Question about diodes

Discussion in 'General Electronics Chat' started by coling223, Mar 27, 2008.

  1. coling223

    Thread Starter New Member

    Mar 27, 2008
    5
    0
    on my truck i put LED bulbs for the taillights and i seem to need a diode for the marker light to function properly, i need a diode with little to no resistance because if the diode has much resistance the LED's get dim
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    A readily available diode, like a 1N4001, takes about .7 volts to bias into conduction, but no resistance as it conducts.
     
  3. Xray

    Well-Known Member

    Nov 21, 2004
    58
    1
    First off, there is no such thing as a diode with no resistance. All electrical conductors and semiconductors have some resistance unless they are classed as superconductors.

    I am trying to imagine the circuit in my mind for what you are trying to accomplish, but I can not. So, it would be helpful to everyone if you could provide a simple wiring diagram of your LED's and marker light.

    Thanks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you want a low Vf diode, you could look at a 1N5817 (20piv) or 1N5819 (40piv) Schottky rectifiers.
    1N5817 at 0.1A, Vf = 0.32V
    1N5817 at 1.0A, Vf = 0.45V
    1N5819 at 0.1A, Vf = 0.34V
    1N5819 at 1.0A, Vf = 0.6V

    They look pretty much like a regular diode, but have about 1/2 the Vf (forward voltage drop). The drawbacks are that they don't have a very high PIV, and you will probably have to order them from someplace. I've seen them selling on E-bay pretty cheaply, but then you have the shipping charges.

    You could go to your local Radio Shack and pick up an assortment of 1N400x diodes for a couple bucks. Use the 1N4001's, because they will have the lowest Vf.
     
  5. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    145
    Although factually correct, after the cut-in voltage then the resistance of the diode is negligibly small that it can for all intents purposes can be considered zero. Look at the reciprocal of the derivative of Ebers Moll for a typical diode - you get milli-micro-ohm resistances.

    Dave
     
  6. coling223

    Thread Starter New Member

    Mar 27, 2008
    5
    0
    ok, while i'm looking for a wiring diagram, i will try my best to explain the setup to you... as i spent 2 hours yesterday tinkering with it. the stop/tail bulb was a dual filament bulb, before i converted it to LED's (2 tips on the bottom, and the side of the base was gnd.)
    now it is still a 2 tip bulb, but when i connect it to a 12v batt to test it, gnd to base touch the + to one of the 2 tips i would get bright or dim, brake and tail accordingly. so, it should work... right? well upon initial install the brake worked fine, but the tails did not turn on dim... i somehow came upon the solution... cut the wire to the brake light and the tail light works fine, but then (obviously) the brake light doesn't work. it's like the brake light wire is shorting the tail light function out... so my solution with my small knowledge of electronics and such, i think that putting a diode in the brake wire would solve the problem... with the voltage only allowed to flow toward the light, making the wire "non existent" when the tails are on... still don't know why this happens... just know it does...
     
  7. coling223

    Thread Starter New Member

    Mar 27, 2008
    5
    0
    ok i have a scan of the wiring diagram... how do i post it here?
     
  8. coling223

    Thread Starter New Member

    Mar 27, 2008
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  9. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    I think the voltage drop across the rectifier diode won't be a problem. It may vary from 0.6V to 1.2V, according to the type of diode and the forward current in use.

    By the way, what is the battery voltage at its peak?
     
  10. coling223

    Thread Starter New Member

    Mar 27, 2008
    5
    0
    battery voltage peaks at about 13.6v
     
  11. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    are you using a common resistor for both the leds? if yes, thats why the one led dims when you try to light both the same time. Put a resistor for each led and it will be ok.
     
  12. tronics

    Member

    Apr 16, 2008
    14
    0
    yeah use resistors, not sure why your diodes to current limit you light emmiting diodes(leds),get the forward volt drop of yuor leds, minus it from you input voltage,divide the rest over a the current needed and theres your resitor
     
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