Using a transistor as a switch for a higher amperage/voltage line

Question

I am looking to turn a motor on and off at very fast speeds (relays won't work), which leads me to transistors. Consider this diagram of an NPN transistor:

Will the 12V 20A line harm the pi since it's grounded to the pi?

Edit

The Motor

As far as pulse rates go, I won't be sure how fast it will need to be until I start testing, but let's say we will be using pulse widths of 400 microseconds, spaced out at 400 microseconds. This is a total period of 800 microseconds (about the fastest you can do on a nema 17 with the A4988 driver).

The motor I am selecting is called the K2G20-12V-BR-04 (G20 is the maximum current draw). On that page, you will find a spec sheet, but take note of this graph:

For no load on the linear actuator, there is a current draw close to 20A. Here is how the motor is wired:

I talked to a representative for this motor on the phone, and he said that this motor will be able to receive PWM. I plan to have two transistors controlling the "extend" and "retract" switches shown in the above diagram. Sadly, I don't believe they provide inductance values.

The Transistor

This will be my first time working with transistors - mechanical engineer here :), so consider my knowledge to be naïve. I believe I have found a MOSFET here that will work. I say that it will work because in the datasheet, it says that it has a "gate source threshold voltage" between 3 and 5 V. The gate source threshold current is 250 microamps. So would the resistor value at the base not be 3.3/250e-6 = 13.2 kohms?

As far as kickback is concerned,

would this not be protecting it?

Answer 1

Your circuit design will work, provided you make a few changes. As mentioned in the comments, switching 20A will be a challenge due to the RPi's wimpy output, but this can be overcome. Before we get into the details however, you need to provide a bit more detail in your question:

What exactly are very fast speeds?

In answering this question, please also explain what type of motor you are using; specifications on the motor will be useful.

Let's address your questions, and a few of the comments in a general fashion now - the details can be covered once you've provided the specifics.

Q: Will the 12V 20A line harm the pi since it's grounded to the pi?

A dc motor is an inductive load, and when you switch it off, it will generate a high voltage spike called inductive kickback. The magnitude of this spike is proportional to the inductance of the load and the rate at which the current is quenched:

V = L * di/dt

A fairly simple snubber circuit is typically used to limit the value of the voltage spike to a safe level, but there are tradeoffs between the shutoff speed and the magnitude of the voltage spike. This is one reason why it's important you define how fast.

The answer to your question is, "No, the 12V, 20A inductive switching will not damage your RPi if you design a snubber to limit the inductive spike to a safe value."

The MOSFET you linked in your comment is probably marginal for its current capacity. Also, to drive a MOSFET, you'll need to add a substantial buffer/driver interface between the GPIO and the MOSFET Gate. This due to the fact that fast switching and low switch resistance in a MOSFET requires much more voltage and current than is available from a GPIO. A Darlington Pair transistor may result in a simpler design, but it will still require a buffer/driver between the GPIO and the Base junction.

Something similar to the following may work. We'll assign values once you've provided your specifications.

^{simulate this circuit – Schematic created using CircuitLab}