what to do with 4 large capacitors

kubeek

Joined Sep 20, 2005
5,682
These caps can store cca 2700J. You should always have connected some bleeding resistor to make it safe after power-off.

Also be careful when manipulating with them, a short circuit could be quite dangerous. Peak RMS power dissipation with four caps in parallel charged to 420V, discharged into 10ohms is cca 5.5kW!
 

kubeek

Joined Sep 20, 2005
5,682
BTW is it possible to damage for example a cell phone RF stage by the electromagnetic pulse from the discharge?

Coil gun: If you manage to transfer just 50% of the energy to the projectile, it will be a seriously deadly weapon. If I remember right a Magnum .500 cartridge has cca 1300J.
 

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
what kind of bleeding resistor should i use? also i have limited resources so the coilguns that i have built have just been the coil and the cap with a switch in between when i used these capacitors it did not work well
 

kubeek

Joined Sep 20, 2005
5,682
The bleeding resistor is more appropriate in the case of power supply and or when building the whole thing.
It is a high value resistor which will discharge the caps in some short time like one minute or so. But yesterday I simulated charging the caps by 250mA and it took 50 seconds to get to 400V. The resistor would dissipate in average 50W to discharge it in that time, so it is impossible to use it.

I think that instead of contactors, you could use an IGBT or mosfet rated for the voltage and current your coil will take.


Also few days ago there was some other topic in this forum involving a coil gun, with a site which looked quite good.
 

Thread Starter

Joeshmoe

Joined Jan 12, 2007
66
could you show me an example of one that would work? the circuit has low resistance and i don't know the pulse time but i am using a simple self made mechanical switch
 

andy73

Joined Sep 26, 2007
10
A mechanical switch is no good, the losses are massive, try IGBT or big scr's nice and simple. you could connect them in para so as to handle the amps flying back and a protective diode bank in anti-para to your coil, so the amps wont get to your caps or switch, but this will increasse your pulse fade-out time, or you could use zener diodes to kill off the pulse quicker!!

the best you can expect to get from a coilgun is about 1 - 6% energy out, so if you put 2500j in and get 1% out thats 25j out, which can move a 2-3 grame projectile well past the 100m/s barrier, that is if your coil and pulse time are designed correctly, if not then "suckback" gets involved and you may as well just throw your bullet...
 

SgtWookie

Joined Jul 17, 2007
22,210
For bleeding off the voltage when the supply is turned off, consider using four of those 7-watt 115v Christmas tree (or night lite) bulbs wired in series with a NC relay contact; the relay being held open by the charging circuit supply.

When the supply is turned off, the relay's coil de-energizes closing the contacts and bleeding the power from the caps. Initially, the bulbs will light up when the caps are at full voltage. As the power bleeds off, the voltage across the light bulbs will decrease, allowing the filaments to cool, which decreases their resistance, thereby accellerating the rate of discharge.

Cheap and effective. The problem will be locating a reasonably-priced relay with the contacts rated at 480v or better.
 

thingmaker3

Joined May 16, 2005
5,084
Some hobbyists have reached a whopping 4% efficiency in their coilgun experiments. Pretty darn good for hobbyists! (The military is up to what, 10%?)

Don't discount power MOSFETs - you can get them for a buck a piece if you buy surplus. On rare occasion, one can find good deals on used IGBT bricks as well.

Here's a Wiki that may help: http://wiki.4hv.org/index.php/Coil_gun


Of course, PapaBravo's suggestion is a gizmo for years & years of fun... how long will you play with a coilgun?
 
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