Simplified mutli-stage coilgun idea

[Scicat111]

Here is a coilgun concept I came up with (and the accompanying thought processes), however I am looking for some input from someone more experienced in the field before comiting to it:
Thyristor switches are hard to use in reluctance designs since they cannot be turned off; this results in either extremely high optimisation requirements for the capacitor bank, or the use of MOSFET switches- those, however, have to be switched on, held in that state, then switched off again- which results in rather complex electronic circuitry.
Since inductance coilguns don't really care when the coils are turned off, would it be possible to use a simple [sensor-thyristor-capacitor-coil] circuit for a multistage design?
One thing which bugs me is that I haven't been able to find any homemade inductance coilguns, wondering whether there is an inherent drawback with them (other than higher power requirements).

[johnnyzoo]

I haven't seen many inductive coilgun designs on the internet. Most of them are disk launchers that have a single flat coil.

Multi-stage design should be possible and it might actually be easier than designing a traditional multi-stage reluctance coilgun. Worth trying definitely.

[Scicat111]

ok, it took some time to get my power supply running, but there are some updates /and a rather unfortunate hurdle/ to report; the hurdle being that I have, regrettably, failed to find any concrete metrics to work with when it comes to current, capacitance, and even voltage required to get a thomson-style inductance coilgun (the injector stage) off the ground. During my ~250V test, a thyristor rated at 300A (2N6506G) was (literally) cracked in half, from which I assume the power has reached the required limit- and the projectile apparently failed to notice. Any base requirements for this system would be greatly appreciated, since as of now I am left with the theory that the small dimensions (3mm OD; 1mm wall) of the copper projectile are in some way detrimental to the thing working as intended (it is a long shot, but I have sort of run out of ideas)

[davekni]

For such a tiny 3mm conductive (not ferromagnetic) projectile you will need a very short high-power pulse.  Thyristors will struggle with the required current rise rate.  Most are specified in the range of 50 to 150A/us.  They burn out if current rises faster.  (Can be pushed a bit for home use if the gate current pulse is high, ie. 1A instead of the minimum 50mA or whatever.)

Here's a video of my little penny-launcher project, 19mm diameter disks using a BTA16 TRIAC switch in the finished version:

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[Scicat111]

ok, updates: i managed to cook the thyristor cell I was using (2* 2n6506g in parallel); found BTA41-600 triacs (on ebay, yay)- however, I managed to fry 2 of those in parallel (should withstand 800A) at ~280V of charge (430 muF capacitor, 450V max)- and, even so, got worse performance than with the old thyristors. not sure what to do, my 2 working theories are: A) the lower R of triacs permitted higher currents, which meant the setup as a whole was limited to lower power (capacitor charge)- which degraded the performance (in particular the length of the pulse) and in the end fried the triacs. working theory B is "don't buy critical parts from ebay"- this has a problem however, as all my other ebay electronics works well- and the triacs do too, until the capacitor banks gets above 250V.

[Da_Stier]

Hi,

did you take a look at the voltages / currents across the Triacs / thyristors and the coil?
I had similar effects with switching elements dying.
(See this thread https://highvoltageforum.net/index.php?topic=1503.0)

Davekni pointed me in the right direction.
The parasitic inductive effects were quite a bit more substential than I thougt.
Maybe you have a similar issue?



Greetings,
Michael

[Scicat111]

wow, thank you- never occured to me. it is true that those triacs have voltage rating 200V lower than the thyristors I used previously, which would explain the ordeal.

[davekni]

Using thyristors, parasitic inductance is less likely to be an issue.  There is no rapid turn-off to create a voltage spike across inductance.  The one situation that could be an inductance issue is if the MT1/cathode connection for gate drive shares a large loop with the high-current path.  That could possible cause excess gate current after firing. 

What is your coil inductance?  One possible issue I'd mentioned before is rate-of-rise of current.

Paralleling TRIACs can be tricky.  If one fires first, it hogs the current, keeping the other from triggering.  Paralleling requires rapid gate current rise and separate gate resistors per TRIAC (unless gate current is high enough that internal gate resistance is sufficient to force current sharing).

BTW, for this sort of (mis)use, TRIACs are generally more robust with negative MT2 voltage and negative gate drive (called third-quadrant operation).  Even when using positive MT2 voltage, negative gate drive is more robust for rapid current rise.

[Scicat111]

I used a smaller capacitor for trigerring (due to my suspicions concerning the current paths), and the current seemed to be rather large (cooking a button shut once; failures, however, occured when trigerring from source directly as well); there have been instances when both TRIACs got destroyed, and instances when only one was- from which I assume there is another problem besides trigerring asymetry. I, unfortunately, have little chance of measuring most properties (as my setup is rather primitive). I tried connecting the TRIACs both ways, with, unfortunately, the same result.
Thank You very much for helping me.

[davekni]

Do you have any resistor in series with the trigger capacitor?  If not, you could be damaging the TRIACs with excessive gate current.  I think most are rated for 1A to 4A peak depending on how short the pulse is.

[Scicat111]

Thank you, that indeed was the problem- after fixing it, and changing the capacitor (~300V, 1mF) I have been able to launch an r=1.3cm aluminum coin ~1m into the air using 3 BTA41 TRIACs in parallel;
the efficiency is bad however, i plan on trying some sort of lateral disc launcher (either of the reluctance, or reconnection coilgun type)
I plan on trying the former first, with iron pieces which would extend the magnetic field to one side- creating the gradient necessary
Has anyone tried something like that yet?
ps. the idea is to circumvent the electromagnetic-aerodynamic tradeoff somehow

[davekni]

Not sure if this is relevant to your experimenting, but here's a video of mine on penny launching showing the advantage of an iron core etc.

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