Been working on a rocket launch control system. I have most of it figured out. But I want to put a fuse in line to protect things should something go wrong. Trouble is I don't know what size fuse to use.
HERE is the thread where I've posted about use of a 555 timer to do an automatic countdown.
What I'm after for the moment is to discover how many amps an Estes Rocket Igniter (electric match) will draw. Obviously that will be the biggest power draw. I'll post the schematic I have so far but in the schem. I don't have the fuse drawn in. It's basically between SW1 and the 12V battery. Likely to be something like a 12V 7Ah SLA battery. In addition to this I've included in the schem. (not yet updated) a diode to prevent accidental polarity reversal. Had considered a bridge rectifier, which can still be included. But I need to get a good idea on what fuse rating should be.

Your assistance is appreciated.

 

Just did a quick update of the schematic. Click on it to enlarge it.
View attachment 358306
Here's the final control panel. I WILL be adding the fuse somewhere on the box. Perhaps the face place, perhaps the back side of the box.
View attachment 358307
Had hoped Laser etching the face plate would have turned out a bit brighter, but - - - .

 

Found an SDS for the Estes Sonic Model Rocket Igniter but it doesn't say anything about electrical characteristics.
https://cdn.shopify.com/s/files/1/0686/0220/0369/files/Estes_Sonic_Model_Rocket_Igniter.pdf

 

As you have a 47 ohm resistor in series with the electric match the current cannot be just over 250mA. I think a 500 mA fuse should be OK. (As long have some very thin wire to protect.

Les.

 

Found some information but am regarding it as less than 100% accurate. On this video the guy measures two types of igniters. One - according to the video - says the Pro Igniter is 1.8Ω whereas the standard Estes igniter is 0.8Ω. He didn't show his test lead resistance and didn't show or state the meter had been zeroed with the test leads shorted together. My meters can easily show 0.2 to 0.4Ω when shorted. That's the time I use the delta feature to zero out the lead resistance and get a more accurate reading. HOWEVER, he also did not say it was a "Low Resistance" test meter. Testing low resistance accurately can be difficult. But for my purposes - the information gives me enough detail.

The 47Ω resister is to limit the current draw when the capacitor is charged. The 4700µF cap should provide enough impulse to ignite either igniter. However, again, after watching the video it appears that some igniters can take longer periods of current draw time to fully ignite. Here's the video:

 

Beautiful project!!

I used to launch Estes rockets about 25 years ago, and if I recall correctly, the igniter current is close to 1/2 amp.

Have you actually tested your circuit with a real Estes igniter, and verify that lights up quickly and not just smolders?

 

12V at 0.8Ω that is 15 amps. May have to rethink the idea of the resistor on the cap.

 

Have you actually tested your circuit with a real Estes igniter, and verify that lights up quickly and not just smolders?

Haven't purchased any igniters yet. Debating between the Estes stock igniter at 0.8Ω or the pro series igniter at 1.8Ω. Also the plan is to use a 12V SLA. The first launch control I had was a plastic battery box with two AA batteries and a long wire. The battery voltage went down quickly after several launches (if memory serves). So at 3V there should be 3.75A on that small controller. Perhaps 12V is not a good idea. But then I have to consider what is the minimum voltage the timer, counters and the AND gates require.

 

Note: When laying out the etchings I didn't take into account the overall diameter of the volt meter. So the "FUEL STATUS" etching ended up getting partially covered. Oops! Oh well. Nobody's perfect.

 

12V at 0.8Ω that is 15 amps. May have to rethink the idea of the resistor on the cap.

I tested it with the AA cells. Thus the cell’s impedance limited the current.
Different story with a 12v lead acid. It will require a limiting resistor, which you’ll find experimentally