Using a LED as a capacitor in a tuned circuit

Discussion in 'General Electronics Chat' started by Shelton, May 6, 2008.

  1. Shelton

    Thread Starter Member

    Mar 13, 2008
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    Hi All

    I have made some minor changes to a circuit I found on the web. What I wanted to try was to modify the capacitance of the tuned circuit by using a red LED. Apparently in reverse biased mode, a LED exhibits a certain amount of capacitance.

    Now when I switch on the circuit, all I hear is a very faint 'rushing' noise like the circuit is not oscillating. Seems like the LED is causing an open circuit. When I short the LED legs with a wire, then I get the healthy rushing noise like its oscillating. Do you know whats wrong with this circuit.

    I have also connected a 1meg Ohm resistor b/w the top of the LED and the center leg of the 1k ohm new potentiometer.

    BTW the zener i used was 5.6v not 7.5 if that matters.
    Thanks all
    Shelton.

    [​IMG]
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Do you have a reference for the junction capacitance of the LED?
     
  3. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    Most likely not unless the OP measured it... I'd say somewhere in the 25 to 75 pF range when reversed by 5V based on my experience.

    @OP why use a LED instead of the varactor? In both the junction capacitance changes w/applied voltage. In LEDs and regular diodes this effect is not usually controlled in their mfg process so it will be non-linear and probably pretty flat. In a varactor the effect is enhanced -- so you have more range to tune with.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Just for grins, I tested a few of the LEDs I have with a capacitance meter. I was somewhat surprised to find such a wide variance. The lowest I found was 5pF for a 5mm standard red LED. Another standard red LED measured 20pF. Tried a Luxeon 4-legged red-orange critter, it was 30pF. A rectangular red LED measured 40pF, while a square red LED measured 25pF. A super-bright 10mm LED measured 15pF.

    Basically, there was no rhyme nor reason to these variances. It all would come down to what the size of the PN junction was.

    I suggest that unless you have the actual specification for your particular LED (which if it's even mentioned, it would be a range) then you would have to measure it with a capacitance meter to get the actual value.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    It is the Radio Shack Special super-regen "radio" by Patrick Cambre. (Until these modifications)
    It has only a single tuned circuit like a crystal radio.
    It doesn't have automatic-gain-control so strong local stations overload it.
    It has an AM detector which can detect FM by "slope detection" if you tune to one side of an FM station.

    Another guy has a site where he uses the documented capacitance of 2N4401 transistors as varactor diodes in his FM transmitter circuits. The capacitance of a 2N4401 is much higher than the capacitance of 2N3904 transistors or 1N4148 diodes.
     
  6. Papabravo

    Expert

    Feb 24, 2006
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    Audioguru is probably humming that rock classic form Stealers Wheel that goes

    "Clowns to the left of me, Jokers to the right..."
     
  7. Caveman

    Active Member

    Apr 15, 2008
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    Is it even a PN junction for these?
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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  9. beenthere

    Retired Moderator

    Apr 20, 2004
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    Yes, but a LED has multiple PN junctions. Bit hard to characterize. Varactor diodes are lots more predictable (as are trimmer caps, for that matter).
     
  10. Shelton

    Thread Starter Member

    Mar 13, 2008
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    OK - the only thing I can think of is that since the LED is in parallel to the cap then if the LED is displaying a very small capacitance, that will provide a very high frequency and hence the failure of the FET to oscillate. So I will try with a varactor instead - but will this work if i try a varactor instead ?

    Shelton.
     
  11. Audioguru

    New Member

    Dec 20, 2007
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    TVs and stereos have used varactor diodes for tuning for years.
     
  12. Shelton

    Thread Starter Member

    Mar 13, 2008
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    yes indeed but here in australia it is very difficult to find them.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Farnell is an electronics parts company who is international. It has offices and warehouses almost everywhere including Australia.
    I buy parts from its American office that was bought by Farnell called Newark.
     
  14. Shelton

    Thread Starter Member

    Mar 13, 2008
    19
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    Ok thanks for that audioguru - i will check them out.

    BTW- getting back to my original problem - how best to solve the problem - or should I just forget about using a LED ?

    cheers
    Shelton.
     
  15. thingmaker3

    Retired Moderator

    May 16, 2005
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    Forget about using the LED. Electrician's pliers will drive in a nail, but a hammer works much better. Why try to redesign the nail?
     
  16. Shelton

    Thread Starter Member

    Mar 13, 2008
    19
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    I bought some varactor diodes - they are of the - SCHOTTKY type - can i use these instead ? it is a BB521.

    Regards
    Shelton.
     
  17. Audioguru

    New Member

    Dec 20, 2007
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