FET driven remote ignition

[twjdeboer]

Hi everybody,

I'm currently in the process of building a remote igniter for a small project in my student society.

I wanted to use an old doorbell in conjunction with a FET, for handling the high power and a 3,3 Ohm resistor.

All the pieces work well. When I press the doorbell, my FET goes to '0' Ohms, and I measure the 4,7 volts of my 3 AA batteries. I can ignite the resistor with the batteries (I measured 8 Amps when shorted).

The thing is: when I use my FET circuit, the output Current (again, shorted with the DMM) is only 400 mA. It was suggested to me that this was because of internal resistances in the batteries and fet circuit, but the math just doesn't check out for me.

I wrote this little matlab script to find out my maximum output current and optimal value for the resistor. Don't know if I've done something wrong.

Code: [Select]

clear all
clc
close all

Ub = 5 ;
Rth = [1:0.01:10] ;
Rl = [0:0.1:20] ;

for j = 1 : length(Rth)
   
    for i = 1 : length(Rl)
        Ur = Rl(i) / (Rl(i) + Rth(j)) * Ub ;
        I = Ur / Rl(i) ;
        P(i) = Rl(i) * I^2 ;
    end
    Pmax(j) = max(P) ;
end

Rth_max = Rth(find(Pmax > 3, 1, 'last'));

    for i = 1 : length(Rl)
        Ur = Rl(i) / (Rl(i) + Rth_max) * Ub ;
        I = Ur / Rl(i) ;
        P(i) = Rl(i) * I^2 ;
    end
   
figure
plot(Rl,P)

I looked for the value where my maximum output power was >3 watts. I need a Rthevenin of around 2 Ohms, which I cannot measure with my cheapie multimeter. I get this plot for power vs. Heater Resistance.

I'm using http://www.irf.com/product-info/datasheets/data/auirf540z.pdf FET. This is my first encounter with FETs so far, so maybe I get the concept wrong, but it kinda works already.

[twjdeboer]

I'll draw up a schematic when I have the time. I'm trying to ignite a paper temple which is our symbol. Nothing malicious

I'm using Duracell AA batteries, fresh out of the pack. Should I get a different FET maybe?

[twjdeboer]

Thanks for helping me out. I had a brainwave and remembered the Li-ion batteries that came with my cordless drill.

I decided to create it anew entirely and it now kinda works. It's still a bit hit or miss, but good enough. Thanks for helping me.

I hope I didn't end up on some NSA watchlist...

[peter.mitchell]

Thanks for helping me out. I had a brainwave and remembered the Li-ion batteries that came with my cordless drill.

I decided to create it anew entirely and it now kinda works. It's still a bit hit or miss, but good enough. Thanks for helping me.

I hope I didn't end up on some NSA watchlist...

you were long ago.