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How to find current from mj/Ohm

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ric_vas

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mj ohmm

Hi,

I have to define the current needed to trigger a squib (you can imagine this as a fuse). The parameters I have are its impedance (1.5 ohm) and the energy necessary to fire it (4 mJ/Ohm).

Is there any way to calculate the current needed (let's say I will connect it to a 1.5V battery)?

Thanks in advance.

Ric

P.S. My interest in here is the math behind it!
 

mj, ohms

Well I would do it this way, but not sure:

You have energy per ohm (mJ/Ω), so to get energy yuo multiply it with R

W=4mJ/Ω*1.5Ω=6mJ, now you have energy...

now you calculate power P=W/t, where t is time in seconds. Lets say t=1s, than P=4mW (watts)...

P=U*I =>I=P/U=4mW/1.5V=4mA...

Hope this helps
 

    R

    ric_vas

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megajoules ohm

It makes sense to me...Does anybody else have something different?

Ric
 

mj/ohms

Current = voltage / resistance
Current = 1.5V / 1.5Ω
Current = 1.0 amps.

Time = energy / power
Time = energy / (voltage^2 / resistance)
Time = (4mJ/Ω * 1.5Ω) / (1.5V^2 / 1.5Ω)
Time = 4 milliseconds.
 

    R

    ric_vas

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mj/ohm

I didnt consider it this way, but it is the same thing... echo47s answer tell you that you need 1Amp for 4milisecs to fire it, but my answer tells you that you need 4mA for 1 sec to fire it...
 

Thanks guys!!

You both helped me. I'll make use of these two approaches.

Ric
 

I've never worked with squibs, but I would expect the 4mJ energy spec to be based upon a particular applied voltage stated in the data sheet. 1.5V seems low, but maybe it's sufficient. The squib won't ignite if the applied voltage (or current) is too low, no matter how long you wait.

A 1.5V battery doesn't deliver 4mA into a 1.5Ω load.
 

Hi,

I have some information for another squib (not the one I'm using right now) but it may be helpful to understand:

Resistence (ohm +/-15%): 1.05
Max current (no ignition): 0.3A
Min. current (ignition): 0.6A
Energy (mJ/ohm): 3.2

For the squib I'm using now the only info. I have is: 4mJ/Ohm to fire it (the resistence I measured).

Ric
 

That's better info. It says the device will never ignite below 0.3A, and will always ignite above 0.6A. Those values probably assume certain environmental conditions such as ambient temperature. 1.5V should ignite this device.

Your 4mJ/ohm device also has such parameters, but it's anyone's guess what the values are.
 

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