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Darlington pair false triggering.....

Discussion in 'General Electronics Discussion' started by Arpan Sarkar, Apr 9, 2017.

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  1. Arpan Sarkar

    Arpan Sarkar

    Apr 9, 2017
    CASE 1: Without any base current (input)the transistor goes on.
    CASE 2: Without any base current the led goes on and stays on with low brightness ,but the relay is off.
    CASE 3: Without any base current either T1 or T4 goes on.
    CASE 4: Above 3 cases sometimes happen or sometimes not.
    This circuit is powered by 12V, 1A ac adapter.
    I have checked all transistor they are good and replaced with new one to be more confident.
    but the same things happening.....
    Whats going on ????????????????
    Please help....................................................
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Add a high value resistor (perhaps 10k) between the base (1) and emitter(3) of your "darling tone pairs" (they're darlington pairs). This should not be necessary because the TIP122 are supposed to have this internally.

    The leakage current of the TIP122 is quite high. Perhaps you can use something with lower leakage (do you really need to use a darlington)?

    If the lakage causes the LED to turn on very dimly, you may need to place a resistor in parallel with the LED which will drop less than Vf of the LED at the leakage current of the transistor.
    Arouse1973 and Arpan Sarkar like this.
  3. AnalogKid


    Jun 10, 2015
    As a test. ground the inputs rather than leave them floating.

    The TIP122 has built-in resistors across both base-emitter junctions. The odds are 99% that you have the transistors connected incorrectly. Check the pinout.

  4. Arpan Sarkar

    Arpan Sarkar

    Apr 9, 2017
    Sorry ,my mistake "darling tone pairs":(:(. thanks for your help steve.
    Last edited: Apr 9, 2017
  5. Arpan Sarkar

    Arpan Sarkar

    Apr 9, 2017
    transistors connected correctly, double checked.
  6. Audioguru


    Sep 24, 2016
    How much current is needed for the relay coil?
    You say you used a 12V/1A AC adapter which is wrong. You need a 12VDC adapter.

    TIP122 darlingtons made by a good company do not turn on when they have no base current. Maybe they are fakes from ebay?
    Arpan Sarkar likes this.
  7. RLP123123


    Mar 30, 2012
    Just viewed this thread. There are major missing pieces of information:What drives the transistor base, cmos/uP PIO? TTL? Switch? What is the environment? Long IO lines? Noisy industrial setting? Evidently your input lines are very near the transistor threshold when in the OFF state. Slight noise can then trigger. You may need a "harder" low/OFF condition. If you are using cmos or uP based PIO, that would be a reason for using the darlington since high drive is usually very low. If you have a tristate driver, then you need to drag the driver lines down using values appropriate for the high drive signal and loading by other devices. What is a high turn on value on the driver lines? 3V, 5V, 12V? For a truly logical solution, much more information is needed. Otherwise it is just educated guessing.
    SPLatMan and Arouse1973 like this.
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