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Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I agree with the use of 'common' electrolytic capacitors, make sure that the voltage is not reversed - (reliable diode)
In terms of capacitance/cost and esr/cost and esl/cost an array of smaller electrolytics is better than one large electrolytic. Commercial dc bus capacitors often consist of many 'push-fit' electrolytic capacitors in series/parallel arrays. (opposite sides of a double-sided pcb as the bus voltages)
To be safe, make part of your wiring using wire small enough to act as a fuse if things go wrong.
Comparing the ESR of lots of small caps in parallel vs a few large ones is good advice Sulaiman and will certainly be an important consideration.
Dr. ISOTOP, I am building this for someone else, so since it's not my money I'm spending I'm prepared to spend the extra if it means there is less chance of a component failing!
A google search brings up manufacturers' data sheets for various capacitors, epcos make them up to 2500uF @ 500v which sound great but I can't find anywhere that sells them in small (~10) quantities.
Where can I buy large (>200J) pulse rated caps?
Assuming you mean the b43415c6258a000, arrow europe has 50-odd of them in stock for about ~$50 each.
Just pick some candidate part #s, and run them through the (fairly lengthy) distributor list.
,locale=en,typeId=distri,countryId=US,stateId=.htm
l
I have those: IMO, only this type of caps are suitable for coilgun "built the right way". As those are non-polar, no diode required for protection, which also gives possibility of short pulse duration which length strictly determined by caps' capacitance and coil' inductance (so there could be NO suckback!). Low ESR of this type of caps contributes greatly as well as low parasitic inductance. Sure, film caps are larger than electrolythics for the same energy stored, and price is higher too. But I assure you, that efficiencies gained by non-polar system do cover those disadvantages. By my exp, the main problem in non-polar system is the connection between the charger and the capacitor bank. During coil's firing voltage of caps is reversed, and high current starts to flow directly through the secondary coil of the transformer, frying out output rectifiers and the trans. The ugly solution is to disconnect the charger by a switch just before firing, then fire using the other switch. Headache for the gunner, used to push one trigger (currently I'm working on automated system driving the charger and mosfet switch). And if sequence is messed up, pieces of rectifiers from charger got accelerated in random directions together with the bullet... With the caps linked above (I used 4 of them in parallel) I have achieved efficiency of 4+% from single coil and 6.66% from 2-stage coilgun (using recuperated 34% energy left from first stage), with second stage showing efficiency of 12+%. Feel the difference from "standard" results of 1%? And I'm sure that it could be much better if I had wire thicker than 0.7mm. So give a try to non-polar caps - they deserve it. But stack-constructed ones, just as on the link above - not the cylindrical-shaped, as those have no advantages over regular electrolythic. Stack-constructed film caps have extrimely low ESR and ESL, which is just what is needed for coilgun, and they allow reverse charging giving an easy way for energy recuperation and letting us free from using "suckback diode". Think about it. And BTW, sorry for offtop about film caps, but I think the topic has such strict name just because noone yet really considered not electrolythic caps as those to have for coilgun, which is not fair, IMHO.
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Where to buy new pulse rated aluminium electrolytics?
