I've got a 27VDC external power supply connected through an NPN power transistor to a DC motor. The power supply and Arduino grounds are common. The transistor base is receiving a PWM signal from the Arduino pin 10 with a 5k pot connected to + and -5V on the Arduino and the wiper terminal is connected to pin A0 to vary the voltage. The motor speed varies but won't get to full speed. The program divides the pot input by 4 to get the right range of 0 - 256 so I think the output is correct. Any thoughts or suggestions to get full speed would be greatly appreciated.
Does the transistor get hot?
Use serial print to see that you are getting the full range of values from your analogue input.
What current does your motor take, it could be that your 5K resistor is too big to allow enough base current to flow.
It isn't a 5K resistor is it.
Is the transistor high-side or low side?
Grumpy_Mike:
it could be that your 5K resistor is too big to allow enough base current to flow.
It isn't a 5K resistor is it.
I think the 5K pot is attached to an analog input.
As usual, we need a schematic/circuit diagram...
fungus:
As usual, we need a schematic/circuit diagram...
and as usual, we have to ask for it 
Sorry I don't know how to display a schematic other than drawing one and scanning it.
As I mentioned above the 5k pot is connected to ground and 5 volts. That would be the two outer terminals on the pot. Wiper, or center terminal, is connected to pin A0 on the Arduino. This should give 0 to 5 volts to the pin.
The emitter of the NPN transistor is connected to ground, the collector is connected to one side of the motor, the other motor connector goes to 27V on the power supply. Both the grounds of the Arduino and power supply are connected.
The base of the transistor is connected to a 680 ohm resistor and that is connected to pin 10 on the Arduino to receive the PWM signal. I took out the resistor to check if that would increase speed but it didn't.
No the transistor doesn't get hot. It's rated at 250V, 16A.
Sorry again for no schematic.
OK It's working now, I swapped transistors and now getting full speed. Must have had a bad one.
Thanks all for the help.
Sorry I don't know how to display a schematic other than drawing one and scanning it.
For any one else reading this, this is a perfectly acceptable way of posting a schematic. Just attache the file with the additional options triangle at the bottom left of the reply box.
Try to keep the size below 900 pixels otherwise it it a bit hard to see if it is too big.