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Calculating amperages straightforward via Ohm's law leads to 300Amps peak on injector and 650 Amps on acceleration stages.
But I'm thinking about something... Won't inductance lower that current, like it does on the inductor on the converter? That would avoid me the need of using diodes with crazy surge currents (and crazy prices too).
This are the best option so far for a non-excessive price:
150EBU04-02. Handles 150A at 400V continuous, surge up to 1.5kA 400V at 25ºC. 14€. 70HF120. This one handles 70A continuous at 1.2kV, surge 1kA for 10ms or 1.25kA for 8'3ms. 7'40€. Stud mount.
So far, these ones are the best option for the high current diodes.
I'm going to re-simulate all the coils on Barry's sim and see what I do. Basically because I doubt that the peak will be that crazy surge current... The banks have very little capacitance for each coil, less than 1.2mF per coil
I'll be posting the results of the simulation in a moment.
In conclusion, I'd better get a pair of those stud mounted and another rated at about 300-400A to be safe, right?
And drop my 120A IGBT idea, because IGBTs with that surge currents are SUPER expensive compared to normal SCRs of the same surge ratings.
I've found this on my dealer:
BT152-600R: 600V, 200A surge for 10ms. Maybe if I shorten the pulse, surge amperage will rise? Or simply by paralleling (common gate) two of them will be enough for the specified 400A rating?
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
Artikbot wrote ...
Calculating amperages straightforward via Ohm's law leads to 300Amps peak on injector and 650 Amps on acceleration stages.
But I'm thinking about something... Won't inductance lower that current, like it does on the inductor on the converter? That would avoid me the need of using diodes with crazy surge currents (and crazy prices too).
It will, but at 10kHz your pulse width is 0-100 microseconds long depending on your duty cycle. The pulse duration through your coil will be determined by your capacitor's capacitance and your coil's inductance (pi*rad(LC))), which is usually in the order of a couple milliseconds. Also, an air core solenoid has less inductance than your boost inductor, so not only will the current rise for a longer amount of time, but it will also rise more quickly.
If you want to know your exact peak amperage, you'll need to use a second order differential equation or a simulation (I prefer the latter :D), but in the end you will get fairly high current (which is necessary for this application) and you will still want a safety margin, so it's best to go with the ohm's law equation to skip the third-semester calculus, simulations, and safety margin calculations.
Artikbot wrote ...
This are the best option so far for a non-excessive price:
150EBU04-02. Handles 150A at 400V continuous, surge up to 1.5kA 400V at 25ºC. 14€. 70HF120. This one handles 70A continuous at 1.2kV, surge 1kA for 10ms or 1.25kA for 8'3ms. 7'40€. Stud mount.
So far, these ones are the best option for the high current diodes.
The second one seems to be mislabeled! The data sheet indicates that it is a standard speed rectifier, even though digikey labels it as fast! I would avoid this at all costs. The first one is good but has very little room for voltage excursions. I would personally use 2x what I am charging my capacitors to, but as long as you keep your connections from capacitor to coil short, there shouldn't be a problem with using that first diode.
Artikbot wrote ...
I'm going to re-simulate all the coils on Barry's sim and see what I do. Basically because I doubt that the peak will be that crazy surge current... The banks have very little capacitance for each coil, less than 1.2mF per coil
They will be close to "that crazy surge current" because that's the only way to get enough energy into the projectile in the short amount of time that it will be near the coil.
In conclusion, I'd better get a pair of those stud mounted and another rated at about 300-400A to be safe, right?
And drop my 120A IGBT idea, because IGBTs with that surge currents are SUPER expensive compared to normal SCRs of the same surge ratings.
Go with the first diode you mentioned. And yes, SCRs offer much high peak surge current ratings for a much lower price and size.
Artikbot wrote ...
I've found this on my dealer:
BT152-600R: 600V, 200A surge for 10ms. Maybe if I shorten the pulse, surge amperage will rise? Or simply by paralleling (common gate) two of them will be enough for the specified 400A rating?
You can't just change the pulse length to meet the needs of your circuitry available, because it will affect your gun's performance. In the end, you may either need a shorter or longer pulse to get the most out of your system. People have paralleled SCRs for coil guns successfully but it makes me nervous because they are still diodes, just a little fancier. I would parallel maybe four to be safe if I couldn't find one with 500A surge.
Alright. Straightforward Ohm law then. Soemthing sitting around 600A peaks then for accelerators and 300ish for injector. Wow, that's a HUGE amount of current for such a small capacitance. I wonder if my capacitors won't go BOOM
According the stud mounted diode, this is the datasheet
It seems to be labeled properly, both datasheet and my retailer (a spanish small familiar warehouse) have the same specifications. And those stud mounted ones will do well with up to 1kA peaks if their ratings are correct, they almost double the straightforward Ohm peak amperage. And they handle twice the voltage, that's a plus considering there will be spikes
If the second one is properly labeled, will it be a better option above the first? Or is the first still a better decision? Asking this because I don't see it beefy enough to justify the 2x price it has. Handles way more surge current, but half the voltage. Also the package doesn't seem so sturdy to be mounted with heavy gauge wires on a massive heatsink.
Wait, you said the speed was mislabeled? Does it need to be fast recovery? Do you mean that current could bypass the diode and go to the caps if the diode isn't fast enough?
So in the end it seems that low energy storage doesn't mean low power pulses, eh? 150kW pulses for the win
Alright, I am going to parallel 3 SCRs per acceleration stage, that's what I can get right now without waiting for weeks. If it turns out to be insufficient, I'll order more SCRs and add them to the banks. Heatsink is a must-have here I guess, since the currents are so ridiculously high. And yep, they're still diodes... That's what makes me think that if people succeed at paralleling SCRs, why can't regular diodes be paralleled aswell? Maybe SCRs are built using higher quality silicon and they are more "similar" to their fellows and therefore it's safer to parallel them. But I ain't gonna play with fire on diodes, they're the most expensive part of my circuit and they are in duty of protecting my whole circuitry. If an SCR burns, it's not a lot of hassle, because it costs less than 2€. But if a 15€ diode burns because I paralleled, or just did stupidities with it, I am possibly gonna get pretty angry, considering I'm not exactly rich
So, if it isn't a lot of trouble... Could you mind explaning a bit that fast switching recovery for the back EMF protection diodes, please? I googled for it and didn't get anything clear about it
If it turns out to be a must (not just because it's fancier to use a fast switching one), I won't have any problem on spending twice the money for a better diode... But if it's possible, I would like to avoid it. I'm sure you can understand what I mean
Thanks a lot again, you guys are seriously making me addicted to this stuff
Edit: I've found that by adding the crazy surge diodes, Farnell is eligible for my products, and is also cheaper.
Check this one:
400V and 600A surge current, just for 1'8€. Ultrafast recovery. Great pricing for considerably good specifications! 400V doesn't leave for much room, but I believe they can easily resist this load. For such a low price, I can get 4 or 5, and see how do they behave.
Also, I've pinpointed some great SCRs rated at 460A surge, at just 2'5€. Two of em paralleled will be WAY above the requirements (one is right below it ), astonishing for such a low price
This is the model:
I doubt two of em in parallel will have any kind of problem to handle the surge current coming from the acceleration stage ;)
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635
Artikbot wrote ...
Alright. Straightforward Ohm law then. Soemthing sitting around 600A peaks then for accelerators and 300ish for injector. Wow, that's a HUGE amount of current for such a small capacitance. I wonder if my capacitors won't go BOOM
If you don't use the proper type of capacitor, it will go boom. That's why paralleled photoflash capacitors are good- they are pulse rated, and the current through each one is divided by however many we have in parallel.
Artikbot wrote ...
According the stud mounted diode, this is the datasheet
It seems to be labeled properly, both datasheet and my retailer (a spanish small familiar warehouse) have the same specifications. And those stud mounted ones will do well with up to 1kA peaks if their ratings are correct, they almost double the straightforward Ohm peak amperage. And they handle twice the voltage, that's a plus considering there will be spikes
If the second one is properly labeled, will it be a better option above the first? Or is the first still a better decision? Asking this because I don't see it beefy enough to justify the 2x price it has. Handles way more surge current, but half the voltage. Also the package doesn't seem so sturdy to be mounted with heavy gauge wires on a massive heatsink.
Wait, you said the speed was mislabeled? Does it need to be fast recovery? Do you mean that current could bypass the diode and go to the caps if the diode isn't fast enough?
Yes, I did. Yes, it does. Yes, it will. The first is a much better option just because of the recovery time.
Artikbot wrote ...
So in the end it seems that low energy storage doesn't mean low power pulses, eh? 150kW pulses for the win
If you use a 50 watt charger for 10 seconds, that would be 500 Joules in your capacitor bank. If you release those 500 Joules in 10 milliseconds, that's 50000 Watts AVERAGE output. But we know the current starts low, peaks high, then drops back down, so to achieve that high of an average value, the peak have to be quite high.
This is an oversimplification, but I hope you get the idea. ALso, I wouldn't consider your capacitor bank's energy storage to be low.
Artikbot wrote ...
Alright, I am going to parallel 3 SCRs per acceleration stage, that's what I can get right now without waiting for weeks. If it turns out to be insufficient, I'll order more SCRs and add them to the banks. Heatsink is a must-have here I guess, since the currents are so ridiculously high. And yep, they're still diodes... That's what makes me think that if people succeed at paralleling SCRs, why can't regular diodes be paralleled aswell? Maybe SCRs are built using higher quality silicon and they are more "similar" to their fellows and therefore it's safer to parallel them. But I ain't gonna play with fire on diodes, they're the most expensive part of my circuit and they are in duty of protecting my whole circuitry. If an SCR burns, it's not a lot of hassle, because it costs less than 2€. But if a 15€ diode burns because I paralleled, or just did stupidities with it, I am possibly gonna get pretty angry, considering I'm not exactly rich
Paralleling = good. Heatsinking actually isn't necessary. It's a lot of current going through it but the pulse is so short that it doesn't generate enough continuous heat to be transferred to the heatsink. That's why you need a powerful SCR, because the die has to handle all the heating in such a fast and short application as this.
The reason you can't parallel diodes is because they conduct better as they get hotter, so when one gets slightly hotter than the other (which it will because no two diodes are exactly alike, even from the same batch of silicon), it will hog the current and make itself even hotter (which makes it hog MORE current), causing a sort of infinite loop until it's conducting way more than the other. In THIS case, we are only using the SCRs for a handful of milliseconds and there isn't enough time for one device to get significantly hotter than the other one, so current is shared better. If you paralleled SCRs for a continuous application, they would experience the same problems as the diodes do.
Artikbot wrote ...
So, if it isn't a lot of trouble... Could you mind explaning a bit that fast switching recovery for the back EMF protection diodes, please? I googled for it and didn't get anything clear about it
If it turns out to be a must (not just because it's fancier to use a fast switching one), I won't have any problem on spending twice the money for a better diode... But if it's possible, I would like to avoid it. I'm sure you can understand what I mean
We all want to save money as hobbyists, but the best way to save money is to use properly rated devices so that a crazy chain reaction of failure doesn't make us have to buy everything all over again, along with the appropriately rated devices.
After your capacitor has emptied itself into the coil, the coil will keep the current flowing and try to recharge the capacitors. This is bad because your capacitors aren't rated to be charged in reverse, so it will destroy them, and it will also lengthen the amount of time that your SCR is conducting high current, so it will probably destroy them, too. We instead give the current another path to take instead of back into the capacitors, a diode antiparallel with the coil. The inductor will empty itself right into the diode, bypassing all your other stuff. But let's say that this diode doesn't turn on quickly. Well, then it will be like the diode isn't even there, and it'll just, like you said, bypass the diode and wreak havoc. But if your diode turns on quickly, it'll stop any harmful amount of current from getting into your other stuff.
Google counter-electromotive force, back-EMF, or inductive kick for more information.
There's also a neat trick using two diodes to use the counter electromotive force to recharge your main capacitors instead of just dissipating it as heat... but we can leave that for another time. =D
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837
The speed of the diode doesn't really matter for the capacitors, nothing much is going to happen to the voltage over the capacitors even with the slowest forward recovery (even when it does matter fast recovery diodes are not necessarily the best, they can have a greater forward recovery voltage ... except for SiC diodes of course, but those are expensive). The coil gun works in milliseconds, the diodes work in nanoseconds.
That Farnell one seems a good alternative to me, same voltage, 600A surge, for about 7 times less price. A bit of a bargain, to be honest What do you think about it?
Okay, I got the point now. Slow recovery= useless diode. So, fast recovery then. The Farnell one is a fast recovery, are 45ns fast enough?
So, power spikes are the kings of the fair. I observed that when using Barry's RLC sim power behaved exactly as you said. Started for about 0.5ms with very, VERY low current, then made a grand spike of several hundred Amps, and then dropped as fast as it rised to almost no current. If we consider the spike duration of 2ms and the rest of the time it's almost idling... That's a hell lot of power, yep
Well, the energy is low compared to some ultrahigh power coilguns around (Saz43's and other fellas' massive destruction coilguns ), but despite that, it's enough to kill a person if used incorrectly.
Okay, so just a small sink on the SCRs to avoid them building heat shot after shot will be enough then. It makes sense, not much heat can build up on the tab if current's only crossing the die for about 4-5ms.
Aaah! Now that you say counter-electromotive, I remembered it! On my language it's called fcem, and yep, it's totally undesirable on LC circuits where high power motors are present because when the motors start to run from a blocked rotor state (starting from 0 RPM) there's an electromotive force going from the motor back to the power source that can easily destroy any polarised component that's present on the circuit. I guess something similar happens when a coil is triggered (on the end, it's like a blocked rotor electric motor ). Good you've reminded it, I had that part of my technology lessons a bit rusty!!
Yeah, the energy recoval tricks will be taken into account for later modifications... I've already planned to build a prototype featuring 2kJ capbank using a modified flyback core and a Mazilli driver to charge the caps. but that will be somewhere in the future, because I don't have the money, the time, or the tools!
Take a look at that diode and the SCRs and throw in your oppinion. If they're favorable, I'll be ordering them tomorrow morning, and hope to have them by wednesday! I can't wait to see this thing firing again
A whole truck full of thanks to you! (I wonder why there isn't any +rep button )
Edit:
@Pinky's Brain:
The thing is that silicon diodes don't switch really fast... The stud mount I posted previously had voltage rise times of around the ms, and if it's not able to reach 330V in 4ms, it will be useless.
Like ScotchTapeLord said, I'd better get fast switching ones, they're not that expensive above regular silicon, and they'll work for sure.
Thanks for sharing your knowledge too
Ooh! And by the way, I've rewound ALL my coils using a much better manufacturing process. I can now extract them from the barrel, move them freely and everything. I've wound them using the wire-tape-cyanoacrylated wire technique, and they are STURDY! They also look much neater and much better built
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Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837
You misunderstood the datasheet ... reverse recovery is not even specced for that diode.
As for a fast recovery diode definitely working ... they have higher forward voltage than standard recovery diodes, so they burn more power while freewheeling. A fast recovery diode could actually fail where a standard recovery diode survives.
In reality in this case it's unlikely to matter, but just so you know.
In reality in this case it's unlikely to matter, but just so you know.
Good. That is, because back EMF spike is going to be as fast (or slow, depending on how you look at it ) as the main spike rise? Or slower?
To me it sonds like it's gonna be of the same speed, but certainly smaller, since most of the current has already been wasted to heat the coils/projectile. That is... a pair of ms for the diode to say "Hey, we have to block this current!" considering I'm talking about injector, when it comes to the accelerators, it has barely a half ms to react.
0.5ms is... 2kHz. I doubt the diode will react, with my current standard recoveries I have installed on the boost converter (about to change, schottkys are ready to be mounted), I'm losing according to sim more than half of the pulses because of the slow recovery time diode has. That's losing a hell lot of efficiency
I think I'm gonna stick to fast recoveries, they're a few cents more expensive and are more likely to work.
Thanks for your advice anyway
@ScotchTapeLord:
Oops my bad! I took the URL of the thumbnail instead of the target link!
Here it goes:
Sorry if it's in spanish... The datasheet is in english though
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Joined: Sun Dec 21 2008, 06:36PM
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The diode looks fine. As for the CEMF spike, a lot depends on how well-damped your circuit is.
In theory, the current of the CEMF spike could range anywhere from 0 (ideal) to to just as high and long as your initial spike (bad), so we prepare for the worst. Try to get your circuit critically damped! If you have differently sized inductors, then you'll need to choose capacitors to fit the equations for damping.
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New to Coilguns - Coilgun Buildlog (Charger apparently fixed)
