1. Tesla00010010

    Thread Starter Member

    Jun 20, 2008
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    good day to everyone,i have a couple of questions about diodes.My first question is,¿is it possioble to measure the resistance of a LED?I understand they can have different voltage drops between 2 or 3 V and currents of 20 to 40 ma.So,there is no way i can measure that resistance because the current they receive will depend on the other components of the circuit,or am i wrong?I tried to use ohm s'law dividing LED s' voltage drop by the current in the circuit,but it gave me different values several times.What i udnerstand is that as the diode isn't just an electric resistance or a capacitor,u can't talk about a LEDs' resistance.And i don't see in the media people talking about diodes ohmic resistance,but the diode should have a resistance value,isn't it?

    Another question i have is that the diode is supposed to conduct electricity when the voltage supplied to it is more than 0.6V for the pn junction barrier to be surpassed and electrons to flow.I thought in using this to make a sort of diode-capacitor circuit using capacitor charging as time-voltage controls for diode to act as switches.Especifically,i wanted to make LEDs to turn on and off in time using DC circuit.I wasn't sure if that was possible to do at all,but my surprise was that besides a capacitor being charged the didoe stills keeps its voltage drop acroos its terminals.I tried to increase resistance in the circuit and i measured 0,4V across the diode,and it still conducted,why?IF somebody could help me with this it would be greatly appreaciated

    Thanks!
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    LEDs are not regular diodes. It's obvious what characteristics have been enhanced, but the structure differences inside an LED is not obvious. There have been several generations in the development of LEDs, and they are all a bit different from each other. From a technicians point of view this is reflected in the amount of voltage they drop, the early generations dropped around 1.5, while the later drop more (2.5V for red, 3.1 for green and yellow, a bit more for blue). The later models are more ESD sensitive too. I've never figured the internal resistance, I just go by the recommened current given with them.

    Even later models, ones I've never gotten my hands on due to prices are incrediably bright, can take a LOT more current, up to an amp isn't unheard of.
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    1,438
    368
    You could say that for any operating conditions a particular LED has a resistance of V/I (typically around 100 Ohm perhaps) but it would be a fairly useless value since, as you say, the calculated value will rarely be the same twice and will differ for every LED.

    If you post a schematic of your circuit it will be easier to understand what you are trying to do.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you have a LED in a series circuit with a stiff power supply and a fixed resistor limiting current, then you can meter the voltage drop across the fixed resistor to determine changes in the LED's internal resistance. Vary the supply voltage to change overall current. Use the fixed resistor to determine current in the series circuit. If it varies from the prediction, then the LED's internal resistance has changed.
     
  5. Tesla00010010

    Thread Starter Member

    Jun 20, 2008
    21
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    Thanks for replying
    What i was trying to do was to use the capacitor charging and discharging to control the diodes(the normal diodes)so they could conduct or no conduct,but my surprise was that even when they have less than 0.7 or 0.6V(the supposed minimum voltage they need to drop for them to conduct)they still conducted and closed the circuit.Do i have a concept error or what am i doing wrong?is not possible to use a diode as a switch in DC?The same question for tjhe transistor.How can i use it as an self-controlled switch?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    We're going to need a schematic to understand how you're using these diodes.

    If I understand your "self-controlled switch", you would want to look into a transistor like a 2N2646. That is a special kind of transistor called a unijunction.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    A datasheet for a diode and for a transistor has a graph that shows the typical voltage drop across a pn junction that changes with its current.
    Therefore since you are changing the voltage then the current must also be changing.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    As Beenthere suggests, the 2N2646 UJT can be used for a simple LED flasher circuit. Unfortunately, UJT's are obsolete, but you can still find them occasionally.

    They are unusual because they have two bases and one emitter.

    Have a look at the attached schematic.
     
  9. Tesla00010010

    Thread Starter Member

    Jun 20, 2008
    21
    0
    Thanks!I didn't know about that transistor,i have more idea now.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    I am pretty sure I came across 2N2646's in a catalog, but can't recall which one now.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    With something like 9500 pages of catalogs, I have to use the little Post-It stickers to find sections of interest.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Tell me about it!
    Although, I'm getting away from the printed catalogs; they simply take up too much space.

    I have far too many reference manuals and books that I'd rather have handy. I do have Mouser and Newark printed catalogs from last year, but I find it much faster to use their (and many other vendors) websites to locate what I need.
     
    Last edited: Jun 21, 2008
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