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  1. #1
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    Varicap driving without current limiting resistor.

    Hi,
    In my received I want to minimize the components count.
    I wonder if I could drive the varicap without the 100k current limiting resistor?

    At the extreme potentiometer end, the varicap will be connected directly to the ground or the VCC.
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!

    •   Alt18th January 2018, 01:34

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    Re: Varicap driving without current limiting resistor.

    That's not for current limiting - it's to keep the variable
    voltage tap from killing Q. Higher R is better. Lower and
    you will **** out all the energy from the crystal tank.

    **** = s u c k, a perfectly valid and appropriate verb.



    •   Alt18th January 2018, 05:50

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    Re: Varicap driving without current limiting resistor.

    Quote Originally Posted by dick_freebird View Post
    That's not for current limiting - it's to keep the variable
    voltage tap from killing Q. Higher R is better. Lower and
    you will **** out all the energy from the crystal tank.

    **** = s u c k, a perfectly valid and appropriate verb.
    I've seen some use even 1M for this purpose, would the varicap still be driven with 1M, or will the current to it be too low?
    Professional engineering is the top, but amateur engineering is more fun.
    It is when you cross the barrier between these two, that things become really fascinating!



    •   Alt18th January 2018, 09:12

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    Re: Varicap driving without current limiting resistor.

    Varicap diodes are run in reverse voltage condition. The voltage you apply 'opens' ('widens') the barrier across the PN junction to decrease it's capacitance so no DC current flows. As there is no current, except maybe a very, very small leakage, the resistor value doesn't matter within sensible range.

    If you look at your schematic with the resistor linked out, you will see the problem when the potentiometer is at each end of it's track - you short out the signal at the crystal. Theoretically, you could replace it with a choke but introducing even more reactive components will probably have adverse effects.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
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