Hello all! I am designing a circuit to bypass a dash switch. When the button on the dash is pushed, the sensing circuit in the ECU goes to 0v which is recognized by the ECU and it disables the function, then illuminates an LED in the switch. I can't use a short pin across the switch because the ECU will recognize no change in state on the sensing circuit across multiple key cycles, set a fault code, and disable the button.

So, I built the circuit in the replacement module below. The 4.7K resistor is to provide current flow similar to the LED (so the ECU still sees current flow on that circuit, and does not set a fault code). The PNP transistor serves to effectively hold the button closed, allowing current flow, until the the LED signal goes high(which indicates the function has been disabled), after which it stops current flow, allowing the sensing circuit to go high again. Pin 9 is for the LED illumination and goes from 6-11 volts depending on whether the dash lights are in day or night mode. Pin 17 is for the sensing circuit and Pin 22 is a common ground.

My big question is how do I choose the best transistor part number to use for my application? I want something that will be comfortably within its operating specifications, with low cost and long term reliability. I am currently using a S9015, which works, but I do not know how to calculate the best (or correct?) part number to use for my application. Looking around there is so many options I am having a hard time sorting them all out to find the ideal transistor to use. Anyone with more experience that has advice would be immensely appreciated!

 

My big question is how do I choose the best transistor part number to use for my application? I want something that will be comfortably within its operating specifications, with low cost and long term reliability.

The most critical spec for any transistor is the current rating. You need to choose one rated well above your actual expected current. Second and related is the power dissipation rating. For long life you want it to run cool.

If I understand your application, neither of these things is going to be a cost driver because you're just handling a small information-level current, not much actual power. So I'd say the next thing to look for is the voltage ratings. Automotive applications call for voltage ratings up to about 60V and I think transistors rated for automotive duty also have wider environmental operating conditions, temperature in particular. I don't know if they're actually built any differently or just tested differently.

If it was me, I'd choose a so-called automotive transistor and not worry much about the rest. You're not doing high-speed switching or handling much current.

 

The most critical spec for any transistor is the current rating. You need to choose one rated well above your actual expected current. Second and related is the power dissipation rating. For long life you want it to run cool.

If I understand your application, neither of these things is going to be a cost driver because you're just handling a small information-level current, not much actual power. So I'd say the next thing to look for is the voltage ratings. Automotive applications call for voltage ratings up to about 60V and I think transistors rated for automotive duty also have wider environmental operating conditions, temperature in particular. I don't know if they're actually built any differently or just tested differently.

If it was me, I'd choose a so-called automotive transistor and not worry much about the rest. You're not doing high-speed switching or handling much current.

Thank you! That's kind of what I was thinking but I don't have a ton of experience working with solid state components.