Issue With Custom Launch Controller

[burritoman1]

Now, that I am using more powerful rocket motors, the standard Estes one wont be enough. Following online tutorials, I have a very basic design: A toggle switch, power button, aligator clips, and a power supply wired together into a circuit. The problem is, none of these designs have any way to control current draw like resistors. I'm just worried without a way to control the current, the battery will draw until it fries the switches.
Some info: Aerotech Igniters need 6v at 3A
My battery is a Vex 7.2v 3000mAh NiMh. It supposedly peaks at something like 12A while being able to provide a stable 6A
I just wanted someone else's thoughts.

 

[burritoman1]

I should mention, this battery is very similar to most 7.2v sub-c rc batteries.

 

[Voyager1]

I have been using basic and more complex relay-based launch controllers for the past decade or so and have yet to fry any switches or relays. These controllers have typically used 12V 1.5 Ah to 7 Ah SLA batteries with no current limiting resistors. However, I have usually employed a 10 A slow blow circuit breaker or fuse to protect against prolonged dead shorts.

Just make sure the switches you do use can happily cope with at least 20 A or more for robustness. With your 7.2V battery you will draw about 4 - 5 A with a typical igniter (~1.5 Ohms). Igniters generally go open circuit when blown, but having a fuse or circuit breaker will protect against the occasional short.

 

[burritoman1]

I have a 5V relay, would this be enough to protect the switches from the (20A) high current?

 

[Voyager1]

When you say 5V relay, do you mean that the coil is controlled with a 5V signal, or that the relay contacts are rated at 5V? Your switches would only be in the low power relay coil circuit (perhaps 7V@100 mA or so) if your using relay-based ignition. The relay contacts would need to be rated for 20A or higher.

The main advantage of the relay-based controller is that you only require high current switching at the pad through short high current ignition leads. The field cable for this controller only operates at low current. The simple non-relay-based controller requires a long high current cable from the launch position to the pad. That presents a higher total resistance to the ignition circuit and wasted power over the cable length.

There are several threads on the Forum that discuss DIY launch controllers. Do a search on DIY launch controller. Here is one of them: https://www.rocketryforum.com/threads/questions-about-my-diy-launch-controller.145459/.

Here is a basic controller circuit below that doesn't require relays.