Understanding volts and amps for reliable igniter firing & testing using resistors.

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kweaver

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So I am trying to verify our inground wiring to our launch pads, to determine if we have enough voltage and current to reliable fire most igniters particularly estes and Aerotech First Fires for E thru I engines.
Aerotech instructions for the FirstFire ignitors advise 12 volts @ >3 amps.
So from my understanding of ohms law: resistance=voltage/current.e. We are using a 12v auto battery that is 12.85 volts at the battery terminals and 12.65 volts at the pads.
Is their a better way to simulate the igniters for a test of the launch system other than using actual igniters.
Ken
 
If you know the resistance of the igniters you plan to use, just purchase a power resistor of that value.

I like to use the RH25 series resistors. (A 1 Ohm version.) Mounted to a heat sink they will dissipate 25 Watts and 5 times that for 5 seconds. Perfect as an igniter stand in. Measure the voltage across a 1 Ohm resistor and it is a snap to calculate the current through it.

A lower tech test is to use Estes igniter trash. The wires that connect to the bridgewire are steel and of relatively high resistance. Connect the launch clips to one of them and test. If it burns through you are good to go.
 
Yes.

1) You need to know the typical resistance of the igniters you're designing for. You could measure them yourself but it's tricky getting trustworthy results when measuring very low ohms (need a good meter and careful setup). Or you could ask others here what they've measured in the past. For instance I've measured a few Estes igniters at about 0.9 to 1.0 ohm; you could use those numbers for a start.

2) Then you need to rig up a 'dummy load' of resistors that simulates your target igniter. For instance, if your target igniter has a resistance of one ohm, you could just use a big one ohm resistor, but that might be hard to find. Or you could use five 5-ohm resistors in parallel to get to the same value.

3) Whatever you do make sure to get pretty big power resistors, as twelve volts across one ohm equates to 144 watts of power. Now, your testing is not continuous but in short bursts of a few seconds duration so you don't really need a dummy load rated for continuous running. Something like five 5-ohm, 10-watt resistors in parallel should work nicely for our kind of use. That's the same as a single 1-ohm 50-watt resistor. Remember it will GET HOT when you're testing! Don't hold it in your hand.

4) You need to measure volts directly across the dummy load (out at the pad) while running your tests. This way you can see the voltage drop of those long wires from the control box. A little drop is perfectly fine (actually it's unavoidable), but in general you should worry if you're losing more than a volt or two in your wiring. You will need to deduce what normal looks like for your setup so you can tell when something deviates on a bad day.

5) Now read rules one through four again and think to yourself "but what about clusters?". Yup, an even more difficult load to simulate and test. Using a one ohm value for a single igniter, two in parallel would be 0.5 ohms, three would be 0.33 ohms, and so on. The currents rise fast, and many launch systems really show their limits under this condition.

6) In short, something like a one ohm power resistor is a suitable 'worst case' test load for pretty much any single igniter out there. The Estes igniter is actually a more difficult load requiring more current than most e-matches, so if your design can fire that it can fire most anything.

Hope this is helpful.
 
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We use a # 1157 auto bulb with some gator clips soldered to it as a test igniter. Draws several amps at 12 volts, makes a quick visual indicator.

Steve G

IMG_0390.jpg
 
We use a # 1157 auto bulb with some gator clips soldered to it as a test igniter. Draws several amps at 12 volts, makes a quick visual indicator.

Steve G

I like that, simple and cheap, the 1157 bulb has been around for about ever (at least to me) as standard taillight bulb for most American made vehicles.
 
I like that, simple and cheap, the 1157 bulb has been around for about ever (at least to me) as standard taillight bulb for most American made vehicles.

Yeah, but now they're all LED's. If a tail light goes out, you're in the tank for a few hundred bucks cuz you have to replace the entire assembly including the bezel. After all, why let the customer spend $2 for a light bulb when you can get $300 out of them...
 
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