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  1. #1
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    Problem with using PNP transistor BC557 and a LED

    I intend to use BC557 (PNP) transistor in indicator circuit, +at pin 1 (E), connect a + pin of LED at pin3(C), and the -pin of LED to Ground, the LED is not glowing (6V input), when positive volt is given through pin2 (B) the LED is glowing, since the transistor is PNP type if the base is biased the LED should glow and the Base is positive the LED should be OFF, the circuit is not working pls help

    •   Alt14th December 2010, 09:55

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    Re: PNP Transistor BC557

    Please put a resistor around 330 to 1k to the LED, if not, you might burn your LED

    ---------- Post added at 10:44 ---------- Previous post was at 10:37 ----------

    Last edited by john blue; 14th December 2010 at 10:43.



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    Re: PNP Transistor BC557

    bushygs
    I suspect you have destroyed the BC557. You need some current limiting, i.e. a resistor, in series with your LED to protect both the transistor and the LED.
    A red LED will have a forward voltage of around 2V when forward biased, and a blue or white one will have around a 4V drop. Add in the CE voltage drop of the transistor (approx. 0.2V say) and the rest of the applied 6V supply voltage must be dropped across a resistor in series with the LED.
    Assume a current of 15mA through a red LED, then 6V - 2V - 0.2V = 3.8V, which divided by 15mA = 253ohms. Choose a 220ohm resistor and the current will be a little higher at approx 17.3mA. Your LED and your transistor are now protected, although you really ought to limit the base current with some resistance also. If you are pulling the base down to ground then you need maybe 47k in series with the base to protect the BE junction of the transistor. Hope this helps.



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    Re: Problem with using PNP transistor BC557 and a LED

    Sorry i had used a 220ohms at the ground connection of LED. i am uploading the image file to show the schematic.
    When the Voltage is below 10V the Zener would not conduct and the BC557 PNP transistor should conduct and the LED should be in ON state, if the voltage is above 10V the Zener would conduct and would reverse bias the BC557 and stop it from conducting and the LED should be in OFF state.

    Please advice where I am wrong



    •   Alt14th December 2010, 14:28

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    Re: Problem with using PNP transistor BC557 and a LED

    reverse the zener ..

    :wink:



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    Re: Problem with using PNP transistor BC557 and a LED

    Absolutely, IanP.

    bushygs - 'conventional' current flow is out of the base in a PNP transistor.



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    Re: Problem with using PNP transistor BC557 and a LED

    Yup, your zener is reversed :)



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    Re: Problem with using PNP transistor BC557 and a LED

    I need the LED to glow when the voltage drops below 11 volt (ie.zener 10 + base current of 1 volt), by reversing the zener the LED glows at 12+ volt and is off, it it is below 11 volt.



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    Re: Problem with using PNP transistor BC557 and a LED

    if you need that condition, use NPN or you have to use BC557?



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    Re: Problem with using PNP transistor BC557 and a LED

    I have used BC557 (PNP) only in the circuit, so as to give the opposite function of BC547 (NPN).



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