Continuity testing of Copperheads

[Mark_1984]

Does anybody know the max safe current I can put through the Copperhead ignitor for continuity testing. Or any other ignitor for that matter - there's no guarantee that I will always use Aerotech motors !!

I'm building a launch controller for up to L1 rockets, and don't want to risk a premature ignition when I throw the key switch and the 'Green for Go' lamp comes on. What do you folks use ?

Thanks
Mark

 

[bobkrech]

With a 12 Volt ignition system, an led in series with a 1 Kohm resistor which limits the current to ~10 ma. This is the simplest solution as 1 K resistors are available everywhere. If LED brightness is an issue, drop the resistor to 470 ohms which will increase the led current by a factor of 2 and limit the current to ~20 ma.

If you keep the continuity current limited to 20 ma or less, you're system is good for all known igniters.

Bob

 

[SCE to AUX]

The launch system I just finished building uses both an LED and a piezo beeper in parallel for continuity indication. Continuity test current is ~18 mA, and it won't even fire an AG1 flashbulb (most sensitive device I know of that gets used for rocket ignition). Certainly no worries about prematurely firing a copperhead, which need a fair amount of current to fire.

 

[luke strawwalker]

Yep... what they said. You can get LED indicator lights, which is an LED and resistor already prebuilt inside a standard bezel at Radio Shack for around a couple bucks each. These keep the current down to safe levels. They are bright enough to see in daylight, because the bezel diffuses the highly directional LED light so it shows up better. You can also get piezo buzzers at RS for the purpose, but I find buzzers to be rather annoying myself.

The main thing is, stay away from bulbs. If your launch controller has a bulb, install an LED indicator in it's place-- it'll lower the current to safe levels and GREATLY reduce battery drain.

Of course most of the better controllers nowdays have either piezo or LED indicators... only the big E seems to be sticking with old fashioned light bulbs... But they can be easily reworked on a rainy afternoon to LED indicators. Remember... if it passes enough current to heat up a light bulb filament, it will pass enough current to heat up sensitive ignitors...

Good luck! OL JR

 

[SCE to AUX]

As far as reworking the old lightbulb controllers, that can be as simple as changing the bulb. There are LEDs available now with built-in ballast resistors and lamp bases to be used as drop-in replacements for incandescent indicator lamps. They cost a bit more than a raw LED and resistor would, but in some cases the drop-in installation is worth the extra money.

https://www.safetybulbs.com/-strse-59/Miniature-Bayonet-Based-LED/Detail.bok

 

[Mark_1984]

Thanks guys. I shall go and find a nice bright green LED for the controller. I'm building one from scratch, so it can have as many features as I want Currently plan is a key switch for safety. Sprung loaded firing switch (under one of those cool looking red flip up covers) Green for Go LED, and a fuse, with red fuse blown (I.e. shorted ignitor) LED. I'm not a buzzer person

Cheers
Mark

 

[SCE to AUX]

In my experience, launch controllers are an application where you may not want to incorporate a fuse. Igniters (especially large clusters) can draw very large currents, often blowing fuses needlessly. Clips short out, wires get stepped on, etc. Unless you make sure to keep spares in your range box, there goes the day's launch, at least until someone makes a run to find a replacement..

If you want to add a measure of short circuit protection, consider a self-resetting thermal circuit breaker (available at auto parts stores--commonly used on defroster and headlight circuits), or one of the Raychem "polyfuse" PTC thermal current limiters. Either of these will prevent burned up wiring, and reset themselves without needing replacement.

 

[luke strawwalker]

Hey SCE those LED bulb replacements are cool! Didn't know you could even get those... drop in replacements for Estes controllers!

I agree on the fuse idea... basically you 'blow a fuse' every time you launch a rocket, as the ignitor is the 'fuse' that overheats and burns the bridge wire, and ignites the pyrogen which ignites the motor. Installing ANOTHER fuse just makes it more complicated as to which one will go, the ignitor or the fuse. Plus the thing will be most likely to blow when you're least able to fix it or when it's the most inconvenient, and from silly random but often occurring things like the clips shorting to the blast deflector, clips wiggling around into contact with each other, etc. You shouldn't really hurt a controller by pushing the launch button and having a short, so long as you don't hold the button down over a few seconds. The wires may warm up a bit but even with a dead short at the clips, you'd have to hold the button down for 15-30 seconds before you started getting things real hot (hot enough to smell the wires or stuff starting to 'burn'. Basically if the thing hasn't fired within 5 seconds of pushing the button it's not gonna go if you hold the button down til the controller melted, so what's the point??

If you REALLY want to have some kind of circuit protection, definitely go with the auto-reset thermal circuit breakers from the auto parts store. I had a pickup with a sneaky intermittent short somewhere in the taillight wiring that would cause it to pop the fuse when I least expected it. Sometimes it'd go for months without a problem, and then bam blow one every other day for several days before going away again. I finally replaced the fuse with a thermal circuit breaker, and I hear it 'pop' out once every few months and then a few seconds later pop back in, but haven't had any problems with it since.

Good luck! OL JR

 

[SCE to AUX]

You shouldn't really hurt a controller by pushing the launch button and having a short, so long as you don't hold the button down over a few seconds. The wires may warm up a bit but even with a dead short at the clips, you'd have to hold the button down for 15-30 seconds before you started getting things real hot (hot enough to smell the wires or stuff starting to 'burn'. Basically if the thing hasn't fired within 5 seconds of pushing the button it's not gonna go if you hold the button down til the controller melted, so what's the point??

That's the approach I took with the controller I just finished. It incorporates a pair of low-current fuses, but only for internal control/logic circuits and an auxiliary 12V power output jack (which I sometimes use for running a downlink receiver for video). If either of these fuses blows, there is an actual problem which needs attention (and there is a spare fuse stored in a clip inside the controller case).

The actual igniter current path from the battery to the pad clips is UNFUSED. All the igniter circuit wiring is done using heavy (14 AWG) wire which will withstand a short circuit for a reasonable time. But the real protection is provided by a time delay relay which drops out the main firing relay if the launch button is held down for more than 4 seconds. Firing again requires manually re-arming the controller by pressing a separate "system arm" button.

If a motor doesn't light within 4 seconds, it isn't going to...

 

[Mark_1984]

Thanks for the comments folks. There are a couple of reasons for fitting a fuse. In the event of a short circuit the firing switch will be breaking a current higher than it's design capacity. It isn't likely to have an immediate failure, but it will shorten its life and reduce reliability. The fuse will also protect the battery against excessive discharge currents. An ignitor takes max 3 amps, so I'm thinking of using a 10A or 13A fuse.

Second, if the motor doesn't fire the 'short indicator' across the fuse will give me a bit more info about why it didn't fire. If it doesn't fire and you don't have a short then there is no point in looking at the igniter clips as it's probably a faulty ignitor or ignitor not inserted correctly.

I'm an electrical engineer by trade...... I go for simple, but safe

It's a very good point about having spares - I shall make sure there is a spare packet in the flight box.

You guys will be the first to know if it bites me in the bum and I have to take the fuse out of circuit !!

Once again, thanks for all the comments.

 

[bobkrech]

Thanks for the comments folks. There are a couple of reasons for fitting a fuse. In the event of a short circuit the firing switch will be breaking a current higher than it's design capacity. It isn't likely to have an immediate failure, but it will shorten its life and reduce reliability. The fuse will also protect the battery against excessive discharge currents. An ignitor takes max 3 amps, so I'm thinking of using a 10A or 13A fuse....

While the all-fire current for many commercial igniters is typically 3 amps or less, most igniters have a resistance between 0.8 to 2 ohms. I=V/R so the current drawn with a single igniter ranges from 6 to 15 amps @ 12 volts. Estes igniters have ~0.8 ohm resistance. We blew a 20 amp fuse on 12 volt portable automotive air compressor/power supply while launching a 3-motor Estes cluster last week at a TARC launch. Copperheads will draw comparable or slightly higher currents.

https://www.esteseducator.com/Pdf_files/1924_launch.pdf

Bob