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phil

Fri Nov 03 2006, 04:53AM

Registered Member #314 Joined: Mon Mar 13 2006, 03:12AM
Location:
Posts: 52

I havent had much time to work on my coil guns due to work/school. But it's slowed down for now. I've been seaching online for a formula to calculate the inductance of a solid core coil. So far I have found one on wikipedia, but in order to calculate it correctly i need to know the permability of the material i'm useing as the core. I'm planning on useing the core of a gutted transformer from an ATX power supply. I need to know if it's possible to find the permability of the material. If not I'll go with an Air core coil becuase i plan on going through the equations as part of my senior project. Also i had another question, I was looking at a website that describes the basics of DC/DC converters. In this illustration Dc F11

there is a transistor correct? If i'm not mistaken I have to trigger the transistor on/off with something like a 555 timer. But what frequency should I use? I checked on "another coilgun site" and saw that he was pulsing it at 1.5Khz in his charging circuit. I need to bring something like a 9 volt battery up to 200 volts, is that even possible with a boost converter?

Steve Ward

Sat Nov 04 2006, 01:06AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Is this a senior project for high school or college?

Anyway, boost converter design from my experience is a very iterative process, and will depend greatly on the switching device used and the type of inductor used (more specifically, the core material). For a boost converter, the inductor must work as an energy storage device, so you must have an air gap in the core if its ferrite (because of its very high permeability). If using powdered iron, there is usually a distributed air gap throughout the core material. If using an old laminated iron core.... well, you just dont use those for this application.

Air core is "OK" but keep in mind, you will most certainly be increasing your losses (or, increasing your copper use). The core increases the inductance, which increases the energy storage at a given current. Of course, cores saturate at some point, so there is a limitation. Air doesnt saturate for practical purposes, but keep in mind the resistive losses at higher currents.

Here is the reasoning i had when designing my boost converter (remember, iterative approach, so you may go through this list several times):

What is the output power?

What frequency does my core material work well at? (generally this will be in the 20k-100khz range)

Should i use continuous or discontinuous operation? (powdered iron? use continuous, ferrite? use discontinuous, why? take a look at the hysteresis loss).

What frequency does my switch (MOSFET, IGBT, or at worst, BJT) seem most efficient? Check out the switching losses for a comparison, conduction losses will be pretty similar throughout the frequency range (your average current may go up or down depending on the current's crest factor). For your case, the power level is so low, you can probably skip this.

Pick an operating frequency, then work out the following calculations:

How much energy (in joules) must the inductor convert to the output on every cycle of operation? (this depends on frequency and output power).

What peak and average inductor and switch current am i willing to deal with? Usually you want this to be minimized, but there is definately a limit as you will find.

Calculate the inductor parameters. Will the inductor saturate at the expected peak current? (better figure out B given the current, inductance and number of turns, then make sure its less than Bsat for the core). If you are working with a core with unknown parameters (i was) then you will have to make a few guesses, and do a few test windings on the inductor to determine the inductance per turn. This may be an advantage for using an air core.

This was probably more information than you wanted, but if you want to design a converter, you will usually need to balance bunch of factors at the same time. Just picking components at random will greatly increase the chances of your converter not working. Though, for such a tiny converter, you could probably get away with it

phil

Sat Nov 04 2006, 03:50AM

Registered Member #314 Joined: Mon Mar 13 2006, 03:12AM
Location:
Posts: 52

Should i just fux the boost converter and use an IC instead? I'm also a lil confused, are there any other ways to charge a capacitor? See whats happening now (becuase i'm a noob) is if i put 9 volts to a 200 volt capacitor the cap only charges to 9 volts. I dont need a full explination of how i could use another way, a link would be fine.

...

Sat Nov 04 2006, 06:03AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Well, no offense but... If you cannot figure out why putting 9v onto a 200v capacitor only charges it to 9v then you might want to look for a project that does not involve lethal voltages.

But in any case...

You have several possible methods:
1. probably your best bet...

Get a small transformer that does what you want it to. I would recommend going down the the local disposable-camera-dealer (ie cvs, wall mart, whatever) and beg them for used disposible cameras (they need to have a flash) and tell them that you need them for school and give them your saddest look. If they don't give them to you either walk off and try another store or cough up the ungodly sum of money (like $8 now!) to buy one, or 2, or 3. If you opt to buy it then you can use all of the exposures, just make sure to wind it fully when you are done, then carefully crack it open, and viola there is a normal roll of film you can have developed. But that is not what you are after.
Once you get home, carefully rip the camera open. Be very careful of any exposed metal parts, as chances are that the cap is still charged and getting shocked by one is no fun (trust me). Find the circuit board, which will have a large capacitor labeled HIGH VOLTAGE on it. Short the 2 terminals of the cap with a screwdriver, it will usually make a loud bang. Do it a few more times for good measure. Then carefully remove the circuit.
You now have in your hand a fully functional 300v power supply. You can parallel several of them together if you need more power.
You also get the caps from the camera (they are ideal for coil guns).

option 2:
Follow one the published boost converter designs (schematics can be found about 1/2 down this page. You can leave out the 1.5A high speed driver if you do not know what it is). This will do exactly what you want.

option 3:
since you seem to have a transformer out of an atx power supply, you could make a small half-bridge smps supply using it and a tl494 and a few other cheep parts. But this is an advanced exercise and I do not want to explain it unless you are actually going to to do it.

good luck and don't blow yourself up!

phil

Sun Nov 05 2006, 04:06AM

Registered Member #314 Joined: Mon Mar 13 2006, 03:12AM
Location:
Posts: 52

I can't promise that i wont blow myself up. But i guess i'm going with a boost converter then.

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