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Series-parallel capacitor bank

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Solidacid

Mon Nov 08 2010, 10:28PM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

i'm currently designing and building a coilgun as part of a high school mechatronic project. i have to launch a 50g projectile(13mm diameter, 52mm long) 10 meters.
i'm using 4 capacitors rated for 450v 4700Âµf.
i pulled the caps from an old defibrillator a few years ago and some of them have minor dents.
how do i figure out how to put together my bank, should it be some kind of series-parallel thing?
i'll be using my ZVS with a rewound flyback for charging, are there any special considerations i need to take when charging my bank considering leakage and/or difference in voltage rating?

i'll be using an arduino to determine the voltage required for the gun to shoot a certain distance but the arduino ADC has a maximum voltage of 5v, should i use a resistive divider or some other thing to lower the bank voltage to a measurable level?

any pointers on coil design? what would likely perform best with such a heavy projectile?

Turkey9

Tue Nov 09 2010, 12:18AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

You're going to want to put your caps in parallel. This is because with such a large projectile the pulse will have to be fairly slow to allow for proper tuning.

For the ADC just use a 100:1 voltage divider right off the caps. I'd use a 1M ohm and 10k ohm to reduce leakage current. You might have to run this voltage through a buffer due to the fact that ADCs don't like high impedance sources (more than 2k ohms).

As for the coil, I'd go with normal solid core pvc insulated 14 awg wire. This is the same stuff used for home wiring and can be found all over. with such high powers it won't matter too much that the insulation is thick. I'd make it about 65mm long and three or four layers to start with. You'll probably need more but it's a start.

Nicko

Tue Nov 09 2010, 02:35PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

If the caps are old (more than a few years out of use), I'd reform them before using them for pulse operations at high voltage. When electrolytics are left unused, they undergo chemical changes which can result in serious failure should full voltage be suddenly applied after a prolonged period of non-use.

For my recent cap bank

, I reformed the caps I had in 50V increments for 15 minutes a time, i.e. 50V for 15 mins, then 100V for 15, etc. all the way to the max for the cap. Took a few days to do them all (I did 12) but they now measure well within original manufacturing specs for ESR/ESL/leakage etc.

Cheers

Solidacid

Tue Nov 09 2010, 07:25PM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

today i made a new coil with the wire Turkey9 suggested, i made it 4 layers thick and 62mm(d'oh) long.
at first when i tried it with a car battery i was really disappointed because nothing happened i then found out my projectile wasn't ferromagnetic....
after making a projectile i tried the car battery again and on the first go it shot about 4 feet :)

i've been doing a lot of testing with one of the caps all day so i charged it to higher and higher voltages, the highest it got was around 360 a couple of times but i think i'll reform em' all anyway.

would it be okay to put all 4 caps in parallel with each their own bleeder resistor an just charge them all at the same time or should i have a diode from the charger to the positive side of the cap?

edit: i'll just get the arduino to reform the caps for me! :D

Turkey9

Wed Nov 10 2010, 03:19AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Do you mean a diode to each capacitor from your charger? You need a diode to rectify your flyback circuit for it to charge but it sounds like you already have that taken care of. Because you're not discharging the caps individually you can just hook them all in parallel with multiple bleeders or just one.

Next thing you'll have to consider is how you're switching all that energy. You could use a mechanical switch but that would be quite a build. I'd use a stud SCR that way you could control the firing with your Arduino.

Nicko

Wed Nov 10 2010, 06:05AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

Solidacid wrote ...

i've been doing a lot of testing with one of the caps all day so i charged it to higher and higher voltages, the highest it got was around 360 a couple of times but i think i'll reform em' all anyway.

Just found this that I measured a few weeks ago when reforming the caps for my bank -

- shows a decay curve for internal leakage for one of my reformed caps charged to approx 500VDC and then left for 32 hours.

Yours decay curve should look very similar.

Cheers

Solidacid

Wed Nov 10 2010, 07:02AM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

i've been planning on getting a stud SCR, i need to find one from Farnell though, any suggestions? i have about 400$ available for the entire build.
about the diode thing, i was thinking something along the lines of This but i just realized thats for multiple coils.

i'm using one of those "modern" black flybacks so it should be rectified already, right?

Turkey9

Thu Nov 11 2010, 12:39AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

I'd get on eBay before you go and buy anything new. It will save you hundreds of dollars. Just brows you should be able to find plenty.

Neuuubeh

Mon Nov 15 2010, 08:31AM

Registered Member #3411 Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33

Turkey9 wrote ...

I'd get on eBay before you go and buy anything new. It will save you hundreds of dollars. Just brows you should be able to find plenty.

Yep, try having a look there, there are really plenty of amazingly cheap high-powered semiconductor devices on ebay, especially ebay.com. We european guys are really envious of that :p.
A recent buying spree netted me several 800v 5500a surge hockey-puck type SCRs for like $6 each. There are plenty of nice deals, however sadly the sellers of the really exotic items (igct with like 60k surge...) either dont ship internationally at all or require shipment via ups or something -> $160 for shipping on an item I dont see getting higher than $30ish... no ty.

Oh well enough ranting, good luck with your project, post pictures please :).
Regards
Nick

PS Ow, first post haha

Solidacid

Thu Nov 18 2010, 12:03PM

Registered Member #1430 Joined: Sun Apr 06 2008, 11:12AM
Location: Ã…rhus, Denmark
Posts: 102

the coilgun has shot the 50g projektile about 15 meters with only 2 of the capacitors i mentioned in my first post.

thank you all for you help!
i'll make a post in the projects section when i'm done cleaning up the design and such.

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