Help with PNP low-side alternative?

Discussion in 'The Projects Forum' started by Buga, Jan 22, 2011.

  1. Buga

    Thread Starter New Member

    Jan 22, 2011
    Hey everyone, essentially right now I'm using a PNP as a low-side switch, with the load on the emitter and the collector grounded. The reason, is I need an active low switch on the ground side, if I moved it to the high side I'd need 8 PNP transistors, to control eight LEDs. I have two groups of 8 LEDs, and want it so one group is turned on when a control signal is high, which an NPN is working great for, but then when the control signal is low I want the other set to be on, so I quickly used a PNP as a low side switch. I know this is inefficient, but can't think of a better way because on the high-side I have a shift register sourcing currents.

    Any suggestions?
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
    Please post your schematic, if at all possible. Verbal descriptions of circuits are not as easy to understand.
  3. SgtWookie


    Jul 17, 2007
    You're using the single PNP as an emitter follower, so you'll get a significant voltage drop across it.

    You can use two NPNs daisy-chained as common emitter saturated switches; see the attached.

    Your circuit is on the left; note the red Rload1 current plot below. The 1st NPN (Q2) will invert the active-low to an active-high. The 2nd NPN (Q3) sinks the current from Rload2; note its' green current plot below.

    Adjust R3 to (Vcc-0.7)/(desired_collector_current/10).

    After you do that, adjust R2 to 1/10 the current required to pull the voltage on the base of Q3 below 0.5v.
  4. Buga

    Thread Starter New Member

    Jan 22, 2011
    Yeah, okay I see how that works. Thanks very much!