I'm trying to run a linear actuator. It's 12 volts and 150 milliamps so I'm using a power supply for it. However, the PWM signal has to come from the Arduino. What's happening when I hook it up is that I'm getting an open circuit instead of a PWM signal (if I replace the actuator with a resistor to ground I get the proper signal). I'm using a Mega so I can sink 200 mA into a ground. Would hooking the ground into the Arduino instead of the power supply fix my problem? Would that mess with the power supply voltage by having a different ground?
I'm using a Mega so I can sink 200 mA into a ground.
You can't get 200ma source or sink from a mega pin. You need to have some type of driver. For the current you are talking about a 2N2222A would work. Emitter to gnd, collector to actuator, drive base via a 470 ohm resistor. Other pin of the actuator to your power supply +. Make sure that you put a clamping diode across the terminals of the actuator.
I do have a question. Do you need to run this in two directions? If so, you are going to need to drive it with an H-bridge so you can reverse the current flow/direction of travel. The L298 motor shield would work well, but more current capability than you need. You could also build an H-bridge from discrete parts(2 x 2n2222, 2x 2n2907, diodes, resistors. You would also need 4 output pins. A lot of work to build your own; the L298 chip isn't expensive.