With some experimentation I determined that it takes about 300 mA to make the heater warm to the touch.
Now I want to computer control the heater using a digital output on the NI USB 6008. The 6008 will control a higher current transistor (or darlington) and the transistor will control the current to the heater.
Seeing that the NI USB 6008‘s output is either +5V or 0V, my first instinct was to try this:
This failed. FAIL.
I looked at the docs for the NI USB 6008 and I found this:
The default configuration of the NI USB-6008/6009 DIO ports is open collector, allowing 5 V operation, with an onboard 4.7 kΩ pull-up resistor.
So basically the digital output is either an open circuit or a path to ground. The 4.7 kΩ pull-up resistor brings the output to +5V when it’s ON, but when you ammeter from DO1 to ground you don’t see any current.
However in this configuration you will see a current turn on and off with DO1:
How do we go from a current to a voltage? A resistor!
This schematic represents the working solution:
Here is a video of computer controlled current: