So why do you want to limit the current to 1-2 A?

 

feedback on what was happening to the system was done visually, and the system was controlled with mechanical switches.

You could combine visual and software. Installing a LED across the igniter would give a visual indicator that Q1 is shorted or there is a problem if the LED is ON. With R7 at 100 ohms should give appx 100mv at the output of U1 when power is applied. This voltage would tell the software to activate a relay to insert R8 in series with the igniter circuit to reduce the current and prevent accidental ignition. If no voltage is detected at U1 when power is first applied the program would goto the standard routine.

 

You could combine visual and software. Installing a LED across the igniter would give a visual indicator that Q1 is shorted or there is a problem if the LED is ON. With R7 at 100 ohms should give appx 100mv at the output of U1 when power is applied. This voltage would tell the software to activate a relay to insert R8 in series with the igniter circuit to reduce the current and prevent accidental ignition. If no voltage is detected at U1 when power is first applied the program would goto the standard routine.
View attachment 281106

Good idea - let me think about it some more. One problem is that everyone stands about 20-30 feet away from the rocket when it is launched, and the ignition circuit is attached to the rocket. I am communicating with the ignition circuit (eg commands to check continuity, launch, etc.) via a cell phone app.

 

So why do you want to limit the current to 1-2 A?

The old data on the igniters says one has to have at least that much current. Newer igniters may use less - no test data. The more current used, the fewer launches from a given set of batteries, so it would be nice to know what is really needed in today's world. If the MOSFET shorts, I don't want to kill the battery by drawing a lot of current uselessly. I also don't want a couple of amps flowing across some bare connectors while someone attempts to setup a rocket for launching.

 

If the MOSFET shorts, I don't want to kill the battery by drawing a lot of current uselessly. I also don't want a couple of amps flowing across some bare connectors while someone attempts to setup a rocket for launching.

That's one reason for the LED to alert when there is a problem then a simple switch at the pad box would allow you to shut that circuit down. You didn't mention if this was a single or multi-launch system but I would design for at least two igniter circuits in case one fails.
On another note I doubt that the mosfet would ever short due to the series 5 ohm resistor.

 

Couldn't find a good link so here's my shot at it for the basic part of the circuit (below):

An op amp has a very high open-loop gain so the input voltage difference is typically less than a mV to cause the output to go from rail to rail.
This means that when the op amp is operating in its linear region due to negative feedback, the plus and minus op amp inputs can usually be considered to be at the same voltage with little error.

So in the circuit below, the op amp will adjust its output to an M1 gate voltage that causes just enough current to flow to generate a voltage across R1 equal to Vin (negative feedback) as that's the only stable condition for the circuit.
Thus the current will be equal to Vin / R1.

Make sense?

View attachment 280993

Zapper,

I have been testing your circuit, and it works very well, except for one small nit. M1 never turns off, probably due to inherent offset voltages. When the DAC is programmed with a 0 value, there is still ~ 70-80 mA of current flowing through R1. That is no where near enough current to ignite the igniter, but still I would really prefer to be able to turn off M1 when the DAC is programmed with a value of 0. Do you have any suggestions on how to do that?

Some observations from testing the circuit and "burning through" a few packages of igniters.
* Igniter resistance is ~1R new and about 0.5R when used, but not broken
* Ignition current is 3-4 A for 1.5-2 seconds
As I said earlier, the data I had on igniters is over 20 years old, and no one has characterized the igniters currently on the market. The igniter ignites, but does not break. The old igniters were just thin nichrome wire, but today the nichrome wire is covered in Pyrogen (i.e. black powder), and that is what ignites when the nichrome wire heats up. I suspect the nichrome wire is thicker than the igniters tested 20 years ago, so I would need a lot more sustained current to break it.

I made a few changes to your circuit.
* M1 changed to IRL44G. Still need to test the IRL620
* R4 changed to 3.3R

Thanks!

 

I would really prefer to be able to turn off M1 when the DAC is programmed with a value of 0. Do you have any suggestions on how to do that?

Connect a 5K ohm resistor in series with the connection between R1 and the (-) op amp input, and a 1 megohm resistor from the (-) input to the +12V supply.
That will give about a 60mV negative offset, which should keep it at zero current with 0V input.
You would then have to increase the input signal by ≈60mV to get the same current when on.