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The normal coilgun must return unused energy (that is what you keep calling "flyback current") back into the cap. Even with a perfect timing and other perfect conditions it is not possible to convert all of cap's energy into the projectile kinetic energy - 8-30% is a maximum of the conversion rate (assuming no suckback and no heat dissipation). To make coilgun operate efficiently, all unused energy must be returned or reused by subsequent stages. The easiest way is to use 3-element circuit - non-polar cap, thyristor and a coil. All unused energy will stay in a cap after a shot, howhether, cap will be repolarized. You may add a second stage to reuse the energy and return cap into normal polarization. I did it once in my first recuperational coilgun and get 6.6% of efficiency from the first stage and 13% from second one; cap had around 25% of initial energy left after the shot. You may try the same - this design is even simplier than classic one. Just make sure charger is disconnected from a cap before the shot, otherwise it will be fried by repolarized cap. The second normal design is a halfbridge. Here coil discharges unused energy back into the power source. Saz is an expert in this field, he can say more. :) All I can mention is that halfbridge is pretty complicated for the beginners and operates on transistors, which cost more than thyristors and generally less powerfull (at least price and size compared).
All other designs which burn out unused energy are monkey designs - inefficient and do not inspire hobbyists to learn electronics. You can easily determine those by damper diode used. But being honest, I did one like that long time ago. :)
Registered Member #8032
Joined: Tue Nov 13 2012, 01:22AM
Location: Chicagoland USA
Posts: 33
Thanks a lot for the explanation Yanderson.
So basically I dont think I can use the first design you mention the "3-element" as I would like to keep my power supply (batteries) connected to the caps. And I don't like thyristors as my understanding is they don't turn off and I need them to before the suck-back occurs.
I would like to hear more about the half-bridge design. Especially if the unused energy can be discharged back into the cap or the batteries I'm using for the power source. I am a bit scared of the complicated design tho but I can start small I guess.
Well, if it is complicated for you to connect a coil to the non-polar cap via thyristor, than just forget about half-bridge - you need to deeply learn electronics for that. Thyristor (the same as SCR) is a simpliest switching element, cheap and powerfull. Once triggered it will stay conductive up untill all current will go through it. When connecting a coil to the non-polar cap through the SCR, you must give it just one short single pulse. Energy goes from cap to coil, than back into cap and SCR closes after that. And it is not possible to trigger it again because the inversed polarity will not let it become opened. But if you add a second stage connected to that cap via another SCR, but turned upside down, this one may be triggered. It is the simpliest coilgun design and it is few times more efficient than classic one. Here is my two-stage recuperational coilgun schematic: Sure don't wonna try?
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Yandersen wrote ...
Once triggered it will stay conductive up untill all current will go through it.
I've a question here....I read (and posted a link) that they stay conductive until the voltage across them is zero (or negative).
Now, if it feeds an inductor, is not the current 90 degrees out of phase with the voltage? Won't there still be current flowing when the voltage is zero?.....(or have I missed something?)
As any semiconductor device, zero voltage means unconductive state at which flow of the current is not possible. Vice versa is true also - till current runs, voltage drop will be around 1-2V. Inductor keeps current flow constant, allowing voltage to jump any way possible, while cap keeps voltage potential at the same level allowing infinite discharge current. All this in ideal case, of course. When cap discharges onto coil through the thyristor, then voltage drop on thyristor is around 2V while current is running. When cap is at 0V, thyristor is at 2V, and coil is at peak of it's current value while maintaining -2V. Running current starts to charge cap again (repolarizing it) and decrease. When current drops to around 0A, voltage on cap is -Vinitial and thyristor closes changing anode-cathode voltage drop from +2V to -Vinitial. Use an LT spice (I never did as I have a real oscilloscope).
"But I already bough all these polar caps spend big money on them too, sucks that I would have to buy new ones now" Don't you know that you can make a fake non-polar cap from two electrolithics? Just connect two of the same electrolithics "minus-to-minus" and put protection diodes for each of them and you have a polar cap with 1/2 capacitance. This way you can keep charger connected to one of the caps without risking to burn it during the shot. Real non-polars polypropylene caps are just better by all characteristics except lower capacitance-to-volume ratio (less energy per volume they store, at least by now - future may be promising).
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Yandersen wrote ...
As any semiconductor device, zero voltage means unconductive state at which flow of the current is not possible.
Now I'm confused. Say the cap is charged to xV, thyristor opens. Current is zero.Voltage on cap falls as cap discharges, current through inductor rises. Current is a maximum when voltage across cap is zero. current continues to flow, gradually decreasing until cap is charged to -xV (assuming no losses), so current is at a maximum when voltage across thyristor is zero. (Voltage and current are 90 degrees out of phase.)
What am I missing? (Apart from the fact that the thyristor will close when the voltage across it is zero, which is when current through it is at a maximum)
From Wikipedia: "A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased (that is, while the voltage across the device is not reversed)."
(EDIT: Post edited.)
Yandersen wrote ...
Use an LT spice (I never did as I have a real oscilloscope).
I have two oscilloscopes, but neither has a 'current' setting, they only measure voltage.
OMG... Current is measured by a voltage drop on a resistor.
What is forcing thyristor's voltage to some value in a circuit with an inductor? Nothing! Inductor voltage changes any way possible trying to stabilize the current. If thyristor is "jammed" between two caps - that's completely different thing. Voltage drop on a thyristor does not change all the way down to the zero current state when it suddenly closes - only then bias changes from +2V to -xV. This sudden voltage change event is filtered via resistor-cap filter in a two-stage coilgun circuit on the picture above, and works perfectly reliable in my practice. Actually, there are two triggering shots for the second thyristor - when first one opens and when it closes, but first time cap's polarity is not suitable for the second thyristor to open, so it got triggered only after the first SCR relaxes.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Yandersen wrote ...
OMG... Current is measured by a voltage drop on a resistor.
Yep, I have come across that before, thanks for reminding me.
Yandersen wrote ...
What is forcing thyristor's voltage to some value in a circuit with an inductor? Nothing! Inductor voltage changes any way possible trying to stabilize the current. If thyristor is "jammed" between two caps - that's completely different thing. Voltage drop on a thyristor does not change all the way down to the zero current state when it suddenly closes - only then bias changes from +2V to -xV. This sudden voltage change event is filtered via resistor-cap filter in a two-stage coilgun circuit on the picture above, and works perfectly reliable in my practice. Actually, there are two triggering shots for the second thyristor - when first one opens and when it closes, but first time cap's polarity is not suitable for the second thyristor to open, so it got triggered only after the first SCR relaxes.
I'm aware that you get voltage spikes on flyback transformers, etc. when the current is suddenly stopped. I can also see that there must be a voltage drop across the thyristor, because it has resistance, but I still don't see how there can be a voltage drop of 2V across the thyristor when the voltage across the cap is zero. I can see that, when the thyristor switches off, and current stops flowing through the inductor, the voltage across the inductor rises to ~ infinity, but the thyristor is already off by then.
I still can't see what happens in the thyristor when the voltage across it is between 2V and -xV. I suppose I'll have to buy a thyristor or two, and 'scope it.
Are you saying that the thyristor switches off when the voltage on the cap drops to 2V?
What I'm saying is written in black on white in the posts above. From the point the SCR opens and up until it closes voltage drop over it is constant: anode is 1-2V over the cathode, no matter the cap's voltage and polarity. Voltage across inductor changes in the way to maintain that drop. Yeah, by a couple of 40TPS80 - my recuperational gauss built on them, so I know they are good.
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Coil-gun Project, questions I couldnt fine answers to
