I learned it on this forum from the cluster expert.

Thanks guys. Well that's interesting. So really the whole difference here is using a soldered joint to interface to the igniter vs. using clips to interface with the igniter. Presumably, if your clips were clean and springy, the clips would be an acceptable method.

Anyone know what kind of current goes through the igniter at launch?

Actually, I'm a little curious about the whole V, I, R profile. The igniters are very low resistance, so the current draw should be huge (relatively speaking) so most voltage sources are going to drop to a lower voltage rather than live up to Ohm's Law and deliver super huge current--although a sealed lead acid (as we discussed in another thread) can probably make a pretty good showing...

But R should increase as the igniter heats up. So there would probably be an initially huge current flow, a largish voltage drop, quickly followed by an increase in resistance leading to lowered current flow and increased voltage (thus somewhat compensating for the lowered current flow) for the milliseconds it takes the igniter to burn through.

Thinking about it, the current may not drop. It may simply behave like a constant current system, with the battery voltage varying to make it's maximum current correspond to whatever resistance the igniter happens to have. Of course, the battery voltage won't go over its rating so if the resistance increases enough to let the voltage back up to nominal, at that point the current will drop.

All of which is my stream of consciousness wondering if wire wrap would work for lead extensions. Wire wrap is usually very small guage so it does not have the current capacity of bulkier wires, but it is also much easier to do wire wrapping in the field than it is to do soldering. I'm also not sure that igniter wires are stiff enough to wire wrap around. I may have to do some experiments tonight.

And reading that post leads to the next question. What is a burn string?

Holds it down until the ignited motor burns it and lets the model go.

They are also sometimes used to deploy wings and other stuff in contest models.

The whole point of soldered wires is so you can put them all together to make a single pos and single neg connection with only one pair of clips, without damaging the igniter. This also eliminates the possibility that a clip will come off before ignition of the motor because the current is going to go through all of the motors, or none of them at all.

As for resistance, Estes igniters and most aftermarket igniters use high resistance thin nichrome wire as a bridge between two more robust lead wires. Even if it was a more conductive metal, the wire is so thin that it has considerable resistence anyway.

Here's a drawing and some pics:
http://home.flash.net/~samily/burn-string/

Doug

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YORF member #11

Will ignitors current share? Maybe. But I woudn't count on it.

I've used wire wrap, too. With short leads, the guage isn't too important. But with wire wrap, two things to consider. One, it's easy to nick it while you're stripping leaving a possible break spot. That's not common, but it is one of the better known failure mechanisms of the technology. Don't let the DoD inspector catch you not using your no-nicks

And two, wire wrap prefers square posts. Going around round wires, it can slide off. So when I've used it, I still ended up putting a dab of solder on it.

Overall, it was comparable in difficulty to using telephone wire.

Doug

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__________________
YORF member #11

Something that never caught on in strip clubs?

__________________
Bill Eichelberger
NAR 79563

http://wallyum.blogspot.com/

I miss being SAM 0058

Build floor: Estes - Low Boom SST Semroc - Marauder, Shrike, SST Shuttle

In paint: Canaroc Starfighter Scorpion Centuri Mini Dactyl Estes F-22 Air Superiority Fighter, Multi-Roc, Solar Sailer II, Xarconian Cruiser Semroc Cyber III

Ready to fly: Estes - Solar Sailer II Semroc - Earmark

Ouch.

I am unclear on what you mean by current share. Any time you have the igniters connected in parallel they are sharing the available current amongst them. It gets divided up in inverse proportion to their resistances. If all the igniters are about the same resistance, then they all get about the same current. That is, for a current limited situation. If the supply has sufficient current capacity, then each igniter just gets its V/Rn where Rn is the resistance of any given igniter 'n'.

Or did you think I meant to hook them up in series? Everything I've read says that's a bad idea.



What is no-nicks? :-) See, it just keeps leading to more questions. But I am enjoying the education I am receiving. I haven't had any trouble with nicks in my wire-wrapping of electronics as long as I am using the stripper tool included with the wire wrap tool.



Yep, the square post issue is a problem. And if it's not possible or at least convenient to fold the end of the igniter wire over after wrapping a wire onto it, then you'll need the solder. Hmmm. Another beautiful idea slayed by a harsh reality.

Do you have a method of soldering in the field or do you do all your prep at home? The only reason I was thinking about wire wrap was my assumption that you wouldn't be able to do launch prep in the field if it requires soldering. On the other hand, I have a 12V powered soldering pencil (cigarette lighter connector) in my car's tool box, so I guess field soldering should be possible. There are also those little butane powered jobs.

Hmmm. Igniter wires are round, so they are not good for wrapping other wire around. What happens if you attempt to wire wrap an Estes igniter lead around a square post? I'm thinking about the header strips which are cheap and are just a line of .1" spaced square posts in a plastic matrix. If one wrapped each lead of an igniter to one of the posts of such a header, then all the connecting could be done to the other ends of posts. And the rigid spacing of the header posts would naturally prevent shorts in the igniter leads.