I am currently designing a circuit that will allow the remote startup of some avionics on-board a rocket. The general idea is that PNP transistors are used to keep the avionics in a default on state, and that an external control box can be hooked up during preflight assembly to keep them disabled until it is time to initialize them on the pad. There are two flight computers, and the design has an ATmega328 to grab diagnostics data from them during startup and drive a small OLED display. I'm pretty sure I got that bit set up properly, so you don't need to look at that part of the circuit unless you want to be extra helpful.
What I am asking for help with is setting up the PNP transistors properly. The way they are set up right now is as if they are controlled by simple digital logic: 0 V to be turned on and +7.4 V to be turned off. However, I am still very new to the concept of transistors, and I have just discovered the need for current-limiting resistors on the transistor base. This has proceeded to get me very confused as to what I need to do to not fry my transistors. So my questions are as follows:
1. If a PNP transistor is in a saturated state, does the current from the base matter? That is to say, once I've got a transistor saturated will it act like a true short circuit as long as the base is connected to ground or do I need to properly throttle the current from the base using the correct resistor? Right now I am using 10k pulldown resistors.
2. If a PNP transistor is in a cut-out state, I assume current does not flow through the transistor, but rather flows past the transistor, through the resistor, and into the battery ground? That is the idea I designed my circuit around, and I assumed that if there is no current through the transistor then I don't need a current limiting resistor on the line that gets toggled to turn off the flight computers.
I greatly appreciate any advice!
Information that might be needed to answer the above questions:
Expected maximum current draw by either flight computer: 1 A
Transistor datasheet: http://www.mouser.com/ds/2/308/D44H-D-111810.pdf
Images:
Rocket-side components:
External components:
What I am asking for help with is setting up the PNP transistors properly. The way they are set up right now is as if they are controlled by simple digital logic: 0 V to be turned on and +7.4 V to be turned off. However, I am still very new to the concept of transistors, and I have just discovered the need for current-limiting resistors on the transistor base. This has proceeded to get me very confused as to what I need to do to not fry my transistors. So my questions are as follows:
1. If a PNP transistor is in a saturated state, does the current from the base matter? That is to say, once I've got a transistor saturated will it act like a true short circuit as long as the base is connected to ground or do I need to properly throttle the current from the base using the correct resistor? Right now I am using 10k pulldown resistors.
2. If a PNP transistor is in a cut-out state, I assume current does not flow through the transistor, but rather flows past the transistor, through the resistor, and into the battery ground? That is the idea I designed my circuit around, and I assumed that if there is no current through the transistor then I don't need a current limiting resistor on the line that gets toggled to turn off the flight computers.
I greatly appreciate any advice!
Information that might be needed to answer the above questions:
Expected maximum current draw by either flight computer: 1 A
Transistor datasheet: http://www.mouser.com/ds/2/308/D44H-D-111810.pdf
Images:
Rocket-side components:
External components: