I have a 18 VDC 5 AH battery with which I can charge a capcitor. The Capacitor need to be able to discharge in 1/10 second at 18 VDC a 40 amp current across a 4.6 ohm resistance. How do I determine what size capacitor is needed?
Thanks.
Karle, sorry but my fingers
Karle, sorry but my fingers typed it wrong, I mean 0.46 Ohms. Perhaps that makes the question more logical.
Capacitor discharge
Dealing with these currents you need to do some research on “oscillatory discharge”. A capacitor does not give up it’s charge as quickly or evenly as you might think. The discharge is more like a pendulum where it overshoots in both directions several times in a fraction of a second before going to zero. If you have a good storage scope you can see this happen. Many people fail to take this concept into consideration when designing circuitry and then wonder why they have frequent failures.
Calculations
Just did some rough calculations and your .46 ohm resistor will need to disipate 1565.2173 watts. Unless you’re going to pull the nichrome element out of a 1500 watt space heater, I don’t know where you’ll find one.
Just my $.02
Your milage may vary. Batteries not included. Void where prohibited.
Karel, I am now totally
Karel,
I am now totally lost. First of all I misspoke regarding the time of the discharge needed. I have ten lengths of the muscle wire in parallel, and the Dynalloy wire spec chart in effect asks for 40 amps for one second across a resistance of .46 ohms. That works out roughly to 750 watts. That would then require roughly an 85 Farad capacitor using the 18 volts available.
I have a 470,000 uF capacitor available and I tried it using just one wire (requires 4.6 ohms, 4 amps at 18 volts or about 75 watts). The capacitor should have the energy of about the same number of watts. When it was discharged across the single muscle wire there was only a minimal response.
Should I come to the conclusion that trying to fire the 10 wire array with a capacitor is the wrong approach or is my math way off?
Best regards,
Robert
Maybe???
I don’t think your capacitor can provide a sustained output for 1 second at that current. You can get multi-farad capacitors but they are generally rated under 5 volts so you would have to series them for the required 18 volts. That presents another problem, by running the caps in series, you are increasing the internal resistance x the number of caps thereby limiting the maximum current available.
You may want to do some research into super caps. These are available in capacities that can be used to replace a car battery but they’re not cheap. Many Youtube videos on super caps.
Switch circuit
1. What type of circuit did you provide to switch the current?
2. Are you sure you need the cap? What about using multiple batteries?
I recall playing the same game about 10 years ago and never got the wire to react as requested. When I moved the wire got trashed.
Project due
As a school project students were allowed to choose whatever project they wanted, were required to state their thesis and document the process. Wrap-up is due this coming week for my grandson. Thanks to your very thorough explanation the “battle bot” weapon (a flipper) will not be able to function as was planned. One aspect of the project documentation of the process is that one can learn by failure. So this project was not all lost as learning was accomplished by my grandson and I too.
By the way the capacitor firing was by an automobile relay G8JN-1C7T-R activated by a signal from a TACTIC TTX300 3 channel SLT Radio, a TR325 micro receiver and a TReX Dual Motor Controller DMC02.
Appreciate all the information and teaching that you provided.
Best regards,
Robert
PS A couple of photos are available here:
https://www.dropbox.com/s/urmz64vl5qicor7/side%20view-s%20.pdf?dl=0
https://www.dropbox.com/s/ld1r3u2d8au0spc/top%20view-s.pdf?dl=0
Project
Looks like Papa bear had all the fun!!!