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Artikbot

Tue Sept 28 2010, 11:21PM

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

Thanks for the links :)

According to the inductor calculator, the higher the frequency the lower the inductor value. If I stick to 10kHz I need a ~220 uH inductor. Great link, thanks!

OMG I simulated my circuit using Grenadier's software and the back EMF killed ALL the circuit but the 555 timer LOL!! I need to filter that EMF as far as possible.

I'm currently trying to make my circuitry not burn out. Let's see what happens with some improvements on the EMF filtering. When it doesn't blow up I'll post it :)

Thanks once more for the help!

1st results: Simulating the circuit shows that the inductor creates (...) a 1.35kV voltage that literally KILLS everything on the board. Not to mention that once the voltage reaches the solenoid it returns to the 555 timer as a back EMF of something as grandiose as 1.35MV that will literally vapourize the PCB itself. Something is going wrong... should I upload my design so you can check it?

Adam Munich

Wed Sept 29 2010, 12:00AM

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

Yeah that inductive kickback is nasty. It won'tl kill everything on the board like the simulator, but it will kill the mosfets. Use a couple 1n4007's as protection diodes. It should solve the problem.

ScotchTapeLord

Wed Sept 29 2010, 12:29AM

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

Simulations are no substitute for experimentation. Real life factors will change a lot... If you want to be safe, just start with feeding the charger a low voltage, and if the rise is too crazy, add more windings to the inductor until it settles out, then work your way up with the voltage and keep adding more windings to keep the currents low.

I would not trust 1n4007 (or anything else radio shack carries) to clamp the CEMF of anything, especially in a coil gun. Go with a good FRED (Fast Rectifier E... um... Diode) with a good pulse rating, and by good I mean perhaps 100A. Any FRED in a mountable package (TO-247 or similar) with a ~30A rms rating should be fine.

That current is coming fast and high, and if you don't guard against it, you can bet it'll go where you don't want it. Sometimes adding a series resistor with the diode can help the waveform (that's what I've heard) and ease the burden on the resistor. However, the higher the value, the more voltage you put in reverse across your capacitors for a short time. No free lunches. =(

It's funny how CEMF is so evil in one area (the solenoid) and so essential in another (the boost charger).

Artikbot

Wed Sept 29 2010, 12:44AM

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

So far my scheme on Yenka looks AWFUL. Well not just awful, but also very very very disorganized.

I'll explain my problem:

I feed the circuit> the inductor (not the solenoid) blows up, killing the mosfet and the main switch (i don't get how can it blow the main switch)

When I click to repair the inductor, it blows up AGAIN destroying everything on the circuit but the stages before it. Cause of destruction: 3.07kV to 16MV flow through the circuit, probably coming from the stored energy in the inductor?

I've changed the solenoid for a resistor so I can test the circuit without bothering about the solenoid. That'll be later.

@ScotchTapeLord:

I have no intentions of blowing up everything for a stupid error I could've solved by optimising on the sim...

Here it goes the issue afaik: Current flows through the inductor, but it is too high for it and blows it up. When it's blown a closed circuit with no resistor flows through the driven MOSFET, blowing it up too. But that little period of time is enough to charge the cap battery a little bit. And when I repair the MOSFET, the switch and the inductor, the remaining current in the capacitor discharges back through the ciurcuit, entering the 555, destroying it, and from there essentially rallies to the inductor, 330V current gets rised to 13kV and then kills everything on the board but a several leds and resistors.

So the idea is... Why does the inductor blow up? Why does the current on the capacitors go back into the circuit? Showing a pic of what I've been experimenting with. Advise: it is REALLY REALLY messy and disorganised. But it's pretty understandable I think :P

Maybe I messed up with the timing's connections, who knows :/
I mean, it's intended for 35W, why do more than 100W cross the MOSFET? The timer (resistors+caps) is set to get 6.8kHz with the respective inductor to get the desired 330V.

Thanks once more for your helpful advise, mates :)

Adam Munich

Wed Sept 29 2010, 12:52AM

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

Yenka is ridiculous with things exploding. The inductor isn't going to blow up, but it will get hot. There's a way to make things indestructible, and I recommend turning it on. It has something to do with right clicking. Just monitor the voltage&current manually and see if anything's astray. You may even need a higher voltage mosfet. Who knows?

But sire scotchtape got it right. Simulations give a pretty good idea of what's going to happen, but it's no substitute for real experimentation.

Can I get they yenka file to play with? You can upload it here. >

ScotchTapeLord

Wed Sept 29 2010, 01:52AM

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

Oh I'm sorry I assumed you were talking about the actual solenoid coil. The boost circuit does not need so powerful of a diode, but it should still be fast.

The weak point of this is the silicon. If you use decoupling, your ICs will be fine. Power transistors are most stressed. Capacitors will fail if you overvolt them, but a suitable bank of capacitors won't charge so fast you can't monitor the voltage with a standard voltmeter.

To be safe, start with a lot of turns and a high frequency (use a potentiometer in series with a timing resistor to vary the frequency, as there is often a "sweet spot" with these types of things, just make sure the potentiometer + series resistor is equal to the resistor that was replaced). Higher frequency and more inductance = lower currents, which means happier silicon and slower capacitor charging.

GhostNull

Wed Sept 29 2010, 03:45AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

I've used yenka, don't trust it, I just makes EVERYTHING go boom. I think if you want to do any proper simulations you need to use a SPICE. Yenka is only good for low power & digital electronics. It is really mediocre.

Edit: also remember the inductor and your solenoid's ESR (resistance)

Adam Munich

Wed Sept 29 2010, 03:49AM

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

Good to know. I'll have to ply with that later.

Artikbot

Wed Sept 29 2010, 08:08AM

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

@Grenadier:

Have a bit of fun making my circuit go BOOM for reasons I can not happen to solve, even having re-checked and re-wired things a million times. Btw, don't trust serial numbers. Yenka hasn't got a single one of what Im using, but I've modified the ratings to fit my needs.

So far the only things going BOOM are the first inductor and its respective control circuit: inductor, MOSFET and main power switch.

@ScotchTapeLord: Great, that means if I put, let's say, a 500uH inductor and start varying resistance values I can happen to reach the point where nothing burns out and the cap battery effectively charges itself to its maximum? And concerning the decoupling part.. How can I apply it to my circuit? I got three loops, the gace control loop, the inductor control loop and the power loop for the capacitor battery. Even tho I guess the power loop to caps isn't really a pure loop.

@GhostNull: Yeah I've experienced that :P but so far it's fine to see if (even BOOMing from peak current) current reaches every part of the circuit, if caps actually charge up, if diodes are properly mounted and that stuff. Once the design on Yenka works with a mimimum of possibilities, I'll order+build everything.

Thanks a lot agan mates! :)

>Edit: Changing the inductance and the resistor that serve as a solenoid for an actual solenoid, and the thyristor with a NO button, the capacitor actually discharges its juice onto the solenoid, but yet again, with the inductor&control circuit having gone bonkers previously.

ScotchTapeLord

Wed Sept 29 2010, 12:27PM

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

Artikbot wrote ...

@ScotchTapeLord: Great, that means if I put, let's say, a 500uH inductor and start varying resistance values I can happen to reach the point where nothing burns out and the cap battery effectively charges itself to its maximum? And concerning the decoupling part.. How can I apply it to my circuit? I got three loops, the gace control loop, the inductor control loop and the power loop for the capacitor battery. Even tho I guess the power loop to caps isn't really a pure loop.

>Edit: Changing the inductance and the resistor that serve as a solenoid for an actual solenoid, and the thyristor with a NO button, the capacitor actually discharges its juice onto the solenoid, but yet again, with the inductor&control circuit having gone bonkers previously.

The resistor I was referring to is one of the ones you are using in your timer circuit, the one that puts out 10kHz. Adding series resistance to your solenoid will also decrease current but the values would need to be very low and it would create a lot of heat. Potentiometers are great because you can experiment while the thing is running. Decoupling is for your chips. It's just a big capacitor (a few uF to keep it simple) across the V+ and GND terminals so that voltage cannot rise instantaneously across the device. The equivalent of decoupling for your semiconductors would be a snubber, but they have no place in this topology.

Your control electronics should not be affected by what goes on in the power section. At worst, if this device were to fail, you would lose your transistor and diode. Honestly, the first time you put it together, if you don't kill something, it's probably not working at all. There isn't much that can go wrong with a boost circuit, though. You're just charging an inductor, cutting off the power quickly, and capturing the output of the inductor (V=L*(di/dt)) across the capacitor (and transistor). The transistor *ideally* would not see voltages higher than the capacitor bank (maybe a few more volts in practice, but nothing significant), so you just have to worry about your timer circuit working properly and the current flowing.

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