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Discussion in 'Electronic Basics' started by Zman, Dec 19, 2004.

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  1. Zman

    Zman Guest

    Hey there

    when placed in the sun, a small led gives off a reading on my voltmeter,
    goes up and down. The only thing is, how do i know what reverse
    current/voltage im going to get just by looking at the information? is there
    a formula to use and does mCd make a diff in voltage or current when giving
    power out?

    the specs are

    15VDC 5mm LED
    (all typical values)
    luminous intensity 120
    Peak emission wavelength 573
    spectral half width 15
    flash rate (freq) 2.4
    operating voltage 5
    forward voltage 5
    reverse voltage .04

    thank you
  2. You have more than an LED. It is an LED and a silicon flasher and
    current control integrated circuit. So any readings you get out of it
    are going to be difficult to understand.

    Any plain LED will generate some voltage (open circuit) and drive some
    current when shorted. There will be some value of output voltage
    between zero and open circuit voltage that allows it to provide
    maximum power (voltage times current). I expect that bluer LEDs will
    produce more open circuit voltage because those shorter wavelength
    photons have more energy, each. The current depends on the die size
    and how efficiently the lens reflector the die is mounted on focus
    light on the die.
  3. Guest

    If you really want to power something, you're better off getting a
    solar cell (available at Radio Shack?) or even a reasonably large
    photodiode. LED's have a very small area, hence the amount of solar
    power collected (proportional to this area) is small. LED's just were
    not designed to be used as a power source.

    In any case, you may want to measure an i-v curve (current vs.
    voltage), either for the LED or a solar cell. To do that, you'll need
    to connect different resistance values across the output terminals, and
    measure the voltage. Then calculate the current for each reading from

    current = voltage / resistance

    What you have done so far is to measure one point on the i-v curve for
    your LED, using a very large resistance, the resistance of your meter
    (perhaps 10 M-ohm?) This reading is the open-circuit voltage.

    Make sure that each point is measured in full sunlight, preferably with
    the device oriented toward the sun for a maximum reading (you can just
    hold the device in your hand, and orient it until the meter reading is
    maximized). At any rate, the device should be kept in a fixed
    orientation for all readings. If some clouds cover the sun during the
    measurements, wait until they have passed by before continuing.

    I would guess that using resistances between a few ohms and 100k to 1M
    would provide a sufficiently complete plot.

    You are correct that changing the amount of light hitting the device
    (LED or solar cel) will affect the amount of current produced. Hence
    the need to do a series of measurements all with the same amount of
    solar power.
  4. LED's were not designed to work as a solar cell. But, because of the way
    that they are structured, they will give off some voltage at a few uA
    (microamps), when the sun is shining directly on them. You can do some tests
    to see how much current and voltage they put out, and then make an array of
    them in a parallel and series combination to give you a useable voltage and
    current output.

    It would be much more feasable and practical to get the proper solar cells,
    and make a solar panel. This would involved mounting them on a panel of some
    type, and properly wiring them up.

    You can purchase a solar panel ready made that is voltage regulated, and
    having a fuse protection wired in. It can be used to run some devices for
    you. This would be most feasable in the end in comparison to something you
    would put together yourself. I have seen these solar panels sold at places
    that sell utilities for mobile homes, boats, and some large hardware
    centers. They are standardly built to put out 12 Volts at several Amps for
    running various small utility devices, or for charging auto and boat
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