Can a diode light up when it's connected backwards?

  1. Femme_physics

    Femme_physics 2,555
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    [​IMG]

    Trying to find the answer to the question if a regular basic LED diode can light up when it's connected backwards? I even made a lil' sketch
     
  2. jcsd
  3. The only way that LED will light up is if the battery voltage exceeds the breakdown voltage of the LED and it destroys itself in a bright flash. :)
    -
    The short answer to your question would be NO!
     
  4. cepheid

    cepheid 5,190
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    Just to complement what was already said: the whole point of a diode is that it is a device that conducts current in only one direction and not the other.
     
  5. AlephZero

    AlephZero 7,298
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    Actually the question of whether you could physically make a light emitting zener diode is interesting (and I don't know the answer).

    But conventioal diodes usually self-destruct very fast if you try to operate them in the Zener region, because the Zener voltage is high (typically hundreds or thousands of volts) so the heat generated in the diode (volts x amps) when the Zener current starts to flow is also high.

    So the answer is "no" for any "mass produced" LED.
     
  6. Well I had one that did, but I think that the manufacture connected the LED backwards when it was manufactured.
     
  7. cepheid

    cepheid 5,190
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    But this can be regarded as a set of two LEDs wired in "inverse parallel" (i.e. cathode-to-anode and anode-to-cathode) correct?

    Either way, whichever one is active is forward-biased
     
  8. Femme_physics

    Femme_physics 2,555
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    So if I have this decoder ->

    [​IMG]

    Current can only flow backwards? I thought the point of a decoder is that signals flow based on the signals defined in A0, A1, and A2. But in this case no signal can be received because the LEDS are blocking the way
     
  9. I like Serena

    I like Serena 6,194
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    Not quite.
    At the Q connections the voltage is either high or low.
    If the voltage is low, the corresponding LED lights up, with current flowing from Vcc to the low voltage Q connection.
     
  10. Femme_physics

    Femme_physics 2,555
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    Ah, so the current flows FROM Vcc, into the LEDS, to the decoder, and eventually leaves as possibly A0, A1, A2 or possibly neither depends on the combination chosen?
     
  11. I like Serena

    I like Serena 6,194
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    No. A0, A1, and A2 are inputs.
    The decoder will have separate connections to a voltage supply that is not shown in your drawing (just like an op-amp).
     
  12. If you change your dc supply to an ac one, it will ;)
     
  13. Femme_physics

    Femme_physics 2,555
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    Such as...

    [​IMG]


    ?
     
  14. I like Serena

    I like Serena 6,194
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    Yes, but the Decoder with have extra 2 connections instead of just one.
    One to Vcc, and one to -Vcc or earth.

    Btw, I have a little trouble with you naming them Vin and Vout.
     
  15. Femme_physics

    Femme_physics 2,555
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    Heh, do you? :wink:


    Well, current has to come in of somewhere and come out to somewhere, so I figured Vin and Vout are the best choices.
     
  16. I like Serena

    I like Serena 6,194
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    Yeah well, my problems are:
    1. Vin and Vout are not currents but voltages.
    2. Vin is usually the input signal (in your case you have 3 input signals: A0, A1, and A2) and not the power supply.
    3. Vout is usually the output signal (in your case you have 8 output signals: Q0, Q1, ..., Q7) and not the power supply.
     
  17. vk6kro

    vk6kro 4,059
    Science Advisor

    It is like this:

    [​IMG]
     
  18. Femme_physics

    Femme_physics 2,555
    Gold Member

    I completely understand now, thank you :)
     
  19. Femme_physics

    Femme_physics 2,555
    Gold Member

    If I have a decoder (same one from page 1) and I am told that the decoder doesn't have a need in a choice signal - CS. What does it mean?
     
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