Power from LED bulb in sunlight

  1. My son is working on his science fair project. I had told him that LED bulbs produce voltage in sunlight and he's been exploring that and is trying to see if he can charge a cell phone using LED bulbs under a light source.

    Using dozens of small LEDs, he is producing about 5v under a light bulb. He's adapted a usb cord to connect to the LED sources and connected it to his phone but it's not charging and doesn't seem to be producing any current.

    I'd like to give him some help, but not sure how. Is it possible to use LEDs in this way to power a cell phone? Or is the current so insignificant as to not be a viable solution?

    Thanks for any help!
     
  2. jcsd
  3. phinds

    phinds 8,300
    Gold Member

    Hm ... I didn't even realize that LEDs could be used backwards like that. They are, after all, Light EMITTING Diodes, not solar cells. They may produce voltage but probably miniscule current, so as soon as there's a load, the voltages goes way down.

    Have him measure the output voltage with successively lower-ohm resistors and see if I'm right.
     
  4. LEDs are somewhat poor photodiodes. The power extracted from a light bulb is probably miniscule.
     
  5. Bobbywhy

    Bobbywhy 1,908
    Gold Member

    Certain LEDs do respond to a light input. A small signal is normally amplified by a transimpedance amplifier to be usable. But the “short circuit current” output (the ultimate measure of power output) would be miniscule. That means LEDs will not generate any significant amount of power, and cannot be used as a photovoltaic (PV) power source to charge a cell phone battery. PV cells are used for this function.

    Here are two papers that describe the behavior of LEDs detecting light:

    Very Low-Cost Sensing and Communication
    Using Bidirectional LEDs
    Dietz, P.H.; Yerazunis, W.S.; Leigh, D.L.,
    Abstract:
    “A novel microprocessor interface circuit is described which can alternately emit and detect light using only an LED, two digital I/O pins and a single current limiting resistor. This technique is first applied to create a smart illumination system that uses a single LED as both light source and sensor.”
    http://www.merl.com/papers/docs/TR2003-35.pdf

    "Using a light-emitting diode as a high-speed, wavelength selective photodetector"
    Eiichi Miyazaki1, Shin Itami2 and Tsutomu Araki3
    Rent this article for $4.00 USD Buy: $28.00
    Abstract:
    “A light-emitting diode(LED) can function as a wavelength selective photodetector. To evaluate the potential for a LED-based photodetector, we have investigated the stationary and temporal characteristics of two kinds of LEDs: a Zn-doped InGaN blue LED and a GaAlAs red LED. The application of a high current produced two peaks on the emission spectra of the blue LED, at 380 and 450 nm. The extinction profile of the blue LED was consistent with its UV-emission profile. The red LED showed an emission peak at 660 nm and an extinction peak at 620 nm. The LED-based photodetector responded within nanoseconds of the onset of the light impulse. The application of a reverse bias to the LED caused the time spread of the output current wave form to decrease dramatically and was accompanied by an increase in peak height. At a 75 V reverse bias, the resultant pulse widths were 2.6 ns in the blue LED and 7.4 ns in red LED.”
    http://scitation.aip.org/content/aip/journal/rsi/69/11/10.1063/1.1149174
     
  6. I'm afraid so.

    Mobile phone is likely to need say 300mA at 5V which 1.5W. In practice you probably need a panel capable of twice that, say 3W to allow for less than full sun. Take a look at the size of solar panel you need for that..

    http://www.portablepowersupplies.co.uk/portapow-3w-solar-usb-charger/

    Compare the area with the area of the LED die. Not only are the LED die a lot smaller (captures less sunlight) but they aren't designed to generate electricity so the efficiency will be very poor.
     
  7. PS One you tube vid suggests that 4 red LEDs might produce 18uA at 1V so to produce 300mA at 5V you would need ..

    5 * 300*10^-3/18*10-6 = 83,000 LEDs !

    Your mileage may vary :-)

    You might be able to make an LED powered light :-) Some low power LEDs need only 0.5mA to light up which might be achievable with say 200 LEDs used as solar cells (if the youtube data is correct). Perhaps only 50-100 LEDs if to do away with the current limiting resistor normally used.
     
    Last edited: Oct 31, 2013
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