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    Tuning capacitor for LC resonant circuit

    Hi I would like to design an automated tuning capacitor controlled by a microcontroller. This tuning capacitor should have a high quality factor and accept high voltage (>500V).
    Here I explain why I need it. I design many different LC resonant circuits and each time I have to tune the capacitor in order to set the resonant frequency. For tuning this capacitor I solder/desolder different SMD capacitors by hand and its annoying when you have many of these. So I was wondering if I could do this in a more automated way.
    So I was thinking about using an array of capacitors and select turn on/off the needed ones with an array of switches/relays/or others. Something that would look like on the scheme bellow.
    Click image for larger version. 

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    Now I'm not sure of what type of switch I should use. As I said previously the quality factor has to be high, so I can't use switches with high serial resistance. Moreover, I would like a solution that is small and compact, so I can't use those big relays that are 1cm thick.
    So what do you think, what sort of switch should I use and do you know any better solution?

    •   Alt14th September 2017, 10:43

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    Re: Tuning capacitor for LC resonant circuit

    A useful specification would include relay voltage and current rating, acceptable series resistance and inductance, signal frequency.



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    Re: Tuning capacitor for LC resonant circuit

    ...500V SMD capacitors and soldering them by hand every time

    If the capacitor values are reasonably low (< 1000pf) I would think of an air-spaced variable capacitor and motor drive.

    Brian.
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    •   Alt14th September 2017, 11:35

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    Re: Tuning capacitor for LC resonant circuit

    @betwixt
    Yes completely agree with your point. However there's still some advantages by using SMD caps, you have a well fixed capacitor and the size is small.
    The capacitor is reasonably low ~100pf. I've already thought about using air capacitor+motor but didn't like this solution because it takes a lot of place and all the metal is disturbing the magnetic field of LC resonator. Moreover air capacitor vendors rarely give you any technical data about the qualité of such capacitors and I have no clue of their behaviour at different frequencies.

    @FvM
    Hard to give you any rating because it's an LC oscillator but I inject 5Watts in it and current can go up to 1 Amp, voltage up to 300V
    For signal frequency or commutation speed, its not a problem as long as it do the job.



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    Re: Tuning capacitor for LC resonant circuit

    These guys are manufacturing highly quality trimmer capacitors.If you connect it a fixed one in parallel, you'll have a freedom to adjust.
    Their trimmer caps have been tested 500 VDC ( there might be some custom designs, ask ..)
    I used a model long time ago, it was brilliant and smooth.Sure expensive..
    http://www.johansonmfg.com/index.php


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    Re: Tuning capacitor for LC resonant circuit

    Quote Originally Posted by SK245230 View Post
    Now I'm not sure of what type of switch I should use...
    Have you considered reed relays? they reasonably small and have low series resistance. The coils can be driven by 5V. Yesteryears, they were widely used in telephone exchanges.



    •   Alt18th September 2017, 03:31

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    Re: Tuning capacitor for LC resonant circuit

    Certainly a combination of reed switches to parallel several capacitors would give optimal value range in a given space but I wonder if they can withstand 300V / 1A or introduce small enough residual capacitance if the required value is only ~100pF.

    Brian.
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    Re: Tuning capacitor for LC resonant circuit

    1000V/3A rating (carrying capacity, not switching power) is well feasible for standard SIL reed relays, contact-to-contact capacitance < 1 pF. Depending on the operation frequency which wasn't mentioned yet, series inductance may matter.



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