Solid core inductance
 

Hello everyone, first off I want to say that I've searched all over the place, and I even asked my mentor/physics teacher about this. For my senior project I'm doing high voltage electro magnetics, and I'm going to build a coil gun. I've built them before but they weren't very efficient. I've been trying to find a formula to calculate the inductance of a coil wrapped around a solid core. I've only been able to find this one.
But the only problem I'm really having is determining the permeability of the medium. I'm using the core of a gutted transformer from an ATX power supply. I checked the markings on the outside but came up empty. I can't just calculate the inductance as if it was an air core coil because the final data would be incorrect. Can anyone tell me how to find the permeability of any given material, or if I could but a core with a known permeability. If not then I guess I have to make an air core coil.

Re: Solid core inductance
 

You've asked a good question. And it is understandable that you are having difficulty finding guidance on inductor design.

An answer will require a little more information. To begin, what voltage and frequency do you plan to apply to this inductor? Was the transformer from the old power supply operated at the same frequency? What are the dimensions of the transformer core? Do you know what the core is made of?

And how will the coil gun work?

Re: Solid core inductance
 

First off this inductor is for a dc/dc boost converter. I plan on bringing 9v up to about 200. I haven't decided on a final frequency yet, but it would probably be something around 1.5KHz. I'm not sure what the original frequency of the power supply is because I don't own an oscilloscope.The core is shaped kind of like El , kinda hard to illustrate. It's 1.5 inches long, 2 inches tall, and half an inch thick. And I have absolutely no idea what it's made of. But judging from THIS website it is probably a ferrous alloy powder E core.

A coilgun is the not so correct name for an electromagnetic projectile accelerator. Basically a bank of capacitors is charged to a high voltage and/or high amperage. Then the current is forced all at once through a coil of wire, kind of like a solenoid. The magnetic field caused by the coil attracts a ferrous projectile that is inside a tube that runs through the coil. The current is then shut off once the projectile reaches the middle of the coil and the projectile leaves the front of the barrel.

Re: Solid core inductance
 

Richard,

A coil gun is a kind of high-velocity, low usefulness gun. You have an inductor, you something on the order of a Farad of charged capacitance through it, and you get a massive magnetic pulse to hurl something out of it. Usually magnetic, for some reason, but I imagine a brass or copper pellet would be likely to work better unless you could manage to instantly kill the magnetic field before it starts slowing down your iron pellet on the exit. Eddy currents are always repulsive, after all, though I'm admittedly unsure a pellet would be large enough to develop sufficient current.

Re: Solid core inductance
 

:yikes: :ahh: You're making something I can only dream of! Haha..
In gaming terms its called a Railgun I suppose. I voted for single coil.. coz I thought additional accelerator stages can be easily added and controlled timing to the microsecond with a microcontroller

Your coil is hollow and the medium is air.. so I guess the permeability of the medium would be some kinda constant of air? Just a quick guess..

Re: Solid core inductance
 

Phil,
It sounds like you are remanufacturing the transformer core from an ATX power supply into a DC converter power supply. If that is the case, there are too many unknowns at this point for you to accurately make calculations. If you don't have access to a scope then you likely don't have access to other instruments either. An easy way to back into this calculation would be to make a coil that would fit over the core. Measure the inductance with the coil and no core and then measure with the core. The difference would give you a rough idea of the permeability of the core. The ATX power supplies normally run at higher frequencies than what you are designing. Likely 100kHz or higher but the core might still usable at 1.5 kHz. In transformer and inductor design there are other factors involved. The object is to know the action of the core but also allow it to operate in a range of currents that does not saturate the material. This occurs when the current/magnetic field causes the core to reach a point where no additional current will cause a higher magnetic field.
The type of core you describe is a typical "E" design for obvious reasons. The core is made this way so that manufacturing costs are lower. If it is a laminated design (many thin strips of iron) instead of powdered iron, it is better suited for your lower frequency design. (Be aware that a design that stresses a powder core can cause it to crack.) The space between the "E" and the "I" part of the core is a magnetic loss and takes away from the permeability of the core material and as such needs to be included in the calculation.
So now the question is where to start. Check the name on the power supply and see if they have a website. You might be able to find a spec sheet on your power supply that will list the operating frequency. It may also list the components. Then search the net for manufacturers of core material looking for something designed for switching power supplies in your operating frequency range. Manufacturers will usually supply all the data for their products on their websites and may even show design applications that would fit your project. Then it is just a matter of winding the core. Make sure the wire you use will withstand the voltage you expect across the output.
As a word of caution, a bank of capacitors can store a huge amount of energy. At 200 volts, a discharge could be dangerous, even fatal. Be sure to include in your design, bleeder resistors to discharge the caps when the power is removed and never work on the capacitor bank until you are sure that the caps have been discharged.

Re: Solid core inductance
 

Thanks for your assitance so far, but please read all the posts before responding, I explained how a coilgun works already. And i've made these before, however they were highly inefficient, and kevin I disagree. I've been able to shoot projectiles through tin cans and drywall sheets, at the moment the technology isn't advanced enough to be usefull, but they could be in the future. Also copper does not work, the projectile has to be steel, or another material that can be attracted to a magnet. The discharge from the capacitors at 200 volts not lethal, but it does hurt a little.

I just want to state again that i need to find the permability of the inductor, that's it at this point. There are only a few unknowns, none of which effect the equation for the indutance. I need to find the inductance of the inductor, NOT the fireing coil. So please only respond if you can help me find the permability of the E core.

Re: Solid core inductance
 

Phil,

We might be able to offer better advice if we know what tools and instruments you have available. Al's suggestion is probably the best approach: (1) wind a test coil (loosely) on the center leg of the E, (2) measure the inductance of that test coil on the core, (3) remove the coil and measure the inductance again in air, and (4) calculate the relative permeability of the EI core as the ratio of the two inductance measurements.

This leaves open the question of how to measure the inductance of your test coil. Again, how best to do that will depend on what tools and instruments you have available.

Re: Solid core inductance
 

Phil,
The energy stored in a capacitor is equal to 1/2 * C (in mfd) * V^2 (in kV). working for a 1 farad stack at 200 volts this works out to 20,000 joules. If you don't think that is enough to be letal than you need to stop and get better advice than we can give you here. The 200 volts is enough to penetrate into muscle tissue and cause permanent damage. Even a small bank of capacitors at 200 volts is enough to be lethal across certain parts of the body.

Re: Solid core inductance
 

200 volts at 500 micro farads is not enough to be lethal. trust me I've shocked myself with them before. I only have a bank of 2 capacitors, now if they were a full farad each THEn i could see taking some precautions, but this is not as dangerous as working at Sequa deli :p. Also, I think I may be able to secure the use of an oscilloscope so i will be able to determine the native frequency of the transformer. How would i go about measuring the inductance of the core. Also, i do not have to construct an air core inductor and measure it, being the value for the permeability of air is readily available on wikipedia.

Re: Solid core inductance
 

Phil,
You need to trust me on this one. 200 volts is considered HIGH voltage. A wall outlet produces a little over 200 volts, peak. If you have been shocked before, consider yourself lucky, very lucky!
You need to measure the inductance in an air core first and then slip the core into the coil to measure the difference. You get an idea of what the permeability of the core is by solving the inductance equation for permeabilty when you know the inductance in both cases. I suggest a trip to the library for a copy of the ARRL Amateur Radio Handbook. You will find some discussion on inductance and measuring techniques.

Re: Solid core inductance
 

I've decided to buy a core from that company that i linked to before. I can buy a core in a variety of KNOWN permeabilities, which will let me calculate the inductance accurately. But i will definatly get a copy of that book and check out those techniques!!

Re: Solid core inductance
 

Ugh... Thanks, I was just starting to get over the nightmares from electromagnetics... Now I'll be woken up in the middle of the night by some sort of vector calculus fourier series.

Re: Solid core inductance
 

Have you thought about using AC current and having it shoot like an aluminum ring? Or do you have to shoot something that is ferrous like a magnet?

Re: Solid core inductance
 

200 volts is considered high? Pfft, 240 volts is the standard in european countries, with 440/480 used for Dryers. Enough to kill you? Sure, likely hood? Almost none. Ive shocked myself with 240 and 110. 240 just bites more.

Re: Solid core inductance
 

The discussion is getting a little off track here.

If you've opened a computer power supply and looked at the component that Phil was thinking of modifying (before he found an alternative that makes it easier to design his inductor), you'll see that the cores are labelled with the usual "DANGER HIGH VOLTAGE" text and symbols. Those warnings are there for a reason.

And yes, 240VAC is the standard in Europe. You are statistically less likely to survive a shock from 240VAC than from 120VAC. That may be why the vast majority of european engineers adhere to standards of safe laboratory practice that make us, their American counterparts, look like a bunch of mad scientists. :rolleyes:

Al is right -- 200 Volts is nothing to trifle with.

Re: Solid core inductance
 

Guys,
Just for the record, I have seen people get dead from these voltages. Why do you think they put all those stickers on appliances? There is an old pilots adage that can be applied here. "There are old electrical engineers and there are bold electrical engineers. There are NO old, bold electrical engineers! One of my instructors on the first day of AC electronics class told us this...
"Learn to identify people who do not have respect for exposed voltage. Stay away from them for when they go, they take others with them." If you don't think (and believe) that 120 volts is high voltage and lethal I can have a couple of hundred electricians and linemen convince you otherwise. Do you know why the International Brotherhood of Electrical Workers was formed in 1894? Because the average life expectancy of an electrician was 34 years of age due to working with line voltage.

Re: Solid core inductance
 

But you have to realize that although this is a high voltage system, the amperage is far less then the electric coming from your outlet.

Re: Solid core inductance
 

While you are correct that a wall socket can deliver more current (typical breaker rating is 20A), it takes far less than that to kill a person.

Look here (you brought up Wikipedia earlier in this thread) for more on the physiology of electric shock.

You also said earlier that you have two capacitors of 500 uF each.

200 Volts x 1000 uF = 0.2 Coulomb, enough electric charge to deliver a peak electric current of 200 milliAmperes to a 1000 Ohm load; your body, for example, might be represented by about this resistance, depending on ambient humidity and how sweaty you are. That current will decay with a time constant of 1 second. Delivered to one hand while the other hand is on the return path, that current pulse is sufficient to put your heart into ventricular fibrillation. Of course you might be lucky and not present such an ideal circuit path to your heart on the first try, but your luck would not hold out forever if you kept trying again and again.

I strongly urge you to read up a bit more, and not to attempt experimental verification of the analysis above.

Re: Solid core inductance
 

Which is the exact reason i use the one hand rule. I only work on the circuit while it's charged with one hand, the other is in my pocket.

Re: Solid core inductance
 

That is an excellent start!

I have to ask why you are not using a linear power supply to charge up the caps. It seems like an easy way to accomplish a 200 volt source.

Re: Solid core inductance
 

Well mainly because the power supply i have is a kit I built a few years ago, and only goes up to 12 volts. But the main reason is i want to be able to finish the coil gun and know that i made it from "scratch."

That, and the fact that i need to drag my presentation out to a full 40 minutes, so I'd rather go through the formulas, and show how the boost converter works rather then just say "well this power supply charges the caps". But yes, using a power supply would be MUCH easier, but just ask my mentor, "easy" is not in my vocabulary :D

Re: Solid core inductance
 

Railguns are slightly different from coilguns. In a coilgun, coils of wire around the barrel are used to pull the projectile down the barrel. A railgun uses two rails which the projectile slides down. In a railgun current passes down one rail, through the projectile and back down the other rail. Railguns have the advantage that they accelerate the projectile evenly down the entire length of the barrel. To do this, a coilgun would require multiple stages.

railgun

Re: Solid core inductance
 

But with each stage comes more complications. I think i will make a multi stage and a single stage. A multistage requires 1 capacitor bank PER coil. They also require an IR detector system to trigger the next set of coils(unless you can figure out the excact timeing, which is almsot impossible). But despite which kind i build, first i need to get the boost converter working, once thats working everything else is easy to make. It'll also give me a chance to use one of those PIC microcontrollers i have :D

Re: Solid core inductance
 

Would a voltage multiplier work??? Abeit, I have no idea if that type of design would applicable since it's a high voltage and low current device.

Re: Solid core inductance
 

The simplest thing to do in this situation would be to get a line operated high voltage transformer with a secondary winding of 300 volts center tap. (I am guessing right now at the numbers) In a full wave rectifier, you might be able to get 200 volts DC to charge the caps or to fire the energizing coil. Transformer cost in the $50 + range new, total supply less than $100 (fuse, line cord, diodes and trerminal strips). This is a common transformer in tube type equipment and might be found in a junk TV or radio or at a Hamfest.
What Phil is trying to make is what some people call a "whistler supply". It uses a low voltage DC power source like a bank of batteries, running an oscilator to produce an AC voltage capable of driving a low voltage primary winding on a transformer. The secondary winding is a high voltage winding much the same as the linear supply described above. The difference is the transformer core. At 60 Hz you need a lot of iron to couple from the primary to secondary windings. At 1.5 kHz or above, the core becomes more efficient and so the need for a heavy iron core is diminished. MOst often these supplies are actually running at much higher frequencies like 10-15 kHz. When they operate, the supply can be heard as whistling. This type of supply is comon in a camera photo flash unit when first turned on. Since you need an oscillator and high power driver (sometimes the same device) this is a much more complex device. The upside is the transformer is a fraction of the weight of a line operated supply. However, 200 volts is still 200 volts regardless of how you get there.
High voltage batteries were plentiful at one time when portable radios were tube operated. Finding them now is next to impossible.

Re: Solid core inductance
 

The boost converter is a fun project in its own right. Learning to build switch-mode power converters is worth the time spent. I'm a little biased here; switch-mode power converters were one of my earliest projects (after audio amplifiers) a couple dozen years ago when I was getting started in electronics.

However, if you just want to charge capacitors, a simple charging circuit using a 115VAC line cord, a full-wave rectifier, a couple of resistors (one in the 50 to 100 Ohm range in series with each capacitor for current limiting, and another in the 100,000 to 200,000 Ohm range across each capacitor to bleed off the charge when it's not in use) and a suitably rated, insulated switch for each capacitor you want to charge would do the job nicely, if you can make the coil gun work with about 170 Volts dc on each capacitor.

If you need higher voltage, or if you need to adjust the voltage to control the gun, or if you just want the fun of building a switch-mode circuit, then go on ahead with the boost converter. :cool:

Also, if you are planning on a multi-stage gun with each stage fed from its own capacitor, you will probably want to have a separate voltage measurement circuit for each capacitor so you can tell when they are charged.

Insulate all the high voltage connection points well with electrician's tape or heat shrink tubing. If you plan to demonstrate the device (and why else would you build it?) then it's a good idea to enclose everything in a non-conducting project box after you get it working. And, yeah, keep one hand in your pocket while the circuit is live. ;)

Stay safe and have fun. :)