Capacitors

[Guest]

Just a basic question, how much energy is one Farad? Is there a way to calculate how many Joules there is in one Farad?

Does this depends on something or is it always a fixed amount?

[bugs]

Capacitors

If ive worked things out correctly its:
Joule = Farad * Voltage^2
(see: http://www.evergreen.edu/biophysics/technotes/electron/basics.htmhttp://www.evergreen.edu/biophysics/...ron/basics.htm)

The big question is, why do you ask?

[Guest]

Capacitors

Many thanks Stefan, much appreciated.

Its just something Ive been thinking of as a weapon for our robot. Very exprimental at the moment, hence I dont want to give out to much information about it.

If you have MSN or something Id be happy to share some of my ideas, but not on a public forum.

[Guest]

Capacitors

Tell you now, a Farad is a big thing to generate.

[bugs]

Capacitors

I have ICQ, you could always mail me ofcourse
Remember that EMP wheapons are not allowed.

[Guest]

Capacitors

Oh yeah, havent thought of that.

Its not an EMP weapon, though it does utilize a couple of big magnets (and Im not talking about solenoids).

[Guest]

Capacitors

I just happend to come by a similar project to what Im doing, and apparantly it should be more like this:

1/2CV^2

Where C is the capacitans in Farads, and V is the voltage in Volts.

[bugs]

Capacitors

If id have to venture a guess, Im thinking youre trying to accellerate something. Or does this have anything to do with you trackdrive system?
The factor 1/2 probably has to do with the inefficiency you experiance when dischargeing capacitors.
BTW: youre 7Ah NiMh batteries are incapable of delivering high currents.

[Guest]

Capacitors

Youre probably right about the inefficiency of discharging capacitors, or Im totally out of ideas.

Youre also correct on the fact that Im trying to accelerate something, but Im not sure about what results to expect. It may work great, which would mean that weve finally got an original weapon for our robot.

Or it may be rubbish, in which case Ill keep experimenting with it at home. In either case, Ive got something to do for the next few weeks.

As for the Ni-Mh batteries, were using NCC60s as speed controllers, it wont require a high current!

[talulah]

Capacitors

The efficiency in charge and discharge of a capacitor should be very high as long as it doesnt have to store the charge for too long (when internal resistances will cause it to slowly discharge itself).

The factor of 1/2 is down to the pure physics of it - its nothing to do with efficiency. (in fact, the equation is the result of an integration of C x V with respect to voltage, in the same manner as integrating momentum (m x v) with respect to velocity results in kinetic energy - 1/2mv^2).

I assume you are using supercaps since you are talking capacitance in Farads. Just remember that these generally have a maximum voltage of 5.5V. They WILL blow up if you exceed this! You could try putting them in series to get around this, but this would reduce the overall capacitance (2 1F caps in series = 0.5Farad).

What are you trying to do exactly?

[Guest]

Capacitors

I havent decided on what capacitors to use yet, but I have a sponsor that have agreed to donate a bunch of them for this project. They have capacitors with a maximum voltage of everything between 10V and 500V (the largest ones are 1 decimeter in diameter and over 2 decimeters tall).

[bugs]

Capacitors

Saft VH-D Cells will do 40A continous. This actually surprized me. I would have thought it would be a lot less. You cant fast-charge them though.

When selecting capacitors to store energy and discharge them quickly make sure they are of the low-esr type. Goldcaps will be completely useless.
Also keep in mind that voltages over 36V are prohibited.

[Guest]

Capacitors

That depends on what you call fast charge.

But we dont use the VH-D cell, we use another type designed for higher discharges at the cost of less capacity. Its not on their website, but it does exist.

[Guest]

Capacitors

At the moment it sound like you are building a guass gun.

Would that be with a reuseable line attached projectile, a raped line discharging projectile or a long bar eg. a electro ram?

I tryed make a few simple configs that look like a SMG Harpoon but i did not think that the high voltages it used would be called system with design rules.
Also the penaration ability was not that good, quite poor realy

[bigsi]

Capacitors

building a guass gun
I suspect you are right! (never heard it called that before though - but the name fits!)

I couldnt resist commenting on this!

Ive built a couple of these for firing aluminium rings. Very effective - fired a 200g ring almost 1/2 a mile!

The biggest trouble I can see is the voltage. Because the energy is derived from the square of the voltage and the max current you can drive through the coil is proportional to the voltage, you need as high a voltage as possible.

The other thing (which may have changed more recently) is the 1F capacitors will mot deliver huge currents - even if you short them out - they have a fairly high effective internal resistance.

I used a bank of 12 x 10,000uF (from a UPS) which were rated to 96v, but did not explode until about 220v! Pumped up to about 190v they seemed fine. In total, I only had 0.12F, but high enough discharge current that it would vaporise an average screwdriver!

The coil was wound out of 8 gauge copper wire (3.2mm) and had a resistance of just over 0.1 ohm, giving a discharge current of about 1000A.

This type of gun works because the combination of the capacitor & coil rings. You get an initial field generated which will oscilate for a couple of cycles. The initial field induces a current in the aluminium ring (which in itself is not magnetic so is not attracted to the coil). The current flowing in the ring generates its own magnetic field. When the field in the coil reverses, the field in the ring does not (or at least there is a delay (hysteresis) so briefly, the coil and ring repel one another - which almost doubles the effective force - and the ring shoots away from the coil (and through your ceiling!).

Be careful with this kind of thing as you can just as easily vapourise the coil and even the ring as you can a screwdriver!

Not sure you could get enough kick with the max allowed voltage - but its certainly worth a go! - particularly if you can get one or more 1F capacitors!

I cannot see any reason it could not be effective. My gun spat the ring out with about 2kj KE (less inefficiencies) - which is fairly respectable in RWs terms! You could get the same with 2 x 1F capacitors at 48v!

Keep us posted about how you get on!

Si