Help with Simple circuit and relay

The circuit will function. You really don't need two fuses, you can fuse the pos. output to both the coil and the ignighter. How many ignighters are you powering at one time? I can't see a switch not handling one, even a NiCr ignighter. You can fuse as high as the wire rating or the switch ratings, which ever is lower.

Tom

Now that you've written the purpose, do you know what your rocket launcher's pyro requirements are?

Just as a possible aid, because I've been personally involved in a different phase of my life:

If you are using the squibs used for the Estes-type engines, I recall a conversation I had with their lead engineer more than a decade back who told me that the experience 100% success rates with the delivery of 3.1 amps for 50 milliseconds. Because this is a heating vs dissipation issue, a higher pulse for shorter times should work. Their final spec is for 3.75 amps for 50 milliseconds to provide a safety margin over the all-fire value they experienced.

He also told me they don't have a maximum "no fire" spec, but that 50 milliamps continuous is safe. They also observed no ignitions using 700 milliamps for 50 milliseconds, but using that level for longer will fire squibs, eventually. An amount below the 50mA continuous figure can be used to verify the existence of a complete circuit.

They chose the 50 millisecond period only because it was the time they designed into their test-fire equipment, and not for other reasons, according to that engineer.

For those considering specific recommendations, those are the details I recall.

Jon

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OK, great. I will make that change as well, The # of igniters at this time will only be a max of 5.

When all is said and done, and I have my project box completed. How in the heck can I measure how many amps are really going through? I have a multimeter which can measure amps, but how do I put a synthetic load on the circuit to measure max amps? Maybe just get a small dc motor with a shaft and make the shaft come to a stall ?

OK, great. I will make that change as well, The # of igniters at this time will only be a max of 5.

When all is said and done, and I have my project box completed. How in the heck can I measure how many amps are really going through? I have a multimeter which can measure amps, but how do I put a synthetic load on the circuit to measure max amps? Maybe just get a small dc motor with a shaft and make the shaft come to a stall ?

Not sure what you mean by a "synthetic" load. If you are looking to see how many Amps triggered your ignighters you will need a fast reading meter with a hold capability. Its also important to know for how long the pulse occurred also. If you are looking to see how much current your unit can supply it will up to the fuse or capability of the DC source if less than the fuse. If the former re-read Jon Kirwan's remarks.

Tom

The squib resistance is about 2/3rds of an Ohm. I think they told me it varied from 0.66 to 0.67 Ohms, with some very very tight variation. The bridgewire is very short and cut in a repeatable fashion, I gathered. It's made of AlCr alloy, though on searching the web I only find an 80/20 alloy and not many references even to that. So it may include other things like nickel, for all I know, these days.

If this is for different rockets, there's really no worry, but one of the problems with firing off squibs in multi- engine models (and I don't know if the OP intends that), is getting all 5 _engines_ to fire before the rocket leaves the pad. This means the timing of the ignition of the pyrogen itself must occur at the same time -- "same" having the meaning of __success__ in firing all the engines before the rocket moves much.

To solve that problem, the energy sufficient to guarantee ignition needs to be delivered at roughly the same instant. A heavy car battery and heavy wires is a brute force method to get there. There are more subtle means. One of the things I've tried to use is a rocket-pad-local flash-lamp system using a 4kV trigger voltage from the remote end. The trigger voltage has almost no current and arrives at the rocket pad and triggers the flash lamps at the exact same moment -- they are _very_ repeatable, that way -- dumping the energy from a cap into each squib.

Jon

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Don't need to go thru all of that. If you just hook them in series and provide more than the required all fire current, if the squibs are consistently made then they all should pop simultaneously. We do this with e-matches all the time and I don't see any difference except the required all fire current.

Series? I think you mean parallel

and

nah series.

if you're worried about one of them self-destructing before the others have lit up parallel each with a silicon diode or two.

but if you use a pulse that's reasonably fast and not excessively energetic you should be able to light them all before the first one goes open-circuit.

putting them in parallel is going to require invonveniently high currents.

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Hi Jon,

Your assumption is correct. I was hoping to build rockets and launch them with multiple single stage engines all firing at the same time.

If I read your last paragraph correctly, you are suggesting that I continue to use a 12 volt relay for max amps, but also bolt on a capacitor also (or many) into the circuit?

Thank you for putting up with me.

I think your use of a car battery, in this application, is good. It will have _no_ problems sourcing the current needed and the squibs will heat quickly. Which is what you want. No need for a capacitor. In fact, ignore my comments there. It was just something I experimented with, with an eye to reducing the need for lugging a huge battery around. (The capacitors allowed a slow build up of the necessary energy from much weaker batteries than your car battery.)

While I never actually tried firing _5_ engines at the same time, I did do _2_ and _3_. I suspect you are probably fine. If you do have a problem (and you will know it, soon enough), then we can discuss what you experienced and consider alternative options. But I suspect you will be fine. Carefully select your squibs for similarity to each other and be sure to position them as uniformly as you can inside the engines. Especially watch out for one or another becoming loose as you set things up. I suppose you might want to keep the lead lengths of your cables to each squib of similar length. But you probably would do that, anyway. Use heavier gauge wire for longer lengths away from the pad. And use engines from the same batch and time of manufacture and storage situations, if possible. Hopefully, there are no microcracks in the solid fuel, too.

I generally got away from multi-engine firings and just used larger single engines, instead. More uniformity and generally less worry and trouble.

Oh, cripes. It's no problem at all. Talk is cheap.

Jon