For learning purposes how can you tell that this would require a pulse rather than a constant voltage from the design of the schematic?

The device connected is a Newport MM-2 motorized mirror. I wasn't able to find the model I have which must be a bit older, but I imagine the new models could be used for reference.

I wrote some code in Arduino to do PWM 50uS High and 1000uS Low, I wasn't able to test it, unfortunately, the oscilloscope got locked in a room I can't get into at the moment. I will test it out next week, but for now, what are your thoughts on using the Arduino to do the PWM?

 

The single thing that lead me to pulsed system was the power rating of that 10 ohm resistor in the between the IRF640 and the output jack.

That 10 ohm resistor looked to be a quarter watt, or at best a half watt. For this example we will disregard the losses in the FET, so our actual results should be a little lower than the calculations

Lets say it's a quarter watt. Maximum power can be delivered when the load equals the source.

Start with the source voltage and the total load of 20 ohms. (that 10 ohm resistor and a 10 ohm load on that resistor). 120V / 20 ohms is 6 amps. 6 amps through a 10 ohm resistor is 360W. Not too good odds for that quarter watt resistor. A pulse of 6 amps could happen if the duty cycle was low.

I thought about the duty cycle. This example is easy to calculate in your head and may not reflect the previous paragraph as it is for illustration only. A quarter watt with a 0.1 percent duty cycle would be 0.25 / 0.001 would be 250 W peak. 250W peak would be 5 A pk in a 10 ohm resistor. 5 A pk would be 50 V pk across the 10 ohm resistor. So 100uS on and 99.9 mS off would be 0.1 percent. Changing the duty cycle to 0.0005 would result in a 500W peak, and working it out to the current of about 7 A. Remember so far we disregarded FET drain to source voltage drop.

Remember the IRF640 can be 11A to 18A according to the datasheet.

You could do DC input, but you would be using a very small input to keep those resistors from smoking. If due diligence is not used, poof goes the resistors.