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Extremely low on resistance LDR?

  1. Feb 29, 2016 #1
    I need a variable AC attenuator and the only thing I can think of is an LDR.
    However I would need the on resistance to be very low, less than 10 ohms.

    I've only found LDRs with on resistances of 100ohms minimum.
    Is it possible to go lower than the listed minimum is I use a really really bright light?
     
  2. jcsd
  3. Feb 29, 2016 #2

    berkeman

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    Can you say more about your circuit requirements? There are other ways to make variable attenuators. What are your voltage and current requirements?
     
  4. Feb 29, 2016 #3
    Hmmm, voltage requirements will probably be very wide, from -600 to 0v maybe. Current will be 0-100ma.
     
  5. Feb 29, 2016 #4

    berkeman

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    You said "AC attenuator". What frequencies? Can you say what the application is? What range of attenuation do you need?
     
  6. Feb 29, 2016 #5
    Audio frequencies. The application is an amplifier where I need an adjustable impedance at a DC bias point. The DC adjustment will be done via a source follower but the signal needs a separately adjustable attenuation.
     
  7. Feb 29, 2016 #6
    I tried using a mosfet in LTspice in parallel with a resistor to create a variable resistance but for some reason it never reaches the source voltage when the resistance becomes zero. For example if the source voltage is 100v then at zero resistance the other size of the resistor should be 100v, but it comes out much lower. Is this a glitch in LTspice or is there some reason for this?
     
  8. Feb 29, 2016 #7

    berkeman

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    Can you post a block diagram of what you are trying to do?
     
  9. Feb 29, 2016 #8
    Screenshot_5.png
    Source follower sets the bias voltage for the load. I need to adjust the AC signal as well, hence my problem.
     
    Last edited: Feb 29, 2016
  10. Feb 29, 2016 #9

    berkeman

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    I'm not able to see the circuit parts -- they are too light for my viewer. Can you post a darker/enhanced version? Thanks.
     
  11. Feb 29, 2016 #10
    Edited.
     
  12. Feb 29, 2016 #11

    Averagesupernova

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    I am not sure what it is but I have wondered about the capacitance of LDRs. I too had the same idea long long ago about using them in pi network configuration as a variable attenuator but someone mentioned that their internal capacitance may not be suitable for this. Something to keep in mind.
     
  13. Feb 29, 2016 #12
    I just did a quick reference of an LDR datasheet, It showed a 5pf capacitance. Doesn't seem like enough to affect the frequency response, the mosfet has more gate capacitance.
    The problem is LDRs don't have a low enough on resistance.
    I'm wondering if I use an extra bright light source I can exceed the datasheet specs.
     
    Last edited: Feb 29, 2016
  14. Mar 1, 2016 #13

    Averagesupernova

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    How low do you need to go?
     
  15. Mar 1, 2016 #14
    Optimally, under 1 ohm. Basically as far low as possible.
     
  16. Mar 1, 2016 #15

    Averagesupernova

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    Care to explain a little more what you are trying to do? Maybe there is a way for the circuit to be changed? I am curious to know more about this.
     
  17. Mar 1, 2016 #16
    Um, I would love to give you a schematic but this is for a product I've spent countless hours on developing so I'd prefer not to.
    I don't think there is another way though, it's a difficult hurdle to get around.
     
  18. Mar 1, 2016 #17

    davenn

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    ohhh dear, how often we hear that cry :rolleyes::rolleyes:

    it's hard for people to help you if you wont let them help by sharing the appropriate info


    D
     
  19. Mar 1, 2016 #18
    I'm sure :-p
     
  20. Mar 1, 2016 #19

    Averagesupernova

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    Well that is one of the pains of inventing. Often times there is a case where many people know that 'if it weren't for THIS, then a whole new process for THAT could be implemented'. It is the one guy who overcomes the THIS who stands to gain. So tell me, do you think someone will solve a problem like THIS on an internet forum? Suspecting you are up against such a thing why would anyone even choose to? I recall a number of years ago I was in your situation and asked a question on this forum about whether a specific part exists off the shelf. I got lots of questions and suggestions. I was not in a position to reveal any more information so I just had to live with doing it the hard way and take a chance of reinventing the wheel so to speak. Lesson learned.
     
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