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    Voltage drop with transistor Base Resistor

    Hi expert,

    Info:
    The pulse signal is 12V to 0V (normally condition it's 12V and whenever there's pulse it'll be 0V for 50 milliseconds). I try to read a pulse based signal (which you can consider it V4 and V5 as per image in attachment) from external circuit into my microcontroller through NPN transistor (which act like a level shifter).

    For the circuit which base resistor is 330 ohm, I discover pulse voltage drop to 0.6V. However when I change to base resistor to 470k ohm, pulse voltage is at 8V which allow me to read the pulse signal at transistor output.


    Can you provide me any explanation why such behavior occur? The cause of voltage drop perhaps? Because as far I know base resistor is fine to be small value, as long as it does not exceed maximum rating of base current. Please share me your thought about it.

    Click image for larger version. 

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  2. #2
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    Re: Voltage drop with transistor Base Resistor

    Hi Hanzhong,
    Check the load driving capability of your external circuit.
    With 330ohms, you are loading your extranl circuit @ 34.24mA ((12V-0.7V)/330) ... where as with 470kohm it is only 23.04uA ..

    Regards,
    Naga.



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  3. #3
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    Re: Voltage drop with transistor Base Resistor

    Hi Naga,
    Thanks for the reply. Yes, you mention I think that it's probably the reason as well. And I just did some simulations, it seems that all along I had misunderstood that transistor were turned on due to current. But it's actually voltage level as long it's more than 0.7V. Just that low base current will result in low collector current and if collector current is low, it might not able to drive the load. I appreciate your thought. Thanks again.



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