Hey, I'm using a UDN2981 to drive a large common cathode 7 segment display. It's connected to a PIC running on 5v and to a 10v supply. However I only get 8.7v out of the driver? the path is PIC -> UDN -> 7-Segment -> 100 Ohm resistor -> collector of 2n2222 Transistor -> GND (connected to Emitter of 2n2222) base of 2n2222 is connected via a 10K Ohm resistor to PIC. Any ideas why I'm getting the voltage drop and any way to stop it? Thanks
I tried connecting the 100 Ohm resistor to ground instead of going through the transistor and there was no change so it doesnt appear to be current limiting of the 10K Ohm resistor on the base of the transistor.
It's normal to loose voltage from the input to the output of ic's, why do you want more voltage? I'm guessing you might want more volts to have the led display at full brightness? Well i think you can just reduce the size of that 100ohm resistor to do that, but i'm gonna need to see a circuit diagram, even a quick but accurate sketch in paint or whatever editor is good. You should also show where you are measuring 8.7v from on the circuit diagram.
if youre measuring from the driver to the transistor, 8.7 sounds perfectly normal. 1.3v difference from the 10V supply, and thats .65V lost in the driver, and .65V lost in the n2222, both of which sound normal. If you want to replace the transistor with a mosfet, you can probably reduce the voltage across the transistor if you get a logic level mosfet with a low rDSon
