npn inverter circuit pulls up but does not pull down completely

Discussion in 'General Electronics Chat' started by przachar, Mar 3, 2010.

  1. przachar

    Thread Starter New Member

    Feb 8, 2010

    As a part of a bigger circuit I am building an inverter using NPN 2N4401. The circuit is very simple as shown in the attachment. My problem is that when the input to the base is 0V, the LED glows brightly and hence is high. The inverter works correctly. But when the input is 5V, the LED only dims but does not turn off completely.

    I tried connecting a 1k resistor to the LED. Didnt work.
    I tried connecting a high value (0.6M) pull down resistor across the collector and the emittor. That also didnt work.

    Any ideas how to fix this?

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Just place the LED in series with the 1K resistor on the collector. The 100 ohm resistor makes a path around the transistor, which may not be turning on as hard as you think (the 10K base resistor is much too high to saturate the transistor).
  3. SgtWookie


    Jul 17, 2007
    Try it like this instead:

  4. hobbyist

    Distinguished Member

    Aug 10, 2008

    I used a red LED with a VDforward of 2v.

    Using 5v. for the entire circuit supply,
    and 2n3904 transistor, I was able to get a vdrop across the led
    of 170mV. wityh a base rersistor of 100K ohms.
    and 60mV. across LED with a base resistor of 10K ohms.

    So are you using the 5v. supply for the LED output also.
    and if so make sure your transistor leads are hooked up and proper.
    Last edited: Mar 3, 2010
  5. przachar

    Thread Starter New Member

    Feb 8, 2010
    Thanks all of you. I just had to reduce the base resistor and it worked.
  6. Audioguru

    New Member

    Dec 20, 2007
    In the original circuit I think you have the collector and emitter pins mixed up.

    With a series base resistor of 10k from 5V the base current is 435uA.
    When the transistor is turned on and the LED is turned off the collector current is 4.5mA.
    Then the transistor needs a current gain (hFE) of at least 10 which is fine if the pins are correct.

    If the pins are mixed up then the transistor has a very low gain.

    The 100 ohms resistor does nothing and should be removed.