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Voltage-controlled switch

Discussion in 'LEDs and Optoelectronics' started by Richard Prosser, May 28, 2014.

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  1. Richard Prosser

    Richard Prosser

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    May 28, 2014
    I have a requirement to put a solid-state switch of some kind in a non-conducting state when its input control voltage is zero - i.e. when a capacitor is fully depleted. A relay may suffice but I would prefer a transistor, mostly for the switching speed.

    I know that such devices exist but so far I have been unable to find one that operates at a threshold of exactly zero volts; most seem to require 0.7v or thereabouts.

    So does anyone know of such a device please, or alternatively can recommend a circuit design that would turn a switch off once a capacitor is fully discharged? Also the switch itself would need to handle large voltage drops, around 20v or so.

    Please bear in mind that my electronics knowledge is very "rusty". I haven't touched the subject since I was at school, over forty years ago!


    Thanks ...

    Richard
    PS I also live in the UK, so American terminology may differ from that here.
    PPS I searched these forums beforehand but found little that was directly relevant.
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    Hi Richard and welcome to Electronics Point :)

    Based on prior experience, I think you will get the most useful responses the most quickly if you give us a lot more information about your project. Your minimalist description leaves a lot of unknowns that we can only guess at, and this tends to waste time as we suggest possibilities that won't work, and overlook possibly simpler solutions that would.

    To start with, explain the overall purpose of your project. Then tell us about the capacitor you want to monitor, what range of voltage it will have across it, what power sources are available, and what you intend to switch with the relay or transistor.

    Don't leave anything out if it could be relevant. This will probably save a lot of back-and-forth and you will get a proper answer sooner.
     
  3. duke37

    duke37

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    Jan 9, 2011
    What is zero? Do you mean less than 1V or less than 0.0001V? A perfect capacitor loaded with a resistor will in theory never get to zero volts.
     
  4. Richard Prosser

    Richard Prosser

    2
    0
    May 28, 2014

    Sorry but this may be commercially-sensitive information and I tried to provide sufficient details.

    All I can add I believe is that the power source will be a battery, probably 9V but it could be less perhaps; that will charge up the capacitor initially but will then be disconnected (in the prototype) and instead the capacitor will be connected to the rest of the circuit, to be discharged.

    So I would be grateful if anyone could point me towards an electronic switch that will be set 'off' with zero input volts (across the capacitor).

    If I have still not provided enough information then I may have to try another tack.


    Thanks ...

    Richard
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    You have failed.
    Yes. If you're developing a product for profit, and you require secrecy, I suggest you find someone who will sign an NDA, and pay them for their advice, rather than asking on a public forum.
     
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