Capacitor discharge speed

[Jo-W]

I have a capacitor of 30.000 uF, 35VDC

I am using a voltage of 30V, the capacitor is connected to a coil which has a resistance of 6 ohm.

This means my capacitor is fully charged at: 30V * 0,03F = 0,9 Coulomb.
With 6 ohm, my coil will use 30V / 6 hm: = 5 A.

And 0,9C / 5A = 0,180 seconds.
My question is: does this mean my capacitor will discharge in 0,18 seconds? Are there any other factors involved? capacitor resistance? -log() discharge speed? etc


Thanks

 

[Roff]

I have a capacitor of 30.000 uF, 35VDC

I am using a voltage of 30V, the capacitor is connected to a coil which has a resistance of 6 ohm.

This means my capacitor is fully charged at: 30V * 0,03F = 0,9 Coulomb.
With 6 ohm, my coil will use 30V / 6 hm: = 5 A.

And 0,9C / 5A = 0,180 seconds.
My question is: does this mean my capacitor will discharge in 0,18 seconds? Are there any other factors involved? capacitor resistance? -log() discharge speed? etc


Thanks

It will also depend on the inductance of the coil.

 

[evandude]

Well for one thing, the capacitor will only be 30v when fully charged, and its voltage will drop off proportionally as it discharges - and by V=IR, the current will drop as the capacitor discharges. Making the assumption that the load is purely resistive:
[latex]Vr(t)=V*exp(-t/RC)[/latex]
where Vr(t) is the resistor voltage vs. time, and V is the initial capacitor voltage.

Since a coil is NOT going to be a purely resistive load, but rather a big inductor, the actual result will be somewhat different.
If you really want a quick-and-dirty way to visualize what's going to happen, put it in some circuit simulation software and run a transient simulation. Doing the math to find a mathematical function for the voltage/current vs. time with the inductance accounted for is also not very difficult.

 

[Sceadwian]

ltspice is free and easy to use for this kind of simple simulation. You can get it from Linear techs website. I have 'simulated' answers to more questions I've had than I've ever asked on this or any other forum =)

 

[Hero999]

You really need to simulate it, calculting it is a pain in the neck.
https://www.electro-tech-online.com/threads/rail-gun-formulae.25613/