launch controller current limiting

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rms59

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New here....

I'm trying to help a few kids in the neighborhood with their Estes model rocket and made a simple launcher for them using a 12v scooter battery, relay, safety switch and launch button.

My question is current limiting. I'm worried that the launch button can get welded to its contacts and draw the full 9 amps through the wiring if the leads to the rocket short out. I notice all the simple controllers don't protect against that. So, short of buying a sophisticated controller, what are other people doing?

Thanks in advance
 
Honestly, I don't worry about it. But if you are worried you can put a 2 or 3 amp inline slow blow fuse on the positive terminal of the battery. It will let you get your full 9 amps for several seconds before it blows.
 
Honestly, I don't worry about it. But if you are worried you can put a 2 or 3 amp inline slow blow fuse on the positive terminal of the battery. It will let you get your full 9 amps for several seconds before it blows.

Thanks. Other than putting my first post in the wrong section those fuses would get expensive. Not knowing that much about rockets I have no idea how much current the estes rockets need (min max). I was wondering about an automobile brake light bulb in series with the output which I think draws two amps.
 
The fuse won't blow in normal use. It will have time to cool back down between launches. I have a 10 amp slow blow on a gel cell battery for my relay launcher, and when the relay is closed the circuit is basically a dead short across the igniter. The fuse does not blow, but I bet I'm getting 20A+ for a second or two each launch.
 
Estes igniters won't take 9A unless they are shorted below the bridge wire (from an installation error). If the rocket doesn't go after holding the firing button down for a second or so, then you need to check for that. From the info above, a slow-blow fuse wouldn't blow under those circumstances.
 
Ok... I will get a slow-blo fuse and try it. I just don't want them to short the leads at the rocket (which dead shorts the battery) and have the wiring burn up when they push the button. I will give this a try.
 
Use an Auto reset thermal breaker. 10 amps. With a short it'll pop open before your wiring melts. After a few seconds of cooling it closes again. No need to replace.
Under normal use, a 10A will handle 20-30A for a short period without opening. Good for cluster ignition.
I've built launch controllers this way for 25 years.
 
Use an Auto reset thermal breaker. 10 amps. With a short it'll pop open before your wiring melts. After a few seconds of cooling it closes again. No need to replace.
Under normal use, a 10A will handle 20-30A for a short period without opening. Good for cluster ignition.
I've built launch controllers this way for 25 years.
Very smart good choice
 
The Estes igniters, the last time I checked, had about 0.6 ohms of resistance. Using a large 12V battery, and large gauge wiring, you could see 12-20 amps through the circuit - a bit excessive (the data I have shows an all-fire current of 2A, no-fire current of 0.5A).
An easy way to solve your problem is to put a small resistance in series with the igniter - a 3 Ohm, 5 watt resistor would limit current to 3.3A. You'll be putting about 30 watts into the resistor; but it'll handle it just fine for the second or two that the current will flow. They'll be about $1 apiece at Mouser or Digi-key, but you can even find them at Amazon:
https://www.amazon.com/uxcell-Tolerance-Resistance-Electronic-Experiments/dp/B07RT6YGR9
 
Oh, and another option is to use a higher current relay - most auto relays that you can pick up at Pep Boys or Napa will be rated at 40A, and you can find them at Amazon also: https://www.amazon.com/Ehdis-Relay-40amp-Spst-Model/dp/B01J53IH5M
One safety item that I like to put in is a 12V beeper on the output of the relay, so you hear it when you launch. More importantly, it'll immediately warn you if there's power coming from the relay (welded contacts or whatever) when there shouldn't be. Something like:
https://www.amazon.com/Cylewet-Electronic-Sounder-Continuous-Intermittent/dp/B075PT19J2
 
To take this thread from being specific to somewhat more general, what is the advantage to having current limited? My own launch controller has no resistors whatsoever in either cuircuit (it's a simple circuit that uses the handheld unit to control a relay circuit at the pad), and I always thought that at least at the pad, more current was more better.

That said, the biggest cluster I have ever ignited is 3 motors.

But that being said, isn't a pad side circuit that could theoretically jump start a a tug boat better than one that could not? I thought the goal was for our pad side circuits to electrically ignite clusters more reliably than Thor's hammer.
 
To take this thread from being specific to somewhat more general, what is the advantage to having current limited?
I had a club pad box have all its internal wiring completely fried by a short, the carnage was quite amazing. We use lipos for power which could easily put 100A through the box without limiting. After that I put a 1-ohm power resistor in the box, but the auto-reset breaker would be a better idea. I agree it's a tradeoff but shorts do seem to happen from time to time.
 
what is the advantage to having current limited?
Apocryphally, some have reported that having too much current run through the igniters can cause the hot wire to fail before enough energy/heat is developed to ignite the pyrogen. Seems unlikely to me, but I ain't gonna argue with some of the people who've reported it (who have a thousand times more experience than me).

mikec's answer is another reason.

When igniting clusters, there is a possible issue where one motor lights early, causing a plasma to develop around the igniter - which will essentially short-circuit the igniter. If the battery isn't capable of sourcing very high currents, the voltage (and thus current) across the other igniters can drop to zero preventing them from igniting. IMHO, the best approach for a relay launcher for clusters is to have individual leads coming off the launch box, each going to an engine. Each lead should have it's own resistor in series - that way, if one igniter shorts, the current drawn is limited by the resistor and the other igniters aren't impacted.
 
When igniting clusters, there is a possible issue where one motor lights early, causing a plasma to develop around the igniter - which will essentially short-circuit the igniter.

Wire igniters in series.
Boost the voltage if you plan big cluster.
 
rocket ckt.jpg

I think this is the way I would like to go. I have a light weight 12v sla battery, the 10amp breaker mentioned above, a little voltmeter https://www.amazon.com/gp/product/B07FZ4ZNSZ/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1 just for show, an audible alarm mentioned above, the 3 ohm 5 watt resistor mentioned above, and a tranmitter/receiver so they can fire remotely https://www.amazon.com/gp/product/B00WT0XUAQ/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1.

I haven't built it yet and I used a 5 ohm 50 watt in place of the 3 ohm until the 3 ohm comes in. But I dead shorted the output (IGN) thru an ammeter imitating welded jumper leads at the igniter and the breaker immediately opened. After a few seconds it closed and opened again. Current got no higher than 4 amps at the shorted IGN.

Anyone spot anything wrong?

Oh yeah, the Manual fire button is a recessed button in case the electronics fail they can stil fire the rocket manualy.
 
View attachment 410642

I think this is the way I would like to go. I have a light weight 12v sla battery, the 10amp breaker mentioned above, a little voltmeter https://www.amazon.com/gp/product/B07FZ4ZNSZ/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1 just for show, an audible alarm mentioned above, the 3 ohm 5 watt resistor mentioned above, and a tranmitter/receiver so they can fire remotely https://www.amazon.com/gp/product/B00WT0XUAQ/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1.

I haven't built it yet and I used a 5 ohm 50 watt in place of the 3 ohm until the 3 ohm comes in. But I dead shorted the output (IGN) thru an ammeter imitating welded jumper leads at the igniter and the breaker immediately opened. After a few seconds it closed and opened again. Current got no higher than 4 amps at the shorted IGN.

Anyone spot anything wrong?

Oh yeah, the Manual fire button is a recessed button in case the electronics fail they can stil fire the rocket manualy.
I assume that the Signal Control block is the remote receiver? I would up the value of R1 to get that continuity current down <50 mA. I would remove the 3 Ohm because you are dumping 2/3 of your ignition voltage across it, assuming that the igniter is about 1.5 Ohms.
 
The continuity test current is going to be awfully high - the no-fire current for the Estes Solar igniter is around 500 ma, so you're half of that; but the no-fire current for the old Q2G2 was only 150 ma. Consider not using an alarm - raising the value of R1 to limit the test current is likely to make the alarm buzzer not work. Instead, consider using a high-brightness (5000-10000 mcd) LED. These will normally have a 2 to 3V forward voltage drop, and a 20ma max current, so an inline resistor of (12V - 2V)/(0.020A) = 500 ohms would be about right, and you won't have to worry about accidentally firing the rocket while you're testing.

If you already have the alarm, consider wiring it in parallel with the igniter leads (from the output of the relay to ground). That way, if the relay ever does weld it's contact or the circuit fails "hot", the alarm will be on continuously to warn you. It does mean that you'll hear it alarm every time you fire - but that's not a bad thing, IMHO.

I disagree with Voyager1. IMHO, you don't care about the voltage and power that are getting dropped across the resistor. What you care about is that the current is sufficient to fire the igniter; your 3 ohm resistor will give you plenty of current to do that and will eliminate the concern about welding the relay contacts closed.

I get very nervous around cheap wireless switches. I have a number of them around the house controlling lights, and they occasionally turn on or off by themselves. I'd hate to have that happen while someone was near the rocket. As long as you recognize this, and have operating procedures that will limit the carnage if that happens ( i.e. Key is off until everyone other than an adult clears the pad, then key on while standing back from the rocket to test continuity, key off again any time you approach the rocket ) then you should be fine. The particular switch you're looking at, with the pairing that it does, is likely to be better about the false turn on/off, but there's no guarantees.

Otherwise, I think your circuit will work fine.
 
With the greatest respect, my point was not about the igniter current. I was referring to the wasted power dumped over a 3 Ohm resistor that is not required in this application. Current limiting resistors have their place in deployment electronics to reduce the possibility of over-current conditions with altimeter deployment channels, but not here. I have been using relay-based ignition systems for many years and I have yet to "weld" relay contacts that are appropriately rated for this purpose.
 
In no attempt to derail your thread, and for clear understanding I am no electrician, but is the complexity of the resistors necessary. I had built a home launch controller three years ago, and recently upgraded to wireless with the keyfob and used a schematic like pictured. I have never had a single problem. IDK, I have hundreds of launches and very few problems, none that I can attribute to the controller.

Wireless.jpg
 
Voyager1 - Yes the Signal Control block is the remote receiver which looks pretty cool BTW. I really want the current limiter because these 5 kids (ages around 14 and one is a nephew) short out the clip leads to the igniter more often than not and by the time they figure out whats wrong the AA batteries are dead. They have no adult supervision and I'm not into rockets. (I fly RC model planes). Their rocket (small estes) is built well and has a video camera on it. I believe they've had one successful firing and launch out of a dozen failed attempts which got them excited enough to want more.

They're launch controller was two wires they shorted together 6 feet away from the rocket. I shook my head because these kids are all thumbs and unsafe. I want to help them by moving away from AA batteries which they either forget to bring, or die before they launch, to a small 12v SLA battery attached to a safe controller as one unit, and teach them to keep a distance. Since I know they'll short leads sooner or later, I don't want the over 100 amps that the small battery can deliver on a short which can fry everything so I want the current limited and an auto reset breaker.

KilroySmith - All good. The 10000mcd white LED, the 500ohm resistor, and the buzzer across the ignitor too. The short sound at launch would only be for a second or two so it won't be a problem. As far as the remote switch, and because of the social distancing in effect the kids are grounded for awhile so it will give me time to make some extensive tests using a noise generator and recorders that I have available.

I actually bought two of those receivers so I may see if there's any interference by using two of them in series. This would require two kids to work together to fire the rocket. Yes, it's over complicated but I have nothing else to do right now and it may teach them teamwork. :)

@roytyson. In your circuit, what happens if you twisted the wires to your igniter in a dead short configuration? Not knowing what battery you have, some are capable of delivering well over 200 amps to that short through the solenoid and your power switch. I'm dealing with unsupervised kids.
 
Voyager1 -
Fair enough. But I'll note that the relay he's using is NOT "appropriately rated for this purpose" without a resistor. It's a cheap relay with a claimed rating of 10A - and hooking directly to the 0.6 ohm igniter, double that current could be getting switched. Using the resistor to bring the current down into the range of the chosen relay is the appropriate solution.

I'm not really following your concern about wasting power in this application. Whether it gets dissipated in the resistor, or in the igniter, the power per launch is going to be roughly the same. Given the 84 W-hr SLA battery he's using, and the fact that he's using about 0.01 W-hr per lpr launch, I'm not concerned about the waste.
 
rms59 -
I wouldn't normally use a white LED in this application. It's my belief that they'd be more difficult to see in the daytime - but that's because I've been using LEDs since long before they were anywhere near bright enough to consider them as light sources. I'd normally use a colored LED in a white or clear package, so it looks white or clear when off, but red or green or blue when on. But I'm perfectly willing to believe that a modern white 20ma LED will be unmistakably on or off standing in the sunshine; I've just never used a white one when I intended to use it outside.
 
@roytyson. In your circuit, what happens if you twisted the wires to your igniter in a dead short configuration? Not knowing what battery you have, some are capable of delivering well over 200 amps to that short through the solenoid and your power switch. I'm dealing with unsupervised kids.[/QUOTE]

Twisting the wires together doesn't do anything...until you push the keyfob Button. THen it smokes the receiver solenoid instantly. I know from experience. So as long as you can control the keyfob until the ignitor is hooked up, it is safe. When I firt started playing with it, I thought that it would smoke the unit even with the ignitor, but so far so go.

Im running 4000 mAh 4s, 30C lipo in the box for both.
 
@roytyson... I hear you. I run 3000 mAh, 3s, 30c (11.1v) in my RC planes and there's a lot of energy there. Anyway, the hypothetical short I gave you caused damage to your equipment and that's what I'm trying to prevent by limiting the current to no more than they need. I don't want these kids to get discouraged and quit because of constant equipment failure like they've been having. Yes.... it's good to have a learning curve but I rather they have rocket trajectory failures instead of equipment failures.

@KilroySmith... Hard as it is to believe, the 10000 mcd white LEDs I have (3.2 fwd current@20mA max which are actually 12000 now that I checked) are really super bright and easier to see than any other color I've ever tested. I use them on my RC planes which turn on with a fast blink rate when the LiPo battery reaches 9 volts so I know when to land and I can see it in bright daylight. I'll use the blink ckt in this controller.
 
Well, I guess I learned something today. Gonna have to try White the next time I need a highly visible outdoor indicator. Thanks!
 
Well, I guess I learned something today. Gonna have to try White the next time I need a highly visible outdoor indicator. Thanks!

This is the LED I use as a wing mounted low battery warning in my RC planes. I have it pushing aprox 170mA, (instead of 120 because I need distance) flashing at 2 blinks a second and I can see it clearly in daylight 420ft away which is limit I fly.

In this controller I'm using 470 ohm which brings me around 22ma and it's still bright enough to see at the controller in daylight.

https://www.amazon.com/50pcs-Straw-...raw+hat+white+led&qid=1585512791&sr=8-2-fkmr1
 
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