cyborg5 wrote:We're also working on a 5 V system. My datasheet says that the maximum continuous collector current is 600 mA.
Check the Vf of the LEDs you are using.
the Forward Voltage drop is probably about 1.5V. Also, the LED's transistor probably has a saturated 'ON' voltage of 0.3~0.6V. Again, check the datasheet.
So, 5V - 1.5V - .6 = 2.9V, 2.9V/0.6A = 4.83-ohm
cyborg5 wrote:I would need some sort of a current limiting resistor in line with that to keep the transistor current under specification.
Yes, that is correct, the ~5-ohm resistor above. Although, it depends on the current capability of the 5V supply.
The TV-B-Gone relies on the internal resistance of the battery to prevent blowing the LEDs and transistors. Depending on how you are powering this thing, you may not need the 5-ohm resistor.
cyborg5 wrote:at least a 50 ohm resistor given 5 V supply and a 100 mA maximum rating.
Just like above, you have to subtract the Vf of the LED and the Vsat of the transistor. (5V - 1.5V - 0.6V = 2.9V, 2.9V/0.1A = 29-ohm
cyborg5 wrote:Is there internal resistance in a fully saturated transistor that limits its current?
Not really. It has more to do with the base current and the transistor's gain. If the transistor can stay "fully saturated" and below it's power dissipation, then the "on resistance" will be nominal because it is related to the somewhat fixed Vsat voltage.
Hope this helps, John