop due to the current running through. Coil resistance should be considered as unavoidable, but zener can be found with better parameters. Voltage drop for different currents are usually given in a datasheet table for TVS diodes.
Simulation seems to look right.

MrFlatox

Sun Jan 20 2013, 02:09PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

So how could I protect the mosfets, and have a quick de-energization of the coil at the same time ?

For example, let's say I am using a 75V mosfet, (or multiple stacked in parallel) and my battery provides 33V, what should I do to prevent the mosfets from blowing up, and have the fastest current decay in the coil.

Another question : in a battery powered coilgun, the maximum current that could circulate is dictated by the discharge rate of the battery, so the load (the coil) should be designed so that its internal resistance matches the max current the battery could supply ? Or is there another way, for a given coil and battery, to limit the current the coil receives and thus the current the battery is giving ?

Thanks

Ash Small

Sun Jan 20 2013, 02:27PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yandersen wrote ...

Please, don't listen to the suggestion above.

Surely a capacitor accross the MOSFET will absorb the spike and protect the mosfet?

It's a fairly standard snubber device.

Yandersen

Sun Jan 20 2013, 05:11PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Hard to predict the exact capacitance value.
Flatox, just put more powerfull zener with a bit smaller clamping voltage.
For 33V battery and 75V MOSFET you have 42V of maximum drop at a coil (so ideally current may decay just a little bit faster than it's rizes). If you limit the current by coil' resistance, than you will have the same 33V drop on coil and zener may be up to 42-33=9V. Zener will not do much difference. The less V drop is on resistance opposite to the zener, the more linear and fast current decay will be. Limiting current by resistance is bad anyway.

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