Yet another coil gun prototype / Electromagnetic Projectile Accelerators / Forums

Forums

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

« Previous topic | Next topic »

Yet another coil gun prototype

first previous 6 7 8 9 next

Move Thread

LAN_403

DerAlbi

Wed Dec 19 2018, 06:54AM

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

Wire length is not really a metric that makes sense to use. It is basically the same thing as making the resistance constant while changing other parameters (except the wire diameter). However resistance is always a parasitic in a coilgun.. so why would you focus on parasitic, instead of the actual parameters that matter

I think you get the point.

[Edit: Off topic content removed]

Zuckerstange47

Thu Dec 20 2018, 06:09PM

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

DerAlbi wrote ...

Wire length is not really a metric that makes sense to use. It is basically the same thing as making the resistance constant while changing other parameters (except the wire diameter). However resistance is always a parasitic in a coilgun.. so why would you focus on parasitic, instead of the actual parameters that matter

I think you get the point.

[Edit: Off topic content removed]

I thought it would be a good idea, because with a fixed maximum switching current of the MOSFET, the resistance of the coils should be identical. But I think I understand, that it does not make much sense, yet I do not know how else to make a "fair" comparison.

DerAlbi wrote ...

I would fully agree. However, please dont be ignorant and at least know that you will damage your capacitor with this circuit. Electrolytics happily withstand -1.5V reverse polarity but you subject them to maybe -15 to -20V.
If you dont care about the life of your capacitor, then this is ok too - they will still last a while. But dont abuse capacitors, and later decide to put them into a bigger capacitor bank. While one failing capacitor (inner short circuit) is likely to be ok, if additional energy is delivered from the outside things can become messy.

Do you know what will happen to the capacitor, when it fails because of the missing snubber? Will it just stop working, or might it explode? I thought I heard a fizzling sound after my last shot, but the capacitors do not look different from before ...

DerAlbi

Thu Dec 20 2018, 06:58PM

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

Hahaha

That is soo good. The typical Coilgun circuit ruins capacitors, and mostly no body knows or even notices it, because before serious damage is done, most have given up. It was always my suspicion (because it must be true) but no one ever confirmed.

The noise comes from is hydrogen forming. The capacitor will degrade over time, but i dont think it will have serious consequences. One fully charged ELECTROLYTIC capacitor on its own will never explode [dont apply that to high energy foil capacitors]. The energy stored per weight is less than (or around) 1 Joule per Gram of capacitor weight.. This energy is easily consumed by the thermal capacitance of the involved material. Even if only a small part of the capacitor fails (as short circuit) it is not different from an external shot circuit which is obviously fine. Capacitors explode when they fail and there is a constant supply of energy - like in a power supply.. if one capacitor is shorted, it may heat up and this continuous heat up can build up pressure that leads to explosion. The same is true for a very large capacitor bank (but not for a small one), if you parallel many capacitors.
As far as i know you can test the health of your capacitor mainly by measuring its leakage current.
What you do is, putting a 1k-Resistor (a high power one, or a low power one with a diode in parallel) in series with the charging circuit. Then you measure the voltage drop across the 1k resistor and wait until it is below sqrt(C*V) in ÂµA (this is a typical value from the datasheet).
First you charge only to low voltages, then you increase the voltage up to the maximum rating.
This process is known as "reforming" the capacitor on this miserable planet..

If you have a bathtub, read about capacitors:

Zuckerstange47

Thu Dec 20 2018, 08:13PM

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

What I noticed is that the voltage drops constantly after the capacitor was charged (about 10-20 seconds from 10V to 9V, roughly estimated). I guess that is a sign of damage as I did not see that behavior before.
I will try a reformation like you said (but without the math part

), starting at a supply voltage of 3V...

When saying the "typical coilgun", do you mean that most people construct their coilguns in a faulty way? Maybe many of the coilgun-makers do not have much more knowledge than me, who only started tinkering around with electronics by getting kicked into touch with the Arduino platform without having a professional background on this topic. Btw: How did you end up being king, did you study this? I hope this question is not too personal or off-topic, again... you do not have to answer

DerAlbi

Thu Dec 20 2018, 08:53PM

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

When talking about the typical coilgun, i mean the SCR design. People like to think up ways to make the current drop faster after the capacitor is discharged to avoid suck-back, but most circuits actually reverse polarity on the capacitors. That is a common flaw, but as said, most people dont notice damages - even if, they dont know what is normal or not; a coilgun is a pretty forgiving project, it will almost always work and deliver results. Also, most coilgun builds are made from scrap parts anyway.. a damaged capacitor is not really a problem and damaging a capacitor takes time (many shots).

Some leakage current is always ok! Discharging from 10V to 9V in 20sec when a 10MOhm voltage meter is connected is actually quite ok.
As said, try to measure the leakage over a 1kOhm resistor. If the voltage drops below 5mV, then you are good. Leakage increases with the applied voltage. Electrolytics have their aluminium oxidized - the oxide layer is the insulation that gets damaged. If it just gets thinner, it will still work as insulator at low voltage, but becomes leaky at high voltages.
Just put it in your calculator: if you have 10.000uF and 50V, you get sqrt(10mF*50V) = 0.7 put ÂµA behind it, and you are done. Now 0.7ÂµA is a very low leakage current, there is always almost an offset to this, like 3..5ÂµA. so anything below 5.7ÂµA (or 5.7mV over 1kOhm) is healthy.

I did not study coilguns

I studies electric engineering at university, but that teaches nothing about practical application of knowledge. It helps to get an engineering thought pattern if you do project at home however. During my university years, i took mostly curses that had nothing to do with what i do at home - just to get the broadest knowledge possible. I learned about information technology, wireless data transfer modulation schemes, error correcting codes, encryption, network behavior, game theory and so on. For some time I was an analog IC designer and worked with 28nm and 45nm IBM technology and now i am the R&D department of a company that builds 5G base-stations. Nothing about coilguns here

I just read a lot, try to distinguish good from bad arguments and i really like to play with simulation. It is very important to learn all the equivalent circuits of all components - all the different level of precision and complexity the component possesses and when to apply which model. It takes a lot of project to learn how to apply models and one should never give up; even if something does not work, the simulation must be brought to a detail until it shows the same problem.
I think the "king" part was a joke and i really do not consider myself as this. I think, i simply have wasted the most money and time on a coilgun. This builds up experience that others do not get, if they change the project after 5 months or if the are stuck with a particular design. I also tend to be a very active user here, because there is not much coilgun community left. I think young people are not into technology anymore; swiping screens is all they can do. So i help anyone i can here (who is interested in following engineering methods)
My coilgun is a very different design so i actually have no practical experience with most of what is discussed here (the SCR design). Everything I know about it is from simulation and the involved math, and seeing patters in the result of the work of others. So take everything with a grain of salt.

Zuckerstange47

Sat Dec 22 2018, 06:42PM

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

Thats interesting information about your background - now I understand how much effort it must have taken to get a solid foundation of knowledge. For a moment I had the impression you considered yourself old, because of talking about the "young people", but incidentally I just read another post about your "rail"gun (in an airgun-forum), where your were described as a "young engineer". I hope to see the video about your design, that was announced there, some time, or have you stopped the project completely? In this thread I was confused that the subject switched towards tripling the energy to reach 60 joule...

Is this just an great dream by your uninformed video-partner, or is it a realistic goal?

Concering the leakage my measurement must have been wrong, because the capacitors were still connected to the inactive circuit. Now I don't see the pointer of the multimeter move at all (it is an analogue one, I do not know how many ohms it has). I already did a lot of reforming, but I am not sure how big the steps of voltage-increase should be, but I guess it probably is not that important.

Zuckerstange47

Sun Dec 23 2018, 11:36PM

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

Today I started comparing the two coils and destroyed the coilgun, twice. After doing 19 shots with the shorter and 9 with the longer coil, I could not recharge the capacitors anymore. When starting the recharging process, it resulted in drawing a big current from the power supply, until it was turned off (even when the charging-switch was deactivated again). After swapping the MOSFET (luckily I had installed it on a socket) it was recharging and shooting just fine.
But two shots later something else smelled burnt, and I don't know what it is at the moment, I will have a closer look at it in the next days. What I know is, that I cannot charge the capacitors anymore when using the usual circuit, but I don't see why and the indications are different from the first time (the multimeter pointer goes up to 1V as long as the charging-supply is turned on, even when the MOSFET is unplugged). Outside the circuit the capacitors do charge perfectly.

However, the resulting speeds of the two coils were very similar, but the shots were not sufficient for an objective comparison. Strangely the speed trap gave faulty results from time to time, but only about one or two for the short coil (that was tested first) and about 50% of the measurements of the second coil. With that I mean that the results make the appearence of beeing wrong, but maybe even more were. Perhaps I should install two speed traps and check that the results are identical...
The speed of the projectile using the short coil was about 8.5 m/s (up to ~0.77 Joule) that was confirmed by many shots that were all done with capacitors charged to 30V. I learned from that experiment, how important the starting position is to incease efficency - previously I only reached about 1 Joule with a much higher voltage.
The longer coil resulted in a resulting projectile's energy of about 0.86 Joule, which was only achieved twice (because of the MOSFET-failure) with a higher efficency than the shorter coil (but I have not calculated it). The last shot with the new MOSFET was displayed as "2.32 Joule", and it felt relatively strong in my hand (

), but I do not trust that one.

So, the next problems are to repair the coilgun and to think about, why the speed measurement is wrong so often, and if the longer coil could be the reason for the failures. Also I am having the snubber in my mind.

DerAlbi

Tue Dec 25 2018, 10:28PM

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

My project is currently on ice. Without money, i cant do anything. (on this miserable planet!) I also would need a partner or 2.. to really get a good thing going. Complicated.

To help you with your problems, we need schematics of your gun and your charger.
It sounds like the negative capacitor voltage swing strains your charging circuit (maybe the rectifier diodes?) Give more detail about the current state of things.
Please with as much detail as possible (Mosfet types, Diode types, Resistor vaues, capacitor values, nominal in-out-oltages and so on; which component has a heatsink?)

V2006

Wed Dec 26 2018, 08:59PM

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

fur DerAlbi: Daher mÃ¼ssen Sie etwas tun, um Investitionen anzuziehen. Ab 1600 Euro werden Sie nicht weit gehen. Die Bewohner dieses miserablen Planeten geben Ã¼berall Geld aus. MÃ¼ssen Sie sie in die richtige Richtung schicken.

Zuckerstange47

Thu Dec 27 2018, 04:43PM

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

DerAlbi wrote ...

My project is currently on ice. Without money, i cant do anything. (on this miserable planet!) I also would need a partner or 2.. to really get a good thing going. Complicated.

To help you with your problems, we need schematics of your gun and your charger.
It sounds like the negative capacitor voltage swing strains your charging circuit (maybe the rectifier diodes?) Give more detail about the current state of things.
Please with as much detail as possible (Mosfet types, Diode types, Resistor vaues, capacitor values, nominal in-out-oltages and so on; which component has a heatsink?)

Thank you for your offer! After it stopped working, I uninstalled the perfboard containing the MOSFET and noticed, that there was an electric continuity between two connectors, that should not be there. Not until motivated by your post I started working on it again and measured a resistance of about 50 Ohms between those connectors. I do not know how the circuit could have changed it's own, but this conductance could not have been there before. I then took a box cutter and cut beween the perfboard pins which seemed to be the reason for the continuity and the problem was solved!
Afterwards I made six shots, which went very well and the speed-measurements were plausible, too (so I assume that they also might have been wrong because of the hardware-failure). I varied the projectile's starting position and reached 0.94 Joule, which is a new record for my 37 mF capacitors charged to 30V (they were discharged to 18.1V by the shot).

Then I wanted to take another shot, but this time I charged to 45V: The result was that the projectile accelerated intensly but was then sucked back and came to a stillstand after about 5cm. The capacitors were completely discharged, so I suppose something broke while the shot was on the way. Now I have the same problem as before and cannot charge the capacitors. I will disassemble it again, soon.

My components are (circuit image appended):
MOSFET: Toshiba TK72E12N1, S1X, N-Ch, 120V, 179A
Driver: MIC4420ZT, 6A, Hi-Speed, Hi-Current Single MOSFET Driver, 10.000pF load Drive
Diode: P600M, 1000V, 6A
Zener: 1N5908, 5V, 1500W
Capacitors: 22 mF and 15 mF, 50V

The charging circuit is a step up converter with a resistor in series. It is only connected before the shooting procedure takes place, using a switch. I do not use heatsinks, but I noticed there must have been some heat at the soldering of the MOSFET mount, as the white, printed PLA had a brown spot there.
Would it be possible that a spark originated there and melted the soldering tin again, so that the short circuit occured and the coil could not be turned off?

I will try to upload the circuit image again, this time I marked the position where there was an electrical conductance with a resistance of about 50 Ohms.

On the subject of your coilgun, I think the marked might be too small for the high price of a commercial product (1500 $â‚¬ when I remember correctly) and most potential customers or investors will not really understand what it is about. When seeing the post of the weapon-expert, who had seen your prototype, I had the impression, that even he did underestimate the effort you put into it, or would have to be done to triple the energy output.

PS: I think I can still not append images here, so:

PPS: Now I am measuring only 10 Ohms instead of 50, but I do not see were the two lines are connected. Maybe I should just rebuild the board completely...

first previous 6 7 8 9 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob