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What is the resistance of a small LED bulb?

Discussion in 'Electronic Basics' started by zalzon, Nov 16, 2003.

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  1. zalzon

    zalzon Guest

    Hi,
    newbie question here that's got me confused. I'm trying to figure
    out the resistance of an LED in this circuit.

    I have a series circuit with a 1000 ohm resistor, a 3mm LED red bulb
    and a 6200 ohm resistor in series. The input voltage is 5.02 volts
    and the input current is 0.46 mA. Using this info, I have calculated
    the LED resistance to be 3713 ohms.

    Is it possible that the LED has such a high resistance?

    On my multimeter, I read 0.46 volts across the 1000 ohm resistor and
    2.91 volts across the 6200 ohm resistor. So i guess it matches up
    with the calculated values approximately.

    I can't seem to get the resistance of the LED using my multimeter for
    some reason.
     
  2. At that current, sure. But for devices that do not hold a fixed
    ratio of voltage to current, called non-ohmic, this resistance has
    little meaning.

    If you change the current through the LED, the resistance (as the
    ratio of voltage divided by current) will be less.

    LEDs drop a voltage that is proportional to the log of the current,
    plus a small drop proportional to the current (an ideal diode in
    series with a resistance).
    That is because the meter probably biases the LED with only a tenth of
    a volt or so, and at that voltage, the current is unmeasurably small,
    so the resistance reads open circuit.
     
  3. zalzon

    zalzon Guest

    Woah. i never knew that. So LEDs have variable resistance based on
    current and voltage. So when designing a circuit, we have to take
    into account the resistance of the LED.
    When I set the multimeter to 200 Ohms resistance and touch it to the
    leads of the LED, it lights the LED up. Is that not enough current to
    make a resistance reading? All it shows is 1 which to me means to set
    the resistance higher.

    When I switch to 2000 Ohms (the next resistance level on the
    multimeter), the LED light is no longer lit. Probably as you said the
    current is too small. The reading still shows 1.

    I keep on going up the scale all the way to 2000K and all I see is 1
    and no light on the LED.
     
  4. A better way to say it is that you have to take the V versus I curve
    into account. For currents within a decade of rated current, the
    voltage varies less than a tenth of a volt. So if the data sheet says
    that the typical forward drop is 2.1 volts with 20 ma, that voltage is
    a good guess for the forward drop for all currents down to about 2 ma.
    If it lights up, it should give a resistance reading, but since you
    don't know the current, that resistance is not much use to you. I
    don't have an explanation for your reading.

    Many meters have a junction test range (labeled with a diode symbol)
    that applies about 3 volts open circuit with a few ma current limit
    (there is an internal resistance). That range displays the forward
    drop at that small (but unspecified) current. But since the voltage
    of diodes does not vary much over a range of forward bias, this
    reading is more helpful when figuring out what supply voltage will be
    left over for the dropping resistor to consume as it sets the LED
    current.
    Higher resistance ranges bias the device under test with smaller
    currents.
     
  5. That's because the current thru the LED is non-linear with respect to
    voltage. The LED won't conduct at voltages much below 2V, and when
    the voltage gets to 2V, it starts conducting heavily, so that it would
    burn out if the current was not limited. This is the way any diode
    acts.

    If you measure the voltage and current at many points, you will find
    that the LED does not have a straight line V-I plot, like a normal
    resistor. So you can't say it has a certain resistance at any point
    other than at that point where the measurement was taken.

    --
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  6. Well, if the digital voltmeter has a maximum range of 1.999V, and it's
    putting enough current thru the LED to cause it to drop 2.1V, then the
    meter is going to indicate that it's out of range, or a flashing 1.
    --
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    ###Got a Question about ELECTRONICS? Check HERE First:###
    http://users.pandora.be/educypedia/electronics/databank.htm
    My email address is whitelisted. *All* email sent to it
    goes directly to the trash unless you add NOSPAM in the
    Subject: line with other stuff. alondra101 <at> hotmail.com
    Don't be ripped off by the big book dealers. Go to the URL
    that will give you a choice and save you money(up to half).
    http://www.everybookstore.com You'll be glad you did!
    Just when you thought you had all this figured out, the gov't
    changed it: http://physics.nist.gov/cuu/Units/binary.html
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  7. dB

    dB Guest

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