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Capacitor switching: Originally an IXYS CS35 - 12IO4 SCR. Changed to 2 Teccor SK8065K (paralleled) after destroying the gate of the CS35 - 12IO4 from over voltage.
Power source:
Battery: 12v/4.7Ah Li - Ion (from portable computer).
Inductance: 100 uH (no projectile) - ~175uH (projectile in the middle).
Resistance: 180 mOhms.
Length: 6 cm (2.34 inches).
Inner diameter: 13 mm (0.51 inches).
Layers: 3.5
Wire size: 18 AWG
Storage Capacitor:
Capacitance: 3900 mF (with a 150 uF as a DC - DC converter filter).
Operating voltage: 450 V
ESR (estimated): 30 mOhms
Projectile:
Length: 15 mm (0.59 inches).
Diameter: 12.7 mm (0.50 inches).
Mass: 15 gr
Material: Type 1006 steel.
Estimated Values:
Projectile Exit Velocity: 46 m/s
Projectile Kinetic Energy: 16 Joules.
Momentum: 0.69 Kg/m/s
Storage capacitor Charge Time: 15 seconds.
Efficiency: 3.9%
Details:
Barrel Length: 70 cm (27.5 inches).
Total Weight: 4 Kg
But before this project come to life, i need some help and i have some questions:
1: I have problems with the booster charger. When i try to charge the capacitor, i can hear the 555 osc but i have no high voltage output. I cannot understand why this is happening. The boost converter circuit comes from Another Coilgun Site (ACG 188 booster charger).
2: I selected the length of the coil to be 4 times the length of the projectile. I know that in a good coilgun project the length of the projectile must be about the same length of that of the coil. I selected that length because i want the pulse from the capacitor to pass before the projectile reaches the center of the coil. But can the projectile be accelerated to 46m/s, even if it's only the 1/4 of the coil's length?
3: I'm searching for a good scematic charger about Li - ion batteries. Can someone help me with this?
Below you can see my current progress. It's only the main body, nothing electronic on it (sorry about the low quality image, it was taken from a really old K700i mobile phone)
After a long time i finally had some results. Below is main assembly construction. Still a lot of work to be done.
Main Assembly (left to right):
3900 uF/450 V storage capacitor. Projectile loader. Coil with thyristor trigger assembly mounted. Dampening switch (Q - switch).
Modules:
Li - Ion battery pack module. Power management module. Li - Ion battery charger. SCRr triggering module.
Projectiles:
Current Appearance:
Scope view:
Small Update:
The Li - ion battery pack was finally changed to implement the ability to monitor the charge - discharge rate. This is neccessery to prevent battery pack from being charged or discharged not equal, leading to battery pack explosion. Still to built the battery pack monitor (shunt type equilizer)
.
Full system simulation (Coil Gun Simulator by William Harriss):
Small Update:
The difference in calculated peak current between the two simulations is caused by the difference of the coil resistance: i wasn't careful enough to notice that the Coil Gun Simulator already accounted for the coil resistance and i added it's resistance as an external (cabling) one. The new simulation is below:
First test fire at half energy (200 Joules - no accelerator coil and projectile position adjustments for best position). Results are very promising although i have not recorded projectile's speed. It left a good mark on a piece of hard wood.
Completed the projectile accelerator module (barrel, firing coil, projectile loader, energy storage capacitor, etc). Added an optical projectile sensor (infrared LED + maching photodiode from an optoisolator) for sensing the projectile existance in the barrel (if no projectile exists, SCR cannot triggered - protection against dryfire EMF and wasted energy).
Completed the Li - ion battery charger design. Awaiting components.
Completed a Rogowski coil current sensor - probe. Calibration required.
Started building a new SCR module (to include the optical projectile sensor)
.
Completed the Li - ion battery charger and protection board. Tests will follow.
Still working on the SCR trigger board. Trigger input PFN (pulse forming network) from the fire switch to be designed and implemented.
Started building a Mazzili ZVS topology capacitor charger (by Uzzor2k). Ferrite core windings are on the way. This will replace the boost DC - DC converter (that never worked as expected). Hopefully, next week i'll have the first test results.
It has been a long time since last update. Work continues, although due to school i have not enough time to work on it.
SCR trigger module is complete (not tested yet).
A serious "space" problem arose: Li - ion battery charger and Mazzili ZVS capacitor charger does not fit into place. I need to redo the battery charger or the capacitor charger and this will cost a lot of time.
I made some tests again with the boost topology capacitor charger. I changed the coil with a bigger salvaged from an ATX PSU (no idea what value of inductance has). This time i had better results (but not what i hoped to): It charged a 1000 uF/450 V capacitor till 400 V. This indicates that i need to find the correct duty cycle in order to get higher voltage output.
After reading about the V - Switch and it's operation, i came up with an idea. The schematic diagram follows (just for presentation purpose - it is not operational). It works exactly like the original V -Switch (i only changed the voltage rating of C2):
D1 and R1 can be omitted if an unpolarized capacitor for C2 is used.
The idea is to turn off the SCR1 not at the time when the current peaks. Instead, we switch it off when the voltage of C1 has reached a certain voltage (here, 100 VDC).
A is the SCR1 turn off point (and SCR2 turn on point). The current drops to 0 (at t = 2 ms) and the voltage of C1 remains at ~100 VDC. That means we have reduced the pulse width time from Toff = 2.7 to Toff = 2 ms, with further reduction of the suckback effect and an increase of the projectile speed.
Small Update:
There was an error of the point A position as drawn (placed on the current graph instead of the voltage graph). The correct simulation is below:
Registered Member #2004
Joined: Sat Feb 28 2009, 11:43PM
Location:
Posts: 39
I can answer your question regarding projectile length...
The reason most people use a projectile the same length as the coil is because it allows the most interaction between the field and the projectile. The efficiency will be higher, but the projectile speed will probably be quite a bit lower. If you want a trade-off you can use a piece of 1/2" steel tubing (think brakeline) and add an iron slug in the frontmost portion. This will increase overall field interaction while reducing weight, and will also stabilize the projectile in flight.
Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
Thanks for the reply Fnord. I tried your suggestion with a smaller coilgun i built (just 150 J - 5.56mm projectile diameter).... instant and successful results!!!!
For the coil - projectile problem i have also thought about the inductive projectiles. That's it, a second coil instead of a ferromagnetic projectile. No magnetic saturation, no projectile slotting, no eddy current losses.... It seems that they must be the perfect coilgun projectiles.
That's one impressive body you have there!! That must be, together with Saz43's massive nuke coilgun, the most impressive body I've seen!
Concerning the charger, why not using uzzors2k's boost converter design? It has a proven reliability and a decent power.
To adaptate it to Lithium batteries I suggest changing the non inverting input of the LM311 with either a 5V zener or a 7805 regulator, and adjusting the inverting input accordingly; or either get an LM317T and adjust it to output 6V, that's what Uzzors designed his divider for. I will deduce you're using 2S batteries (7.4V nominal), but if you step up to 3S you'll see higher powers and a notch higher efficiencies.
If you still want to use your charger (which I understand, I've ran across similar issues), to start with, why don't you check that your diode can recover fast enough? Sometimes it is simply slow and blocks the current.
Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
This one year was dedicated building the body of the coilgun. 12 months of carefull design, obtaining pieces, cutting, milling, drilling, checking for corrections.... I am planning to post it to World's Coilgun Arsenal page and so, i tried to make it as good as i could (P.S. : i still have many to do before the body is completed).
For the charger problem, i use Uzzor's2k boost converter, but i changed the inductor with the one of the ACG booster charger. The value of the inductor i constructed is unknown, so there is a chance that the problem is the inductor. The diode i use is an HFA08TB60, specifically designed for such applications (diode was taken from an ATX power supply and has been fully tested).
The voltage that the charger stopped was about 200V. I should also check the duty cycle of the 555 osc. I've read that the Voutmax of a booster charger is found using the equation Voutmax < Vin/(1-max duty) and maybe the problem is the duty cycle (but i don't believe it). Uzzors2k booster charger FAQ has an answer for that problem, but it's referred to battery power, not the inductor value.
For the Lithium battery charger i've followed instructions from many lithium battery charger IC's and i found that the LM317T solution might be the only cheap and easy and i'll stick with it. I doubt if i can make it work efficient but it's the only way.
Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
After a long time i finally had some results. Below is the test bench for the energy storage, projectile loading, switching, EMF dampening sub - assemblies and also power management and thyristor triggering sub - assemblies.
Li - Ion battery pack * Power management module Li - Ion battery charger * Thyristor triggring module
(*: subject to change)
Projectiles:
Current appearance:
Scope view :
UPDATE : The Li - ion battery pack was finally changed, to be able to monitor the charging - discharging sequence. This is neccessery to prevent battery pack from being charged or discharged not equal, leading to battery pack explosion. Still to built the battery pack monitor (shunt type equilizer).
UPDATE 07/09/2011 : Added RLC and full system simulations.
RLC simulation (by Barry of course ):
Full system simulation (Coil Gun Simulator by William Harriss):
UPDATE 16/09/2011
Completed the projectile accelerator module (barrel, firing coil, projectile loader, energy storage capacitor, etc). Added an optical projectile sensor (infrared LED + maching photodiode from an optoisolator) for sensing the projectile existance in the barrel (if no projectile exists, SCR cannot triggered - protection against dryfire EMF and wasted energy).
Completed the Li - ion battery charger design. Awaiting components.
Completed a Rogowski coil current sensor - probe. Calibration required.
Started building a new SCR module (to include the optical projectile sensor). Capacitor energy dump trigger topology.
UPDATE 27/09/2011 :
Completed the Li - ion battery charger and protection board. Tests will follow.
Still working on the SCR trigger board. Trigger input PFN (pulse forming network) from the fire switch to be designed and implemented.
Started building a Mazzili ZVS topology capacitor charger (by Uzzor2k). Ferrite core windings are on the way. This will replace the boost DC - DC converter (that never worked as expected). Hopefully, next week i'll have the first test results.
UPDATE 28/09/2011:
Minor update. Mazzili ZVS topology capacitor charger current progress:
The 10K resistors and the 15V zener diodes are under the board.
UPDATE 24/10/2011:
It has been a long time since last update. Work continues, although due to school i have not enough time to work on it.
SCR trigger module is complete (not tested yet).
A serious "space" problem arose: Li - ion battery charger and Mazzili ZVS capacitor charger does not fit into place. I need to redo the battery charger or the capacitor charger and this will cost a lot of time.
I made some trials again with the boost topology capacitor charger. I changed the coil with a bigger salvaged from an ATX PSU (no idea what value of inductance has). This time i had better results (but not what i hoped to). The results indicate that it is necessary to find the correct duty cycle in order to get higher voltage output.
Registered Member #3792
Joined: Sun Mar 27 2011, 06:07PM
Location:
Posts: 136
Yes, that's why i'm posting the simulations.
I trust Barry's RLC simulator, so it must be a bug with the Coilgun Simulator. Does anyone know why is this happening?
Also yesterday i made the first test fires with very good results (even without any adjustment for the projectile - coil position and with only the half energy - 200 J). Results are very promising, and i may reached or passed the 2.5% efficiency.
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Reluctance Coilgun Sniper Rifle
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