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Transistor Array And IR Emitters...


Hi all,
I may be being silly here so bare with me... I am designing a circuit which will drive some IR emitters from a dsPIC. The PIC runs off +3.3V and the diodes require 100mA each. I intend to use a ULN2003A transistor array to drive the necessary current. Now at 100mA the diodes have a typical voltage drop of 1.4V (1.7V max). The transistor array has a Vce saturation of 0.9V (1.1V max). So if I add the total voltage drop I get 2.3V leaving 1V across the current limiting resistor, hence I require a 10R resistor. Is this correct?

Now the second question is that if I wanted to drive two IR emitters from a single I/O port could I just place them in parallel with a smaller limiting resistor?


You're not leaving yourself much headroom and no, never connect LEDs in parallel, use a separate resistor for each LED.

Headroom as in current limiting or voltage? It looks as if I'm going to need a 5V supply for something else in the design now so that will solve the problem.


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