Hi,
I'm playing around with a 7 segment numeric LED. I've got resistors on the anode side that allow for 25ma to pass with my 5v supply. If I ground the cathode all 7 segments come up to 179ma or about 25.6ma for each one.
I have a package of NPN switching transistors laying around I got from radio shack. I measured the hfe of each one and found them to be all over the map. Here are the way they came out, 73, 85, 91, 93, 99, 101, 102, 108, 182, 185, 185, 226. The package says 200 hfe typical. Question # 1 : Is this range normal or are these not very consistent???
Question # 2, I tried a 101 hfe in my circuit. I have a 1K resistor connecting the base up to the 5V, so 5/1000 = 5ma. Shouldn't it allow 5 * 101 hfe = >500 ma to flow ??? In my case it only allows about half or around 102 ma to flow instead of he full 179ma. Am I missing something here??
Thanks,
Alan
I'm playing around with a 7 segment numeric LED. I've got resistors on the anode side that allow for 25ma to pass with my 5v supply. If I ground the cathode all 7 segments come up to 179ma or about 25.6ma for each one.
I have a package of NPN switching transistors laying around I got from radio shack. I measured the hfe of each one and found them to be all over the map. Here are the way they came out, 73, 85, 91, 93, 99, 101, 102, 108, 182, 185, 185, 226. The package says 200 hfe typical. Question # 1 : Is this range normal or are these not very consistent???
Question # 2, I tried a 101 hfe in my circuit. I have a 1K resistor connecting the base up to the 5V, so 5/1000 = 5ma. Shouldn't it allow 5 * 101 hfe = >500 ma to flow ??? In my case it only allows about half or around 102 ma to flow instead of he full 179ma. Am I missing something here??
Thanks,
Alan