Questions about my DIY Launch Controller

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Sounds like you need a little resistance in the place where the multimeter is measuring current (10A fuse in the meter I assume?). That means when you hit the button it happily applied full voltage across the meter and a large current flowed. Based on the 10A fuse blowing I'd say you had probably over 20A. Had there been an igniter there it probably would have worked :cool:.

The meter has a very low resistance and will let quite a large current flow if it is otherwise unimpeded. Adding a little resistance in the place where the igniter should be should bring the current back to sensible levels.
Say the meter is 0.1ohm. At 12V you would (discarding other circuit resistance and battery impedance) get 120A of current.
With a 1.2ohm igniter in series, the current comes in at about 10A. Your 10A fuse should take that for quite a few seconds before blowing, but your meter may go overrange temporarily.

If the 10A fuse was not in the meter and the meter is not where I was assuming, let me know!

Given the high currents expected when firing up an igniter it is wise to have a fuse somewhere in your launcher.
 
a $300 picture tube will protect a 10cent fuse...by blowing 1st. an automobile turn signal bulb (1157) will work if you want to discharge a 6 - 12v battery, remember to moniter the voltage, you don't want to go below 10.8v (for a 12v battery).
Rex
 
Sorry, I blew the 10A fuse in my launcher.

Yes there is also a 10A fuse in the meter, that was (already?) blown. But I happily had 6 brand new fuses right there. So I had just replaced that.

I checked that one too afterwards, it was actually a little steamy, but still good. (I tested w/ the multimeter).

I was measuring Amps.

So I should try an igniter?
(can I do that indoors? Maybe inside a coffee can?)

EDIT maybe start w/ just a piece of igniter wire?
Should I be measuring the amps w/ the igniter in the circuit?
 
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Yep. Hook up an igniter. Safety glasses please when testing.

As a safety measure we usually scrape the ends of the crocodile clips we put on to the igniters together, before connecting to the igniter. If there is a failure of the launch controller that means the igniter won't go off in your face. If no sparks, then connect igniter and try it.

FYI there are many different sorts of fuses. If you replace the one in your mulitmeter make sure you replace it with a specified type. They can usually break quite high currents. If you just put in any old glass fuse it can set up an arc (say you had the probes set up for 10A and tried to measure the mains voltage). One blows safely and saves you and the meter. The other one is much more destructive.
 
Yep. Hook up an igniter. Safety glasses please when testing.

As a safety measure we usually scrape the ends of the crocodile clips we put on to the igniters together, before connecting to the igniter. If there is a failure of the launch controller that means the igniter won't go off in your face. If no sparks, then connect igniter and try it.

FYI there are many different sorts of fuses. If you replace the one in your mulitmeter make sure you replace it with a specified type. They can usually break quite high currents. If you just put in any old glass fuse it can set up an arc (say you had the probes set up for 10A and tried to measure the mains voltage). One blows safely and saves you and the meter. The other one is much more destructive.

Teee heee heee heee
well that worked. :D

That burnt hair smell makes me glad I did it under the cap of some rattlecan clearcoat.

So I guess all thats left is mounting all this in the box. :)
 
I have a 1 Ohm RH-25 resistor that I use for this sort of thing. Measure the voltage across it to check performance.

So long as you limit the exposure time this resistor type will handle large overloads.
 
Teee heee heee heee
well that worked. :D

That burnt hair smell makes me glad I did it under the cap of some rattlecan clearcoat.

So I guess all thats left is mounting all this in the box. :)
It might be a good idea to bench test your complete circuit with the three diode-isolated ignition outputs before you box it all up - unless you’ve already done that.
 
It might be a good idea to bench test your complete circuit with the three diode-isolated ignition outputs before you box it all up - unless you’ve already done that.
Not sure what you mean.

Fire 3 igniters at once? I don't even have three cables. (Pads "B" and "C" are at this point just dirt roads running to the middle of empty fields. I have plans and future capacity, but I'm not there yet.)


Help me understand what happened w/ the multimeter:

The low power circuit measures 22mA. That's way below the 100mA no fire rating of a Quest igniter, and way way below the 1A no fire rating of Estes. So that's safe and good, right?

The high power circuit *can* blow a 10A fuse?
But not always? (and under what conditions?) Is that OK? Is that overkill? Should I turn that down (somehow)? or is that just "plenty o' juice?"

edit: I was expecting (guessing) i would have like 4 to 8 A on the high power circuit.
 
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The low power circuit measures 22mA. That's way below the 100mA no fire rating of a Quest igniter, and way way below the 1A no fire rating of Estes. So that's safe and good, right?
Correct. Safe and good.

The high power circuit *can* blow a 10A fuse?
But not always? (and under what conditions?) Is that OK? Is that overkill? Should I turn that down (somehow)? or is that just "plenty o' juice?"
It is up to you as designer to define the operation under normal and abnormal situations. You could add a little resistance in the circuit permanently. What resistance is the problem, and what power dissipation? If somebody want to put their finger on the button for a long duration (how long?) then the resistor must be able to survive sustained power dissipation.

Personally I would add a little resistance so that blowing the fuse is not a semi-normal situation. I would probably use in a high-power resistor to be able to take a reasonable time at shorted-output condition, but not be too big or expensive. Probably around 10-50W dissipation at a rough guess. Remember P=V*V/R.

What value of resistance would you choose BM?
 
Make sure your diodes to the pads are well and truly rated to take the short-circuit current. If they are underrated then they can go short-circuit and at future launches, when you have more than one rocket racked up, you can end up with an unintentional drag race.
 
View attachment 357945
This is what I mean. From your original circuit.

Exactly, Right now I'm working on the red circuit only. Diodes are part of the Phase 2 upgrade.

It is up to you as designer to define the operation under normal and abnormal situations.

Fair enough, although slightly more responsibility than I feel I'm ready for. :)

OK the question is; what's worse case scenario here? I pop the fuse. No chance of fire or electric shock, or anything like that (right?).

And the condition that would cause that would be clipping the two alligator clips together (or to any other very low resistance electrical device, like a multimeter) , arming the system, and pressing the ignition button.

So don't do that. And carry spare fuses (or maybe replace it w/ a circuit breaker??)

I don't see a need to restrict the circuit, especially considering I am going to need that extra juice if I'm ever going to do the (hypothetical) multiple launches. (right?)

Now I'm off to the HD for material for the control panel (and some spare fuses) :)
 
Make sure your diodes to the pads are well and truly rated to take the short-circuit current. If they are underrated then they can go short-circuit and at future launches, when you have more than one rocket racked up, you can end up with an unintentional drag race.

Voyager spec'ed the diodes for me. They're huge and heavy. (I wouldn't have guessed at that)
 
Brain,
A multimeter is a high impedance device in the ammeter setting only. It needs to be to affect current minimally.
When measuring voltage it’s a high impedance device. It needs to be to affect voltage minimally.
Personally, I would do nothing to limit the current the launch circuit can deliver to the igniters.
 
Fair enough, although slightly more responsibility than I feel I'm ready for. :)

OK the question is; what's worse case scenario here? I pop the fuse. No chance of fire or electric shock, or anything like that (right?).

And the condition that would cause that would be clipping the two alligator clips together (or to any other very low resistance electrical device, like a multimeter) , arming the system, and pressing the ignition button.

So don't do that. And carry spare fuses (or maybe replace it w/ a circuit breaker??)

It is your design, and it is a valid way of doing it ;)

Be aware that wind blowing the clipleads around can cause them to short to each other or to any metallic parts of the pad. That's the usual cause.
 
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