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Proper capacitor bank construction

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Kizmo

Tue Mar 04 2008, 12:13PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Im making fairly large capacitor bank from 3300uF 350VDC capacitors (Total 6600uF @ 1050V, in future maybe over 10000uF..)

It will be constructed like this:

Protection diodes will be 200A @ 400V each but how about those balancing resistors?

teslacoolguy

Tue Mar 04 2008, 03:48PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

the resistors all depend on the value of them i would use no less than 1meg ea at 1w each but the higher the better.

David.Lightman

Wed Mar 05 2008, 03:49AM

Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38

what is the purpose of wiring the caps in series parallel? Why not total series? I am guessing that again, math is keeping me back. Series would increase the farads, and the parallel increases voltage too?? I have mine wired like this: caps

D.L

Shaun

Wed Mar 05 2008, 04:44AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

David, all of your caps are wired in parallel. Voltage stays the same, but capacitance adds.

When you put capacitors in series, the voltage you can charge to adds, but the capacitance decreases.

rp181

Wed Mar 05 2008, 01:58PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

say you have 2 500v 1000uf caps. in parrallel it comes to 500v 2000uf. In parrallel, it comes to 100v 500uf.

uzzors2k

Wed Mar 05 2008, 02:10PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Potential energy remains constant in either case, so it's just a matter of design or practicality.

The size of the resistors is non-critical, as long as they are all the same size. Their exact size depends on whether they are bleeders or just for balancing. Just use the RC time constant rule to estimate a size if they are for bleeding, and then select a power rating so they don't burn up. For balancing I'm not sure how to find the ideal size, but since the reverse resistance of a diode is huge pretty much any resistance will work.

ramses

Wed Mar 05 2008, 03:16PM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

agreed, and all series would increase the equivalent series resistance beyond what you can tolerate. all parallel would give lower series resistance, but lower voltage. the deepfriedneon.com MMC calculator(although designed for tesla coil capacitors) is a great way to determine you options.

ramses

Sulaiman

Wed Mar 05 2008, 07:48PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Such a cap bank can be LETHAL
The most important parameter is safety, so I recommend

Blast-proof enclosure. You're dealing with more energy than a Smith & Weston 0.500 bullet - hopefully enough said !

Electrocution - you MUST ensure that there is not a lethal charge on the cap bank before you get near it
this means fitting a safe method of discharge, and monitoring the cap bank voltage.

I would start with a time constant of about one minute, which means, about three minutes before the cap bank is 'safe'
each parallel row of capacitors should have a discharge resistor that forms a time constant of 60 with the total parallel capacitance of the row.
e.g. Each row = 5 x 3300 uF in parallel = 0.0165 Farad, T=RxC so R=T/C=60/0.0165=3636 Ohms .... say 3k6
The power lost in each resistor is VxV/R which for 350 V is 34 Watts ... use a 3k6 50W resistor per row.
You could reduce power loss (and resistor cost) by using, for example 36k 5W resistors
BUT you will have to wait at least 30 minutes for the cap bank to be 'safe' (about three time constants)

Even better would be to use several higher value resistors in parallel per row
this would prevent the failure of a single resistor leaving a dangerous voltage on it's row.
The simplest is one resistor per capacitor.
So for the above example, instead of one 3k6 50W resistor per row
you could use one 18 kOhm 10W resistor per capacitor.

Don't worry about exact values - 10k to 33k should be ok.

David.Lightman

Wed Mar 05 2008, 10:35PM

Registered Member #1327 Joined: Mon Feb 18 2008, 12:13AM
Location: North Ridgeville, Ohio.
Posts: 38

Uzzors wrote ...

Potential energy remains constant in either case, so it's just a matter of design or practicality.

On that note, I think I got too much for my own good. I have 40 caps @ 2400mfd 450vdc.. I'm going to have to take it slow. Maybe sell off a few to get funds for a power supply. I have to find a way to put more than 24vdc into them. Maybe a simple way to rectify 120vac.

Kizmo's thread, but I am am finding it very informational..

D.L

rp181

Wed Mar 05 2008, 10:58PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

where did you get those caps? im looking for some

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