# led's

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

1. ### ZmanGuest

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. ### John PopelishGuest

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. ### News.ca.inter.netGuest

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
batteries.