Transistor switch to send 12v low current?

I'm a noob.

SITUATION: - Arduino Nano under the dash of a car. It gets 5v through a buck converter.

  • Required to send a 12v signal to trigger a solid state controller. Current = 75ma.

  • Would like to control this signal through a digital out pin (D7 is available).

This work? |411x500

I THINK THE FOLLOWING: - I can use a PNP transistor.

  • BASE to pin D7 through a resistor.

  • EMITTER to 12v side of buck converter.

  • COLLECTOR to load (solid state controller 75mA) which will be switched on and off from logic at D7.

PARTS: - P2N2907A : I'm not sure, this is why I'm asking

  • Resistor between D7 & BASE = 10k which will flow 1.2mA which is enough to saturate?

QUESTIONS: - Is this proper?

  • HOW do I turn it off??? If I set D7 pin output to LOW, it will ground. But... do I set it to high? Then, the R1 resistor needs to ensure that it's big enough for the new voltage difference not to flow enough current to saturate, but sometimes the 12v is higher in a car, could be a problem? Can I somehow just set the pin to float so nothing flows?

  • I have seen large value resistors from base to emitter voltage, says to ensure switch turns off? Should I include this? Shown as 100k.

THANKS for any help. Trying to learn...

You can't turn it off. Even if you switch the port pin to input, so it won't directly sink any current, you still have a protection diode from the pin to Vcc, and current will flow from the 12V supply through the emitter and base, through the protection diode to 5V, and that will turn on the PNP. So you need to use an NPN transistor instead, with the load placed between the 12V supply and the transistor. Then the transistor will turn on when the port is high, and turn off when it's low.

ShermanP: So you need to use an NPN transistor instead, with the load placed between the 12V supply and the transistor. Then the transistor will turn on when the port is high, and turn off when it's low.

Problem with car stuff is often that the ground side of the component is the component's frame which is used to secure the thing into the car, sometimes with many screws. If that's the case (and OP didn't describe the load physically) it can be well nigh impossible to "break" the ground to put the npn in there.

I wonder if that's why OP has proposed high-side switching in the first place?

To switch high-side with a higher voltage than the logic supply requires level shifting and then a PNP or p-channel switching device.

http://forum.arduino.cc/index.php?topic=298588.0

Other option would be a signal relay or reed relay.

neiklot: I wonder if that's why OP has proposed high-side switching in the first place?

You're right. I cannot access the ground side of load. The load is internal to an OEM Mercedes fan controller. It has two very large wires (power and ground) to power the fan, and it has 2 small wires for instructions what to do. One is a PWM from the Nano which I have working, and the other requires a high side switch from 12v (will not trigger from 5v). It internally grounds through the large non-switched ground wire. The reason I don't just tie the high side switch directly to ignition power is a couple reasons: 1) I need to ensure PWM is established first, it requires the PWM arrive first otherwise it goes into fail safe and full blast the fan until ignition turned off. 2) Even though it seems I can establish the PWM fast enough and get it to work as long as it always receives PWM from Nano, I'd like to have it run for a minute after car is shut off and then shut off both high side trigger and PWM.

MarkT: To switch high-side with a higher voltage than the logic supply requires level shifting and then a PNP or p-channel switching device.

http://forum.arduino.cc/index.php?topic=298588.0

Other option would be a signal relay or reed relay.

Thx! I'm on the scent...

Any other suggestions or comments welcome.