coil gun power supply

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kennyenstrom

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hi im looking to wire up a microwave transformer to a standard 120 volt outlet
ive calculated that a 480 w, 30 ohm resistor will drop me to 4 amps. I have already done this with ceramic resistors with no success as they just overheat and short out. Do you have any recomendations on resistor types i have been looking at wirewound resistors from ohmite. could these work? heres the link
**broken link removed**
thanks
 
a heater would be to large for the small 1 foot x 1/2 foot x 1/2 enclosure allowed for the charging supply
 
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you mean because of the high voltage. of coarse. or the capacitors blowing up haha its happened and made holes in my garage or of the projectile?
 
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What about using an inductor or capacitor to limit the current?

An 100µF motor run capacitor should to the job fine be ware that if it's in series with an inductive load possible resonant effects need to be taken into account.

A 100mH inductor will also do but good luck finding one rated to 4A, it might be easier to wind your own on a transformer core.
 
ya i think an inductor would be out of the question and it is ac out of the wall. i know the capacitor wouldnt gain voltage so what would be the resistance?
 
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If I were you I would look at a different approach for charging your caps.

Google 'Xenon strobe schematics' for information on charging high voltage caps from mains.
As already stated, you're talking ridiculous power here, microwave transformers won't just give you a sharp zap, they'll send you straight to your maker.

Unless you've got experience at most I would order a mains xenon strobe kit, then remove the xenon lamp and trigger, and add a form of cut-off to prevent over charging the cap (which you shouldn't need to do if you've got the voltage right).

Blueteeth.


Also note: Coil guns don't necessarily work best at 300-400V capacitor voltage. The coil gun designs on the internet are woefully inefficient....going for raw power rather than any calculated methods. It may be prudent to start out charging caps to a lower voltage for your experiments, them ramping up the voltage later to see the effect. Sure you'll dump more energy in the coil, but magnetic field is based on current (capacitor value) rather than voltage.
 
But a higher voltage increases the current rise which is what you need to shoot things long distances.

You also need to abruptly cut the current when the projectile has left the coil or it will start pulling it back.
 
I built one of these, just used a chunky voltage doubler (rectified) direct from mains voltage. The charge current was fairly low, but that doesnt matter really, just takes longer to charge, there is no need for a transformer for the charge circuit.

As hero says, the timing is critical, as I found. That proved to be hard to control, because dumping huge amounts of current into a coil, a relay/contactor just wont cut it (far to slow to turn off, and would be destroyed by arcing, both on and off), I ended up useing a massive surplus SCR (1200v, 1000A), It gave an instant dump without arcing or bounce, but of course stayed on untill the caps were discharged. Thats OK, the discharge time is pretty short, but the only way to contol exit velocity and prevent pull back, was carefull, experimental selection of the projectile weight and position in the coil before launch.
 
i was taking about using an inductor as a resistor before the microwave transformer. i wasn't ruling out inductance from the whole circuit as the microwave transformer itself uses inductance between the primary and secondary. haha no need to worry. i just tried using two metal pipes and sticking them in salt water and varying the molarity of the salt. it worked but splashed water all over the place. all i really want to know is could a 30 ohm 480 w wirewound resister take the load.
 
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I'm not sure if this will help, but here is an idea to control the primary voltage across the transformer.:

Make a variable heavy duty inductor by taking some #12 insulated wire and wrapping it around a tube form .....maybe 50 turns or so.... whatever works.
Then find a length of iron ...rebar might work. As you place the iron slug into the wire wrapped tube, the inductance of the component will increase and the voltage drop across it will increase, while at the same time, the voltage across the other circuit component, say the primary of the microwave transformer, will decrease.

I think the original application of this scheme was to control the heating/temperature of a electric clothes iron....Have not tried it myself.

If you have a chance .... post a circuit of what you are trying to do....
If it works, don't for get to post a video clip of the coil gun...
 

Kenny, the MOT is overkill. Unmodified, a 120V MOT has a secondary of over 2kV. You could remove the secondary windings, re-wind it with 18AWG solid insulated wire, and keep checking the voltages as you wind it till you get what your looking for. This is very time-consuming.

For Coil gun/Rail gun cap bank charging, I built a Boost Converter. It runs from any 12V 2A wall-wart, and charges my 400V 2000uF bank just fine. Just be careful with selecting the diode and the inductor. My diode (connected to the inductor) is rated for 1kV 5A, and the inductor is 130uH ferrite cored salvaged from an old computer CRT. Oh, and DON'T run a boost converter without a load (cap-bank), unless you want to fry the 555 and LM311.

Small 1/4 Watt inductors explode, too small ferrite core inductors get really hot and short.

How many stages are in your planned CG? What are you using to control them?
 
could i use dc to power the transformer maybe if i used a car battery i wouldnt have to woory about blown breakers. are ac transformers capable of running on dc or do i have to get a specific transformer
 
You can power an AC transformer from a DC source, but you need an oscillator in the middle. Properly rated transistors and resistors are all that's needed. If you send straight DC to an AC transformer, you get an electromagnet.

Disposible camera flash circuit boards use D1960 transistors and a couple small 1/8W resistors to make the 1.5V DC oscillate to run the small transformer. Search for a schematic on these flash boards, and scale it up for your supply/transformer.

I have successfully built a board using 4 transformers with 4 transistors, and 8 resistors to power up 1.3kV (AC) from a 12VDC source. Fed that into a 16 stage Cockroft-Walton Multiplier to put out 2.7kV for a "pocket" Tesla Coil.
 
wow reading through this project makes me wonder why kenny haven't posted after the 5th...
Kenny? are you still alive?

Either way, you know, you have 30 ohm, and 480w, do the math
 
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