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Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
I was thinking about switching the stages in a multi-stage coil gun and I came up with the idea you can see in the schematic attached. It's a really messy schematic as I drew it very fast in paint. It's probably already been done if it works, but I haven't seen it and would like confirmation if it does work.
The triangle is the projectile and shows the direction of the projectile and the rectangles are the coils.
Update: Or do I have to put R1 and R2 at the collector side of the PNP transistor?
Wow, have my respect for non-standard thinking! So the idea is "the coil must be ON when projectile dims the gate, but as soon as the next gate is dimmed, coil goes OFF no matter the condition of the gate". Good idea for coilguns with projectiles longer then the coils, but the schematic, even if would be drawn error-less, would not reliably work in practice unless the comparator with big hysteresis is used - otherwise with the same levels at + and - you will have noise at output. Unless resistors for positive feedback is added. Besides, you do not need pnp transistor - simply switch positions of phototransistor and ballast resistor to invert the output.
Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
Thx for your reply Yan, but shouldn't I keep the PNP transitors and feed them via a voltage divider so you would have different voltages when both fototransitors are off so the igbt's are still deactivated? or is this already fixed by the resistors for positive feedback. Also, would you just place those resistors over the output and the +? If I'm wrong, can you draw the schematic the way it might work? and maybe give a simple explanation? I'm sorry, but I don't know very much about electronics, especially not when it comes to comparators, but I can read something about them and see if I can learn some more about them.
Edit: Can't you use another voltage divider at the - of the comparator so it will get a slightly higher voltage than the + when they are both at 0? This way I don't think this positive feedback is neccesary, but maybe a really small positive feedback could still reduce the small amount of noise? Correct me if I'm wrong.
Here is my recommendation (not schematic, cus I am lazy to draw it): Phototransistor and ballast resistor (say, 1KOhm) are the same as in your schematic, and let's call the connection point between them as "control point". Then, from the control point to + goes 100KOhm resistor. And from output to + goes 1MOhm resistor. The - goes directly to the control point of the next gate. Draw it nicely, post it.
The opamp works simple: if voltage level on + is higher then on -, output is high. Otherwise - low. Mention that there is no default value on inputs - left floating it may have any potential, resulting in unpredictable output behavior.
Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
What would be the voltage to supply to the circuit? 5V? I think 5V would be nice as it's commonly used and it's not far above the voltage the average LED takes. Also I understand that part, but didn't fully understand how those resistors for feedback work. I think I do know after watching a video on youtube. I'm currently drawing.
5V is good for this part of the circuit, but too low to drive the IGBT, but you need a MOSFET driver IC for that purpose anyway - do you understand that driving the IGBT's gate by opamp is inappropriate due to low output current and slow speed of output change? Anyway, there is no guarantee that the schematic will work as expected - need to be tested first.
Use non-inverting MOSFET driver and connect output of the opamp directly to the input of the driver. Simple as that. But first, you need to build 3-4 gates of the schematic we were discussing and check if it works at all - it may not. All I can say for sure is that yours was surely wrong. ^^
Registered Member #4932
Joined: Thu May 17 2012, 01:42PM
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Posts: 59
Here's the schematic. thx for your help btw even though it might not work (the first time) Schematic
Btw, how do you do those green arrow links?
Edit: Would it be possible to make some kind of huge spear cannon this way which uses a very long and thin projectile? Of course you can still load it with normal, small ammo and you wouldn't have to make the entire spear ferromagnetic of course.
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Multi-stage coil gun, switching the stages.
