Oscilloscope could not capture waveform

Discussion in 'General Electronics Chat' started by oookey, Oct 22, 2013.

  1. oookey

    Thread Starter Member

    May 24, 2010
    62
    0
    Hello, :)

    I patched up the circuit as shown, however when it's ON, with the oscillator probe at point A, I could see only DC voltage (straight line), I was expecting some on/off action displayed on the screen. because PNP was used .
    Any suggestion I could capture the waveform at point A?
    I’m using a 20MHz oscillator scope.

    Datasheet of BC327 is attached.
    The LEDs were 1W, forward voltage 3.2V~3.6V

    Thanks
    oookey
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,972
    3,219
    The PNP has the collector tied to the base which means the transistor is fully on (actually acts like a forward biased diode. Your oscilloscope is seeing exactly what is happening. Why would you expect to see anything but DC at point A? You don't have an oscillator circuit.
     
  3. oookey

    Thread Starter Member

    May 24, 2010
    62
    0
    quote: "The PNP has the collector tied to the base which means the transistor is fully on "

    I agreed its fully on, the PNP will turn on when the base is 0 or more -ve, if base is +ve, the PNP off. When current passes through R, voltage builds up & off the PNP.
    My idea is to let the LEDs on & off rapidly that our eyes never realised, the LEDs been off.
    would the 20Mhz oscillator not capable to capture the freq?
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    A frequency of 0 will look a lot like DC...:)
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,972
    3,219
    The transistor is biased so it turns on and stays on. It's not the base voltage but the base-emitter voltage that determines the state of the transistor. The base-emitter voltage is a constant value once the transistor turns on. The transistor is configured as a diode. You need to read a tutorial on transistor operation.
     
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