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Artikbot

Mon Nov 01 2010, 11:05PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

I put in what I had available (and ready to destroy to be honest

), calculated a bit under a watt, and considering the resistor is rated at 1W, no problems there. But well, the resistor was dead before I installed it, so it wasn't doing anything at all Realised about this when felt that it was cold and the voltage was dropping so slowly. I'll go and get some 1M 1W resistors to put as bleeders. I need to get as well IC sockets for the 555 and the op-amp just in case any of both burns and I have to replace them without too much hassle.

Yep, the Op-Amp goes low when 5V threshold is passed and resets the 555. The 555 starts oscillating again when voltage on the inverting input falls below 5V.

Decoupling capacitor... Apart from the very tiny 10nF capacitor I have installed between ground and pin 5 of the timer, the other capacitors are relatively big 100uF and 10uF wired to the 7805 VR IC, that is a more possible source of power for the 555, since they're wired directly (no diode) to the +7.4V rail of the circuit.

A more exact description of the idea would be this one:

1: Powered up and let the circuit charge up to 296V, then cutted power because the MOSFET heatsink was starting to feel dangerously hot (in the 70-75ÂºC temperature range, it definitely needs a small fan attached to it). I was charging the full 3.5mF bank, it charged for about 20 seconds. The diode was hot aswell, but that's understandable because I'm working with switching times way below its recovery time. I'm gonna order a 3A 600V Schottky diode to replace the 1N5408, and it's perfect because the new diode has a package that allows for a heatsink to be installed, great because I'll be playing with lower temps allowing higher powers to be achieved.
2:Cut power (disconnected battery leads directly) and let capacitor bank discharge slowly.
3: When voltage was sitting around 210V I heard a beep in the same pitch range as you hear when the circuit is working, lasted for about 0.2s-0.3s
4: Let the circuit discharge completely, and when power was re-applied no beeping was heard and output was exactly the same as the input one (no oscillating, and maybe the diode wasn't even working? It should've dropped at least from 7.54V to 6.8V if it had been fit).

Resistors are in a good shape, everything shows the correct value. It's just that no power is coming out of the circuit. My 555 deduction comes from there.

The idea of connecting RST to GND is great!
I'm thinking about getting several mini relays to switch off the circuit and disconnect capacitor bank from the circuit at the same time, I might as well get another one to isolate the 555 from any possible power source and avoid the circuit to go BANG if something goes bonkers.

Thankies!

Artikbot

Wed Nov 03 2010, 09:25AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Ops, sorry for the early bump, but I've started to think in the second stage of the coilgun development (we can say that the charger's finished, it just needs a bit of optimisation and troubleshooting an issue involving buying another kind of diode), and I've got some ideas for it.

Since I have so little energy (close to 200J, that sir, is a bullsheet of power) I'm definitely going for a multistage coilgun consisting of:

70J injector coil (manually triggered)
60J primary accelerator stage (optically triggered)
60J secondary accelerator stage (optically triggered)

The optical triggering circuit consists of a phototransistor + IR diode couple for ultrafast response.

The phototransistor will be used coupled with a resistor to act as some kind of a voltage divider and will feed an LM393 comparator (2ns switching time, that sir, is FAST) via the non inverting input, with the 5V fixed value going through the inverting. That way when the phototransistor is not being lit, it'll trigger the coil SCR and start the coil.

The circuit looks a lot like the one described on this page (phototransistor section), mainly because it's very very very simple yet extremely effective:

The first coil stage consists of 8 layers of copper-enameled wire with about 410 turns in total, wound in 20AWG wire.
Since the other stages must be faster than this one I'll probably wind them on 18 or 16AWG, with about 16m wire each stage (each wire cylinder consists of about 32m of wire), with the layers it represents.
I'll be using another manufacturing process for this coils, instead of layer>tape>layer>tape without any glue to keep coils arranged (had a very difficult time to avoid layers dismantling and going out of the coil), I'll simply wind a layer>glue it>thin masking tap layer>glue the wire>wind>glue>tape and so on, I hope this way it'll be WAY easier to wind them.

Basing on some very estimated calculations, if I get a 3% efficiency on the 1st coil, a 5% on the second and a 7% on the third (considering the projectile enters with speed, that allows for much greater efficiencies), I'd get a total kinetic energy of 9.3J. Based on the kinetic energy formula, that's 55.8m/s muzzle speed, considering I'm using a 5.5gr type-1012 steel rod.

And hey, 55.8m/s muzzle speed isn't crappy at all for those 200J electric energy.

I guess I'll have to play with iron laminations and coil positions relative to the phototransistor/diode couples in order to achieve maximum efficiency, but if it's worth it, I don't have any problem on spending weeks optimising my coils

Greetings!
Marc.

Edit: Parts (SCRs for the coils, phototransistors and IR diodes) are going to arrive on wed-thurs, so I hope to have the optical stages built by next monday

Edit2:
Project retook!

Artikbot

Sat Nov 13 2010, 12:56AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Phew.

This is stressing!!

After calling more than 10 retailers including Farnell, I've FINALLY managed to get the parts. A kind guy from southern Spain holding a small familiar shop can supply me the parts, no minimum expense, and 24h shipping for just 7â‚¬. Cool eh?

Here we go:

LM311 comparators. 200ns response time, pretty fast for $.80
600V 100A IGBTs. Screw SCRs, I need a VERY precise timing on my secondary and third coils, so I've switched to beefy IGBTs instead.
Optical couple, 950nm wavelenght, phototransistor 2'3ns rise/fall time. Astonishing, just for $1 each matched wavelength pair.

And the rest, just precision resistors and other precision components I will be using to ensure the most precise of the timings. Efficiency is what counts, in the end :)

This is the idea of the concept (thanks James Paul from Coilgun Systems for such a great website):

On those squares will be mounted the optical couples. Circuitry is completely apart from that sketch, possibly mounted under it. Very heavy (10AWG) wiring will be used from the IGBTs to the coils, as well as from the main triac (powering the injector) to the main coil. Why heavy gauge? Less inductance, less heat, more conductivity. Efficiency, in other words

Oh and btw, I've found some Schotky 600V 1.5A diodes while scavenging an old TV PCB, I'm so happy! More efficiency to the charger ^^ I'll place two in parallel and get 3A superfast switching 600V diode battery. Great! So I can use all my 1N5408 to protect the capacitors and the circuitry from back EMF.

Thanks to everyone who has put interest on this project!

ScotchTapeLord

Sat Nov 13 2010, 01:16AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

Hate to be the bearer of bad news but paralleling diodes isn't as straightforward as you'd think. A little bit of searching will explain why... It's best to go with proper current specs on diodes.

Adam Munich

Sat Nov 13 2010, 01:19AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Just to explain why you can't // diodes, they don't share the current evenly. One will hog some more than the others, then in a runaway cycle it will hog more and more until it goes boom. Then the other ones are overloaded and they pop one by one until none remain.

Artikbot

Sat Nov 13 2010, 12:18PM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

ScotchTapeLord wrote ...

Hate to be the bearer of bad news but paralleling diodes isn't as straightforward as you'd think. A little bit of searching will explain why... It's best to go with proper current specs on diodes.

Yep, I read something like that on the nets... They said something about being lucky if you get 30% the current out of the second one.

Anyway, there shouldn't be any problem with the 1.5A alone, since it's on the HV part, and 1.5A at 330V means... 495W, about fifteen fold what the circuit's supposed to deliver :P

Thanks for reminding that ^^

@Greandier:

There's something I don't understand. If paralleled, how can it be that they don't share the current, if they're identical? You mean that they won't switch from conducting to breakdown state at the same time?

Adam Munich

Sat Nov 13 2010, 12:50PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

That's just it, they aren't identical. Every diode is not made equal, and some will have less 'resistance' than others.

ScotchTapeLord

Sat Nov 13 2010, 03:50PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

Artikbot wrote ...

Yep, I read something like that on the nets... They said something about being lucky if you get 30% the current out of the second one.

Anyway, there shouldn't be any problem with the 1.5A alone, since it's on the HV part, and 1.5A at 330V means... 495W, about fifteen fold what the circuit's supposed to deliver :P

The only device that should ever have 330V across it and 1.5A going through it is a 220 ohm resistor (and maybe some old thermionic stuff...). Diodes are either on or off- on, in which case they have full voltage across them and their leakage current through them (negligible) or their voltage drop across them and full current through them.
Also, during switching, there is a bit of overlap with voltage and current, and that's why switching is kept fast.
Datasheets are your friend!

However, the diodes still might work for charging the capacitors. I've used absurdly small diodes for this purpose. However, you can't use diodes this small for CEMF (antiparallel with the coil). The pulse may be short but it's an obnoxious amount of energy, and if your diode fails, then your SCR fails and your capacitors fail in a horrible chain reaction. That's why it's worth it to spend on some diodes in heatsinkable packages that can handle a few hundred amps pulsed.

Artikbot

Sun Nov 14 2010, 01:29AM

Registered Member #3247 Joined: Mon Sept 27 2010, 09:42AM
Location: Spain
Posts: 137

Yeah, the Schottkys are intended to replace the 1N5408s I use as rectifiers on the boost converter. Just because the 1N5408 gets ridiculously hot because I am going WAY above its frequency limits, and it also causes efficiency to drop below any logical levels. It's gone about twice as high since I've switched it for the Schottky. And it doesn't get nowhere as hot as it used to do

Back EMF diodes... My current coils are going to handle somewhere about 80A during the very, very short duration of the pulse (they're the acceleration stages, the injection and main stage is going to handle something bigger... thats why I have an 850A surge triac

).

So, heatsinked packages... I'll see what can I get for a reasonable price ;)

So far I've found those (reasonably priced):

BRY29/7000. Capable of handling up to 800V 66A surge, for 8'3ms. More or less the length of the main pulse, about 10 amps less. 1'5â‚¬, TO220 package.
BYX61200. This little monster handles 1kV at 75Amps for 10ms. Can be bought for about 3â‚¬. TO220 package.
MUR880E. This one handles 1kV at 100A for 8.3ms. RRP of 1'5â‚¬, TO220 package.

All of them are superfast switching diodes, the last one is the fastest (75ns recovery time). I wouldn't care at all if I had to get another extra one for the boost converter, if that would mean improving reliability

I'd get the third, what do you say?

Thanks!

ScotchTapeLord

Sun Nov 14 2010, 03:15AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

I've had good experience with MUR diodes.

BTW, 80 amps sounds a bit low for your max solenoid current. Usually they are closer to 500A... Capacitor voltage divided by coil resistance for your upper-bound amperage. Due to the fact that the capacitor loses some of its charge while the current through the inductor builds up, you won't actually get as high as that number, which is why it can be considered a safe upper limit.

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