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Basic Circuit Question

Discussion in 'Electronic Basics' started by [email protected], Oct 28, 2005.

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  1. Guest

    I am trying to debug a kit that I bought that isn't working. I am
    trying to test various links on the board with a an LED and resistor
    soldered together.

    When I hit the put the ends of the LED/resistor at a point on the board
    with a direct link to the power supply, it lights up (so the obvious
    isn't a problem).

    If I move my LED/resistor pair to the next in the chain, a diode I get:

    (-V9) ------- (- Diode +) ---- (- LED + ) --- (Resistor) --- (+9V)

    and the LED no longer lights. Should I expect it to light up,
    suggesting there is a problem with the Diode? Or is there something
    more to diode that I am unaware of (I just thought they were one-way
    current blockers -- suggesting the above should work).

    The Diode is a TN4001. Resistor is 267Ohm. Foward voltage on the LED is
  2. I agree that it should work. The diode isn't a perfect one way
    current blocker. It requires that there be a fraction of a volt used
    up across it, before conduction takes place, but a 9 volt battery
    should have plenty extra to provide this voltage and also the more
    than 3 volts needed to do the same thing for the LED, with enough left
    that the resistor has some voltage across it, to limit the current
    (via ohm's law) to about (9 - 0.6 - 3.7)/2670 = .0018 A. That almost
    2 mA should be producing a visible indication. Try turning the diode
    around and see if you are confused about which way it conducts.
    Remember that the diode will conduct when the end with the strip is
    more negative.
  3. You'd see some visible light with this chain although much less then you
    would without the diode. So I see three possibilities for the error:
    - Bad contact (soldering for instance)
    - Diode is reverse connected
    - Diode is defective.

    petrus bitbyter
  4. Guest

    would without the diode. So I see three possibilities for the error:
    Me do a bad soldering job? ....Definite possiblity.
    Definitely not the case.
    I'm going to walk over to radio shack and get a replacemnt for $0.57

    Thanks all.
  5. Rich Grise

    Rich Grise Guest

    You have one end of the diode marked "+" here. Did you know that the
    band on a diode is usually the cathode (and with the 4001, always),
    which in schematics is sometimes marked "+", which that end will be
    _inside a power supply_. Outside the power supply, the diode will
    conduct when the banded end is more negative than the blank end by
    a diode drop, about .7V.

    Hope This Helps!
  6. ehsjr

    ehsjr Guest

    Are you saying that you disconnected whatever was connected to the +
    side of the diode and then connected your LED/resistor tester?
    You said you moved the *pair*. That is a little ambiguous. I assume
    you left the resistor end of your tester connected to +9, and moved the
    LED end to the + side (non-banded end) of the diode.

    Yes, it should light up if the diagram is correct. And if
    it is correct, it does suggest a problem with the diode. It
    could also be a bad solder joint, or bad connection from the
    tester to the circuit.

    Or is there something
    How do you know the resistor is 267 ohms?
    (The implication is that you have a multi-meter.
    If you do, it makes a better teste device than a
    tester made from a resistor and a LED)

    Check the diode again. It should be a 1 N4001, not a T N4001

  7. Jasen Betts

    Jasen Betts Guest

    I'm guessing that you've got either the diode or the battery in backwatds,
    for the LED to light the banded end should be towards the negative battery

    if the diode is between the battery and the rest of the circuit to protect
    the circuit from a reversed battery then either it or the battery are

    if so it may be faulty or the faullt may lie elsewhere.

    if not it may be behaving correctly.

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