A transistor used in emitter follower mode has the "load" between emitter and ground. Therefore "all" of the output voltage appears across this load. There must always be a drive voltage between base and emitter to produce the base-to-emitter drive current. Hence the emitter voltage can never be greater than the base voltage (both relative to ground). If the base voltage is the PWM output of the arduino - a nominal 5 volts, it follows that the emitter voltage can never be greater than the arduino voltage.
Thanks for the explanation. That makes sense. It's nice to understand why I was seeing what I was seeing. It had me stumped, and I had not yet found an explanation elsewhere.
Now the question is--how can I get the 0 to 12 Volt range that I want? Obviously it is not going to be in the common emitter mode. I'll keep looking for an answer. Any help will be appreciated.
I need to send a signal of between 0 and 12 Volt to an electric-over-hydraulic trailer brake.
I stumbled, why not put simple question in subject line "how to switch my trailer brake?"
Low impedance line ?
I didn't mention what the load was because I did not know that information was important. Plus I did not want to spark a lot of discussion along the lines of: "Just use a commercial in-cab brake controller." or "It's dangerous and stupid to use a homemade brake controller on the public roads. You'll kill us all!"