12 and 5 V signal in one wire-protection against self induction and high voltage

Hello!

I am doing a bigger project which includes measuring resistance of car injector and testing them by ~12V PWM signal.
Measuring resistance is made by using this schematic:

I would like to ask the following questions:

  1. Fuel injector I am testing are devices based on electric coils. Coils are known for self indcution when voltage drops. How to protect properly transistor and arduino against this phenomenon? Will diodes be ok? How to chose right backward-voltage value?

  2. PWM singal will be generated by arduino, and will steer transistor which will be connected to 12V power supply. Which transistor will be better for this - BJT or MOSFET?

  3. To keep functionality of device I am building, I want to clip only one connector to each injector. But, as I wrote upper, i need to firstly check resistance and then apply PWM. Is it possible to do this using just one connector? I thought about using another transistor activated by digital pin in voltage-measure circuit. I also attached very, very simplified scheme of what I am talking about.

Thank you!

Inductive loads need a flyback diode. Any diode that can handle the normal drive current of the load as peak current will do. For pwm you better pick one with a somewhat higher rating.

Probably best to use a MOSFET.