Common cathode to common anode

Discussion in 'The Projects Forum' started by hsherwood68, Dec 29, 2013.

  1. hsherwood68

    Thread Starter New Member

    Oct 29, 2013
    4
    0
    Hi all,
    i have a project that switches on a variety of LEDs for a scientific instrument. Three are UV, IR and White and the remaining three are packaged in an RGB led with a common cathode. The problem I now have is that as seen in the circuit attached, my MOSFETS are wired (and soldered...) in such a drive that the LEDs have a common anode.

    My question is this: what do I have to do to make this circuit operational? I have heard that one can do this with PNP transistors?

    Thanks
     
  2. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    If you can invert the gate drive, you can then ground the common cathode connection and use the MOSFET in shunt with each anode such that when the MOSFET is driven on, it shunts the current away from the LEDs and keeps the LED off.

    Otherwise, as you suggested, you can use a PNP:
    Emitter connected to + power supply. Collector through a resistor to LED anode;
    A resistor from base to emitter (10k);
    A second resistor from base to the drain (4.7k).

    The resistor values above should work reliably for something like a 2N2907 and 20 ma through the LEDs.
     
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