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Yet another coil gun prototype

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Zuckerstange47

Fri Nov 30 2018, 12:37AM

Registered Member #61926 Joined: Wed Nov 22 2017, 04:08PM
Location:
Posts: 34

DerAlbi wrote ...

Well, if your mosfet is on, then there is just little voltage drop across the mosfet (since Drain sopurce is virtually shorted), so the specs make sense. Also if your mosfet is off (Drain-Source not "shorted"), which allows the 75V across drain.source, then NO current will flow, so you are below the 0.5A rating as well.
The only critical thing is, switching from On to Off. There is a transition period where there is actually 200Amps AND 75V across the mosfet. the faster you switch the shorter the time. And you will switch faster than 10Âµs. So dont worry about the 10ms rating.
However the energy from the coil must go somewhere. The current will continue to flow even if the mosfet is off. A freewheeling diode wont conduct instantaniously but it will also take time for it to "turn on". This effect (forward recovery) does not exist in Schottky diodes, but they often dont have a beefy current rating and only 2x overload safety - in contrast to silicon diodes which suffer from slow forward recovery but are much more resistant to overloading.
Why am i telling you that? The time it takes the diode to take over the current is time that the mosfet spends in the transition state between on and off - this is what is (mainly) shown in the SOA diagram.
Honestly, just try and see what happens.

I guess my question must have been really stupid

I am very grateful for your patient explanation!
For the switching to be as fast as possible I was thinking about using a MOSFET-driver for each MOSFET. My driver has a rise time of about 35 ns for my MOSFET, so I assume that should be fast enough.
As a suppressor ("Schutzbeschaltung" in germon, is this the right word?) I am using a Zener diode (1N 5908, 5V, 1500W) and a diode (P600M, 1000V, 6A) but I have not figured out how to calculate the time for the transition state.
Does your last suggestion indicate that I will notice the MOSFET beeing destroyed if my suppressor it not good enough, or do you mean to see the result on the oscilloscope (which I do not own)? So far there were no defects on my prototype (as far as I know).

( Zener and diode:

)

V2006 wrote ...

Zuckerstange47 My offer is still valid.
Buy coils from me and become the first designer on this miserable planet.

I do not really understand what the designer-part means, but my coil construction line is ready for production

the_anomaly wrote ...

Zuckerstange47 My offer is still valid.
Buy coils from me and become the first designer on this miserable planet.

The joy of learning is in the journey not the final result. Zuckerstange47 is learning by doing.

Exactly!

DerAlbi

Fri Nov 30 2018, 08:05AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Dont worry about the life of your mosfets right now. They will blow anyway at some point, so just buy enough instead of being overwhelmed by complexity.
1.5kW-Zener: sounds sketchy. at 5V that gives your 300A, but a (is it really a Zener or is it a TVS?) Diode will suck up that heat energy in its silicon - once that gets hot enough it will die.
TVS diodes with 1.5kW will never dissipate the 1.5kW, its only a theoretical rating because a pulse load is assumed. This pulse load (Transients) arent supposed to be longer than 10Âµs at some points.
There are people who built Coilguns with Zeners and it worked, but i think you need to use a much bigger Zener.
For a start, you can simply use a normal diode in forward conduction and see where it gets you. But try your Zener first. But check it after every pulse.
Regarding your switching speed: dont worry. Knowing the usual Coilgun setup, the actual switching speed of your mosfet wont be an issue. The time it takes for the current to stop flowing through the mosfet is another issue. I guess your mosfet will be fine.
Just try. Post a picture of your setup first ^_^

V2006

Fri Nov 30 2018, 09:08AM

Registered Member #61550 Joined: Thu Apr 06 2017, 03:23PM
Location:
Posts: 86

Zuckerstange47 The part is a coil of unique design.
But everything else is possible
There are no more such coils on this planet.
Better learn from the best tutorials
Good luck

DerAlbi

Fri Nov 30 2018, 09:42AM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

WTF.
V2006.. sometimes... i am just not sure if you are a troll or actually that ...... weird.

V2006

Fri Nov 30 2018, 05:37PM

Registered Member #61550 Joined: Thu Apr 06 2017, 03:23PM
Location:
Posts: 86

DerAlbi mutually))
here 15 years they write the same thing
I just wanted to help

V2006

Fri Nov 30 2018, 05:45PM

Registered Member #61550 Joined: Thu Apr 06 2017, 03:23PM
Location:
Posts: 86

they have been discussing the same thing here for 15 years - mosfet , diode ...
I wanted to help

Plasma

Sat Dec 01 2018, 11:08AM

Registered Member #61406 Joined: Thu Jan 05 2017, 11:31PM
Location:
Posts: 268

This might help limit the damage to the MOSFET,,a diode will only protect against forward emf not back emf. If you place a capacitor and resistor in series and then parrellel across the coil it will asobe the spike if it has a large enough storage

Edit picture isn't showing

V2006

Sat Dec 01 2018, 12:33PM

Registered Member #61550 Joined: Thu Apr 06 2017, 03:23PM
Location:
Posts: 86

I know it.
I wanted to say that in the discussion of small details, the very essence was lost here
So you can do another hundred years the Electromagnetic Projectile Accelerators
No success
It's not like you think
You lost the essence here the Electromagnetic Projectile Accelerators

DerAlbi

Sat Dec 01 2018, 03:18PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

The essence of the hobbyist coilgun is learning electronics. Communicating about effects in electronics and how to read datasheets, understand the SOA and so on, is therefore extremely on topic. And everyone has to learn it again and again, that is natural.

The essence of a professional coilgun is very different. Its technically a linear reluctance motor which is very complicated. Its metic is power densitiy - nothing else matters; the often discussed efficiency is only the means to achieve a good power efficiency.
So "Shots per second" * "Kinetic Energy per Shot" / "Device weight" gives you a number in Watt per Kilogram. This is generally the figure of merit a motors engineering is judged on.
I have never seen you having any credible scientific description of what you achieved except shaky 240p camera footage shooting at a door or tin cans. Actually, seeing tÂ´any built in action is the least interesting thing: numbers are the only thing that counts in the professional league. I didnt find anything relevant from your side until now.
I dont really understand how you complain about the state of things.
It seems strange to reply to Plasmas "put a snubber circuit around the mosfet" (thats what he tried to say - just paraphrasing for better google words

) with "i know it" as if the thread was made for you.
Generally you hijack a lot of threads and post your coilgun videos that are without any information or relevance to the thread.
The spam in one of your previous posts for example linked to 2 videos:

and

.
Whats the point?
Do you think that video without any numbers or description of how its built or what exactly is innovative is any contribution to the community?
All anyone sees is that something moves. How is that any superior to what Zuckerstange is trying?
Please make your own thread, provide the world with the wonders you engineered there and please provide real numbers including the procedure how you obtained them. If you have done anything ground breaking, people will acknowledge it.

Zuckerstange47

Sat Dec 01 2018, 03:25PM

Registered Member #61926 Joined: Wed Nov 22 2017, 04:08PM
Location:
Posts: 34

DerAlbi wrote ...

For a start, you can simply use a normal diode in forward conduction and see where it gets you. But try your Zener first. But check it after every pulse.

I have been using the zener + diode for about 20 - 50 shots with no problem so far, but the voltages were not very high yet. I will post an image when I plan to change the setup.

Plasma wrote ...

This might help limit the damage to the MOSFET,,a diode will only protect against forward emf not back emf. If you place a capacitor and resistor in series and then parrellel across the coil it will asobe the spike if it has a large enough storage

Edit picture isn't showing

I cannot upload pictures as well, thats why I used this uploading service last time:

(It belongs to the blender 3d-software community)

I would be very interested in learning more about that topic.

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