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series/parallel capacitor bank

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cboehm

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I want to build a high voltage capacitor bank for various high energy experiments. I've found plans for banks of various specifications, many of which contain resitors placed in various locations. From the reading I've done I've gathered that the resistors play a role in evenly distributing charges across capacitors wired in series. Seeing as how dangerous working with an EXTREMELY high energy circuit is, I would like to have more understanding of the theory behind the bank in case I need to call BS on the plans before I accidently kill myself. I understand how capacitors store energy, as well as the formulas for the effect of series/parallel arrangment on capacitance and voltage rating. What I really need to know is how to calculate the resistor values needed to properly do the job of balancing charge, where they go in the circuit, and why.
 
The resistors have two functions: to drain the capacitors when ther power is turned off and (as you said) to ballance the voltage in two or more capacitors connected in series.

The problem is that is several capacitors are connected in series, they'll each have different leakage currents and capactiances to the voltage across each capacitor will differe significantly. The resistors form a potential divider which ensures that the voltage is distributed more evenly.

Don't worry about the value too much, to use something high like 1M or 10M.
 
Also be careful about not exceeding the max voltage ratings of the resistor packages you are using. You may have to use several across each cap (in series) so that you say within a safe margin.

Lefty
 
thanks

thank you both for the explanation as well as the additional tips. Onward to eBay and victory!
 
in reality you cannot choose 1mega or more because it realy depends on the leakckage of the caps if one leaks more then another a 10mega res will fry both. yes if the Energy is there stored and you are not carfull you can get fried once the the skin breakdown is reached the blood is sodium METAL. you can feel 28v and that depends on moisture your skin calouses other things. open wounds and 1.5 volt from a D CELL you are dead if it goes trough your heart. it is funny but most people that die from electrocution the path is invariably the penis FULL OF BLOOD
 
Lol a 1.5V cell can't kill you even if the skin was peirced.

The lowest voltage I've felt a slight tingle from was about 56V (from a fully charged 48V battery bank) and that was because I had sweaty fingers. A friend of mine has touched 75VDC and hasn't had a shock.

Your blood doesn't contain any sodium metal (or any other elements in their metalic states) but sodium ions and other ionic compounds which make it conductive.

Generally, if you have moist skin, you should avoid contact with any voltage above 30VDC or 12VAC, if your skin is dry then you're alright up to 25VAC or 60VDC (see the IEE regulations).

Capacitors are a totally different story. The potential for electricution depends on the capacitance as well as the voltage. A 100pF capacitor charged to 25kV will not harm you but a 100:mu:F capacitor charged to 250V could kill you so it's a bit more difficult to estimate.

Generally capacitors charged up to 60V are safe (even if you have moist skin the shock probably won't last long enough to kill you).

At higher voltages, it's the energy which kills, not just the voltage so the capacitor size is more of a factor than the voltage. When working on capacitors that could be charged above 60V, make sure that all the electrolytics are discharged before working on the circuit. Ceramic capacitors tend to have lower capacitances and are less harmful.
 
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I'm a HVAC service technician, which means I get electrocuted on a fairly regular basis. I've been hit with 460V on many occassions, only stopped my heart once, then only briefly. They teach us to take measurements with one hand in our back pocket, that's probably the only reason any of us are still alive. I've been juiced by several motor run capacitors, usually under 80 uF though. Even then they were charged by AC, no telling what part of the wave they were at when they were disconnected. Wakes you up though. You've told me to use 1-10 Mega Ohm resistors, connected across the leads of the capacitors. This will discharge the capacitors automatically when disconnected from the charging circuit, and balance the voltage drop across each of them. I'm inclined to believe 1-10M will do the job and don't doubt your expertise, but I'd really like to know the science behind it. Could you perhaps point me in the direction of a good resource with which I might better educate myself?
 
capacitor bank

if i had 20 1000uF 100V capacitors and wanted to make a bank of them for a disk laucher or some other fantastically destructive project, but needed to wire them in series/parallel branches to increase the voltage rating, would i still be storing the same amount of energy as if they were all wired in parallel?
Say I had 5 parallel rows of 4 caps in series. I'd have 400V nominal at 1250uF. If I put them all in parallel i'd have 100V nominal at 20,000uF. Which arrangment stores more power? I'm wanting fantastic current in a very brief pulse.
 
The energy stored in a capacitor is ½CV² and so,
First case,
0.5 * 1250 * 10^-6 * 400² = 100 Joules
And,
0.5 * 20000 * 10^-6 * 100² = 100 Joules

Pretty obvious really as all the capacitors will be charged to 100V and so will all contain the same energy. One big difference between the two is that the 400V one will kill you.

Mike.
 
thanks. the bank i'm working on will actually be charged to 5.2 kV, which is why i'm intersested in the actual formulas and safety interlocks i'll need in order to understand and survive the project, as opposed to ending up laying on the ground dead with no hands next to a freshly exploded apple or bent hard drive disk.
 
I have a related question. How can I charge capacitors in parallel the discharge them in series without using spark gapping?
 
Depends on the voltages involved Tigrathi, normally you just use mosfets or transistors as switches.
 
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Starts of at 120v then quickly goes to 500,000v. Then it gets passed to a 2nd bank which further increases to values that burn out most high voltage equipment. I don’t like using spark gapping because I loose a lot of power with each arc.
 
Good luck finding another type of switch that will work at 500kv...
You could control the atmosphere around the spark gap, but I have no idea what gas you would use to optimize conduction during the spark and still provide enough isolation. It'd be hit or miss and getting the gasses would be a pain.
 
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Tigrathi, they're called resistors =O
 
Resistors produce way to much heat at those amp levels. Unless you know of resistors that can limit a circuit to 100amps without burning out even if the fuse is rated to 250 amps.
 
So what is this thing that you are building that takes over 100 amps and produces 500 Kv?

What circuit do you have now if any?
 
Well I have a early sketch in my notebook of the initial power part of the circuit.
 

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