At the risk of contributing some useful rocketry information, I want to follow up on a discussion from the last meeting. I found an older Estes launch controller of mine, one with an incandescent bulb for continuity check and battery clips for a 12v battery. If I recall correctly, it was what I used to launch rockets as a kid using a lawn and garden tractor. I replaced the bulb with one of the 12v LEDs that I have. If you remember the discussion, we need to limit the current that a continuity check uses to less than around 100ma or so to safely do the check without a chance of firing ANY commercially available initiator/starter/igniter. LED's usually need only 10ma or so to light, so that provides a 10x margin for anything that is used as a starter/initiator/igniter. Incandescent bulbs can draw more than 100ma, and so are not safe for things like e-matches. If anyone has an old Estes (or any other manufacturer's or scratch built controller) that uses an incandescent bulb, I can either give directions on how to do the modification, or when face-to-face meetings are in vogue again, I can help do the modification.
Last edit: 3 years 9 months ago by guyw. Reason: Missing hyphon
When I built my homemade LCS I did worry about this stuff and then forgot all about it.... but then I went ahead and checked it out... since in my home-built LCS I used a 12V incandescent bulb. When I built it, I was shooting for a system that would not fire an Estes starter with a published max no-fire of .5 A (500 mA?). I was pretty sure of my calculations when I found out a 12V incandescent bulb will be about 1.5 Ohms and it would not fire an estes starter when it was all put together. Tonight I put the clips together (a zero resistance igniter shall we say?) and hooked up my voltmeter/ammeter to measure the current and flipped the continuity switch. It was 60 mA. An estes starter is around 0.7 ohms...
Well I can attest that it doesn't fire a starter on continuity check... wondering if my volt meter is off...
The e-matches that Curtis bulk ordered (which I have a box of) are rated with a max no-fire current of 300 mA.
I would not trust it with much else...
I also know that first-fire mini and jr.s are ok with my system however I rarely use it since the continuity switch is not great for the purpose.
But I feel that the moral of the story is: if you build your own... check the current you get out on continuity.
Last edit: 3 years 9 months ago by DeltaVee. Reason: getting the facts worng
The equation for power in terms of current and voltage in a circuit is:
Power (P) being in Watts, Current (I) being in Amps and Voltage (V) being in Volts. So for example, a 3 Watt incandescent bulb on a 12V supply would draw 250 ma of current to fully light. With the resistance of the wires, battery, and whatever initiator added, you would get a dimmer light and somewhat less current, but still close to the minimum fire current for e-matches. It gets even more complicated, as initially the bulb has a high resistance when cold and unlit which decreases as it heats up and begins to light, increasing the current draw, which causes the bulb to get hotter, which causes the resistance to go down, and so on. So you could have a time delayed launch!
As an aside, since Electrical Engineers use I or i as the designation for Current, in College we had our own Physics course because we use j instead of i for imaginary numbers.