I picked up three 7 segment numeric LED's for my first AVR tests where I am going to attempt to multiplex them.
All 7 segments have a 100 ohm resistor which at 5V gives each one about 26ma. The package says 30ma max. They have a common cathode. So I have 7 places on my breadboard that go to a 100 ohm resistor, then to each of the three segments. If I tie all the cathodes to ground and apply postive voltage to each of the 7 segments I see all three digits same segment light up as expected.
Now I am trying to implement the transistor control of the cathode. I have a NPN transistor I am connecting the collector to the LED cathode, emitter to ground, and I am trying to use the base to turn it on and off.
With the cathode shorted to ground and all segments on, it runs about 200ma. The transistor says 200 hfe so does that mean 200x gain ? If I take .200 and divide it by this gain I get .001 (1ma) and I read something about adding 30% to it, so 1.3ma. My voltage is 5V so 5/.0013 = 3846 ohms. Would this be the correct resistor to connect the base to +5V ?
I have to think I've done something wrong because a 100 ohm resistor yields about 170ma instead of 200ma and a 460 ohm resistor yields about 56ma. It looked like about 1/4th of the current was flowing into the base. Would that be 4 hfe?
I must have something wrong, please correct me!!