This idea is strictly for LPR use and black powder rocket engines. I have no idea if this would be suitable for composite propellant rocket engines and the igniters used to ignite them.
The idea came from discussions in this existing post:
https://www.rocketryforum.com/threa...imation-eyes-on-and-opinions-welcomed.179560/
Objective: Connect a 12 volt, sealed lead acid battery to Estes-type igniters using a compact system that lets me stand 20 feet away from the launch pad while pushing the "launch" buttons.
Here is what I came up with:
There is one box near the launch pad. I will call it the "transistor box". It contains two NPN BJT transistors configured as a "Darlington Pair" to achieve high current gain.
A separate "controller box" is attached to the "transistor" box. It contains an on/off toggle switch to completely turn off the system. It also contains two normally open, push button switches, wired in series. Both push buttons have to be pressed at the same time to complete the circuit. The on/off switch also has to be in the "on" position. All this redundancy is to minimize the possibility of an accidental launch.
The gist of the idea is this: When you press the launch buttons you connect the 12 volt battery to the base of the first transistor in the Darlington Pair through a 470 ohms resistor. This turns on the first transistor in the pair which, in turn, turns on the second transistor in the pair.
The second transistor is a high-current type which conducts the high current from the battery to the igniter.
The whole process should not take more than three seconds to launch the rocket. Then you release the push button switches and current stops flowing.
Both transistors in the pair are fully saturated, meaning they are fully "on" when you push the buttons.
I chose a TIP41 for the first transistor in the pair and a 2N3771 for the second transistor. The second transistor needs to have a high current rating (preferably more than 12 amps). The first transistor can be a medium power transistor like the TIP41. Both transistors are NPN types.
The current from the battery cannot get to the igniter unless the second transistor in the pair, the 2N3771 is turn on. The current flowing through the igniter has to flow through the collector and out of the emitter of the 2N3771 to get to ground. If the 2N3771 is not conducting, as it is not when the push buttons are not pressed, no current can flow. The circuit is "open". The 2N3771 is acting as a solid state "switch" to turn the flow of current on and off.
There should not be a lot of power dissipation by the transistors so I don't think they need to be mounted on heatsinks. Yes, 12 amps flowing from collector to emitter in the 2N3771 is lot of current, but when the 2N3771 is fully saturated, the voltage drop from collector to emitter will be less than a volt, I think, so the wattage (power dissipated as heat) will be just 12 watts or less. And that will be for three seconds or less.
The key is the high current rating of the second transistor in the pair. It has to be high enough to handle the current flowing through the igniter(s) which can be a lot - 12 amps if the igniter is 1 ohm in resistance and the battery is 12 volts. It has to have a nice, high current rating for the current on its collector when current is flowing to the igniter. I chose the 2N3771 because I have some in my parts bin and because the current rating on the collector is 30 amps. That should do it.
The controller box can have fairly low power switches in it and the wire from the controller box to the "transistor box" can be fairly light gauge, like it is here (22 gauge). All the controller box switches do is connect the 12 volt battery through the 470 ohms resistor to the base of the TIP41 transistor. That is a pretty small amount of current - about 26 milliamps (.026 amp).
So, what do you think? Is this schematic ready to prototype on some perfboard? Am I missing something?
The idea came from discussions in this existing post:
https://www.rocketryforum.com/threa...imation-eyes-on-and-opinions-welcomed.179560/
Objective: Connect a 12 volt, sealed lead acid battery to Estes-type igniters using a compact system that lets me stand 20 feet away from the launch pad while pushing the "launch" buttons.
Here is what I came up with:
There is one box near the launch pad. I will call it the "transistor box". It contains two NPN BJT transistors configured as a "Darlington Pair" to achieve high current gain.
A separate "controller box" is attached to the "transistor" box. It contains an on/off toggle switch to completely turn off the system. It also contains two normally open, push button switches, wired in series. Both push buttons have to be pressed at the same time to complete the circuit. The on/off switch also has to be in the "on" position. All this redundancy is to minimize the possibility of an accidental launch.
The gist of the idea is this: When you press the launch buttons you connect the 12 volt battery to the base of the first transistor in the Darlington Pair through a 470 ohms resistor. This turns on the first transistor in the pair which, in turn, turns on the second transistor in the pair.
The second transistor is a high-current type which conducts the high current from the battery to the igniter.
The whole process should not take more than three seconds to launch the rocket. Then you release the push button switches and current stops flowing.
Both transistors in the pair are fully saturated, meaning they are fully "on" when you push the buttons.
I chose a TIP41 for the first transistor in the pair and a 2N3771 for the second transistor. The second transistor needs to have a high current rating (preferably more than 12 amps). The first transistor can be a medium power transistor like the TIP41. Both transistors are NPN types.
The current from the battery cannot get to the igniter unless the second transistor in the pair, the 2N3771 is turn on. The current flowing through the igniter has to flow through the collector and out of the emitter of the 2N3771 to get to ground. If the 2N3771 is not conducting, as it is not when the push buttons are not pressed, no current can flow. The circuit is "open". The 2N3771 is acting as a solid state "switch" to turn the flow of current on and off.
There should not be a lot of power dissipation by the transistors so I don't think they need to be mounted on heatsinks. Yes, 12 amps flowing from collector to emitter in the 2N3771 is lot of current, but when the 2N3771 is fully saturated, the voltage drop from collector to emitter will be less than a volt, I think, so the wattage (power dissipated as heat) will be just 12 watts or less. And that will be for three seconds or less.
The key is the high current rating of the second transistor in the pair. It has to be high enough to handle the current flowing through the igniter(s) which can be a lot - 12 amps if the igniter is 1 ohm in resistance and the battery is 12 volts. It has to have a nice, high current rating for the current on its collector when current is flowing to the igniter. I chose the 2N3771 because I have some in my parts bin and because the current rating on the collector is 30 amps. That should do it.
The controller box can have fairly low power switches in it and the wire from the controller box to the "transistor box" can be fairly light gauge, like it is here (22 gauge). All the controller box switches do is connect the 12 volt battery through the 470 ohms resistor to the base of the TIP41 transistor. That is a pretty small amount of current - about 26 milliamps (.026 amp).
So, what do you think? Is this schematic ready to prototype on some perfboard? Am I missing something?
