Need help on transistor/mosfet selection

Discussion in 'General Electronics Chat' started by zgmfx20a, Sep 21, 2009.

  1. zgmfx20a

    Thread Starter New Member

    Sep 16, 2009
    5
    0
    I'm trying to short a bunch of signals on a circuit to ground - these include clock signals, data interfaces, and pin resets to see how the board behaves.

    Its worked all ok with an npn transistor for DC signals, but as soon as I try to fault a high speed clock signal, the transistor seems to do nothing at all. Its a simple npn transistor (PN2369A- Fairchild), with the base resistance at 1k @ 3.3V, collector connected directly to the signal I'm trying to short, and emitted connected to ground.

    Is an npn transistor suitable for shorting clock ac signals? If not, what should I be using? I could simply short it with a wire, but I'm looking for a safe way to do it instead of touching the board while its in operation. I can trigger the voltage at the base using an external power supply.
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your base current is (3.3V - 0.7V)/1k ohms= 2.6mA so the transistor should be able to short 26mA signals to ground. The datasheet shows that the max saturation voltage loss is 0.25V and is typically 0.15V.
     
  3. zgmfx20a

    Thread Starter New Member

    Sep 16, 2009
    5
    0
    Does that assume the signal is DC ? When I try to do it for a clock signal, the same setup doesn't work. The ac signal has a Vpk-pk of 2.4V, high 2.1V, low 300mV as probed from an oscilloscope. I use a series resistor of 100ohm to get the current at collector ~21 mA.

    It should switch to ground, instead its only switching to 1.32V high, 300mV low, and Vpk-pk 1.06 V. Unfortunately, I don't even have the schematics for this board so I can't check out the rest of the circuit.

    Sorry for sounding like a noob, but I'm not very experienced with this stuff. I'm simply trying to validate some code to detect devices failure, and was hoping to test it out in hw to help :(
     
Loading...