# Capacitor/Railgun Question



## Vintageman (Aug 21, 2011)

I think I'm going to build a railgun for an upcoming science fair if I find out that it's allowed. I might just build it anyway for fun. However, when building a railgun, what is the most important thing about the capacitors? Should they be high voltage or high farad or both? I'm a bit confused also on the whole joules measurement thing too, is that a derived measurement? What is it a measure of?


----------



## octaneman (May 13, 2009)

The most important thing is your power supply and the capacitors ability to able to store the electrical charge until a a large current is accumulated. The ratings in microfarads on the capacitor determins how much charged is stored at a given voltage. The voltage rating is the maximum voltage the capacitor can hold before it goes up in smoke. Both numbers are equally important if your planing that kind of project. 


*WARNING*! : *THE ELECTRICAL CHARGE CAN LAST FOR MONTHS EVEN WHEN NOT IN USE. IF NOT HANDLED PROPERLY HIGH VOLTAGE CAPACITORS CAN KILL IF THEY ARE NOT DISCHRGED. *


----------



## Vintageman (Aug 21, 2011)

Here is what I'm thinking I'll use for my power supply and capacitor. Will they work together?

*Capacitor*
-40V DC
-130,000uF
-85° C
-20% Tolerance

*Power Supply*
-Form Factor: Benchtop
-No. of Outputs: 1 o/p
-Range programming modes: Auto
-Output 1 Max. power: 60 Watts
-Output 1/Range 1 Max. Current: 1.5 A (0 V to 40 V)
-Voltage Ripple and Noise: 200 uV
-Current Ripple and Noise: 500 uA
-Load Regulation, Voltage: 2 mV
-Load Regulation, Current: 250 uA
-Line Regulation, Voltage: 2 mV
-Line Regulation, Current: 250 uA
-Transient Response Time: 50 us
-Programming Response Time: 45 ms
-Remote Sensing Measurement: No
-Multiple Unit Combinations: Auto-Parallel,Series
-Overvoltage Protection: Yes
-Overcurrent Protection: No
-Over-temperature Protection: No
-Local Lockout: No
-Self test: No


----------



## Done_Fishin (Oct 10, 2006)

take a look at this .. 

HowStuffWorks "How Rail Guns Work"

some very basic info but I'd say that you have to decide and define just what you want to do .. 

a very simple type of gun (IMHO) would be a simple solenoid which would move a projectile just a few centimeters. In the article above they state that very large currents are needed and that the size of the capacitors would take up several cubic meters ..


----------



## Vintageman (Aug 21, 2011)

I already understand how railguns work, and that if I am only propelling a .2 gram metal BB about 40 feet instead of a projectile around what looks to be 60 pounds for 250 miles, I will need a lot less current. All I would like to know is if this power supply would fully charge this capacitor, and if this single capacitor would release it's voltage fast enough.


----------



## gcavan (Aug 13, 2009)

> I already understand how railguns work


That's good. Now you need to learn how capacitors work.

Any power supply will charge any capacitor to the maximum rated voltage of the power supply. Every capacitor will discharge itself within 5 Time Constants (τ).

RC Charging Circuit Tutorial & RC Time Constant

PS: The above deals with a simple series RC circuit. You are more than likely going to be dealing with a series-parallel RLC circuit so the mathematics will be somewhat more complex, but this will get you started.


----------



## Done_Fishin (Oct 10, 2006)

one of the things I remember reading somewhere was a maximum amount of capacitance sitting behind a diode or bridge rectifier ... a capacitor appears as a short circuit to a power supply when it is drained and the power applied .. likewise a coil appears as a short circuit when a voltage is applied .. it is of course the fact that the voltage across the device changes as it is charged that stops the device from burning out or burning out your power supply .. 130,000 uF is a lot of capacitor 

Good luck with your project .. :wave:


----------

