R C Circuit with 3 specific output parameters, and specific physical 1

[JoeSmurf]

http://files.engineering.com/getfil...7a-a194-c07501bf7599&file=Circuit_problem.pdf

Hello , I have included my best efforts at a schematic, and related formulas for my problem as an attachment to this post. Please take a look at it if you have the time.

My requirements are to produce 2 volts, and 13 amps, for 30-40 Milliseconds.

My constraints are
* a very minimal physical presence in terms of weight, I have 3 ohm resistors I need the requirements passed through in parallel ( I think they need to be in parallel, to receive the requirements simultaneously, simultaneity is a constraint)
* the power source must be physically disconnected for at least 20 seconds before the requirements are delivered to the resistors.

My variables are
* Just about any power source financially accessible less than a car battery ( expense )
capacitor Farad value can vary
Additional resistors or small lightweight components can Vary, (lightweight in the grams magnitude)
wire gauge has to stay pretty small


I've tried to model the probem myself by thinking about the Time constant in the V(t) = V(o)e^-(t/RC) equation, but I find that the one ohm resistors force the time constant into the nearly instantaneous condition, whereas I need 30-40 MS.

I have been stumped by this problem for months, and have asked many people, I wouldbe very impressed to find a nice elegant solution, and EXTREMELY appreciative. If you would like to reach me directly my email is joexsmurf@gmail.com I will check the forums often for a response.

Thank you for your time.

http://files.engineering.com/getfil...7a-a194-c07501bf7599&file=Circuit_problem.pdf

 

[IRstuff]

Student posting is not allowed.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[JoeSmurf]

-IRstuff,

I am not a student. That's kind of a rash assumption. I am a Civil Engineer, I do structural design engineering. I also do model rocketry for a hobby. My goal with this circuit is to charge a capacitor on the ground that will discharge at a certain moment in flight. That moment is after the first booster has burnt out, but instead of lighting just one additional engine after that, I want to light 3.


The problem has 3 aspects..

-Simultaneous firing.
-no battery going up ( too heavy)
-Firing has to be timed, this will be done physically triggered by the seperation of the lower stage.


Anyway, hopefully someone else has a more productive response.

 

[Skogsgurra]

The first problem is that you do not have enough charge in the capacitor. The available charge is 15*2200 uAs or 2.2 mAs while you need 13*30 mAs, which computes to 390 mAs. So, no matter what clever circuitry you put there, the energy produced in your resistors will be barely noticable even if your circuit is 100 percent efficient - which it will not be.

A much better approach is to use a small hearing aid or clock battery. They can easily do what you want. Switch control is a separate issue. Get the available power right first.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[JoeSmurf]

Skogsgurra,

Thanks for the reply, I have purchased a small hearing aid batteries that seemed rated in voltage and amperage to what I need. However this is a path I'm reluctant to use, even though 3 batteries of that size are not much more weight than the capacitor, it would mean having to change the batteries by deconstructing and reconstructing the rocket. Vs if I use the capacitor solution, I can just use any battery source outside the model, and replace it as I need to without deconstruction.

However I think I understand what you are saying about the energy being minimal, is there no way to modify this by using different capacitors, resistors, or any other circuitry ?

 

[Skogsgurra]

You need a capacitor that is around 500 times bigger than what you have. Even then, the varying voltage will be a problem and you will probably not get anywhere even if you build a sophisticated switch-mode voltage regulator.

I cannot see any problem with that battery. Wires to a batteri compartment that is accessible from outside. That is how it is done.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

How about a low ESR Supercap? Example products (*) can be rated for 4.5 volts with an ESR of 40 milliohms and capacitance of anything up to about a farad.

*

http://www.cap-xx.com/products/products.php

e.g. GS2

 

[Skogsgurra]

Yes. Have used them as memory back up. Not very good at high current. Why not test?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[VEBill]

Has to be the low ESR type. Supercaps for memory backup have (very) high ESR. The linked website says that they're good for high current pulsing. Not tried them myself.

 

[IRstuff]

VE1BLL's cited capacitors should be fine for the currents specified. We've not used that particular brand, but we have used Evans caps for high current applications, ala 150A with slightly higher ESRs, but we had way higher voltages.

The specified load has only 154-ohm resistance, so circuit parasitic resistance options are very limited. Even the specified 40 mohm ESR will drop 0.5V at specified output current.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[VEBill]

Might need two supercaps in series. With only one, even at 4.5 volts (the Vmax for some supercaps), over 0.33-ohm load, probably just fails to get to 13A (considering the other voltage drops). It depends how close to 13A is close enough. Experimentation beats specs.

Another option would be to separate the three circuits into three separate loops. This would drop the current and make things a bit easier. It's unusual, but I think one could still use just one high current switch to close all three circuits at once (provided they're all at the same state of charge). The reduced current in each loop might bring little batteries (x3) back into the realm of possibilities.

 

[IRstuff]

Pretty interesting article about the vagaries of igniters. However, it presents some very contradictory information w.r.t. to the OP's stated current requirements. Assuming the three igniters are in parallel, then their individual resistances are more like 0.46 ohms, and if I believe the article, the current requirements are more like 10A PER igniter, so more like 30A total. If that's correct, then the minimum voltage must be at least 4.6V, and you need to be looking at something closer to a 12V system.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize