Science Buddies: "Ask an Expert"

Posted: **Mon Jan 23, 2012 6:46 pm**

Hi,

I have very limited knowledge of electricity but I am attempting to help my 8 yr old with a solar project. She is trying to light an LED with a solar panel. She has trouble shot the problem but has not found the answer. Below is an outline of method and materials:

Materials:

LED from a string of Christmas tree lights.
2 volt solar panel
capacitor
alligator clip wires
75 watt light bulb/fixture

Problem:

She hooked up the positive wire from the solar panel to the Positive wire from the LED and the Negative to the Negative. The LED did not light. (She tried reversing polarity and it did not work either).

She then tried hooking the LED to a battery (with a resistor btwn the Positive battery lead and the positive LED lead). The LED worked. (She tried the solar panel with the resister and it did not work).

She then tested the the output of the solar panel (w/LED btwn the meter probes) and it measured 2.3 volts. The battery circuit measured 2.7 volts.

Then she tried adding a capacitor to the solar configuration. The LED still did not light.

I am out of suggestions for her to try. Any help would be very much appreciated.

Thank You

Posted: **Mon Jan 23, 2012 10:15 pm**

bobbickle - You might be right on the threshold of the LED to conduct forward current. Different LED colors (and technologies) require different forward voltage potential (Vf) to conduct forward current. Infrared LED's are usually the lowest Vf devices and will conduct forward current (and therefore emit IR radiation at just over 1 volt). Red LED's are generally the next lowest voltage devices to turn on going up through green, blue and white LED's which require the highest Vf, sometimes over 3 volts. If you have access to a multimeter, check the current flow on the 20 or 200 mA scale. LED's tend to operate at high light output at 20 to 30 mA forward current. They might be just visible at currents as low as a few mA in a darkened ambient room. If you have access to a red LED from your string, try that. It sounds like you are using the 75 watt bulb to illuminate the solar panel. You also might try bright sunlight to see if that raises your output voltage to verify your LED action. Measure the voltage across the LED to understand what the threshold voltage is. You may eventually need a series resistor to limit the current to 20-30 mA, but the use of a capacitor in this circuit will not be helpful.

Rick Marz

Posted: **Mon Jan 30, 2012 9:07 am**

Thank you Rick.

We followed your advice and confirmed the power was below the threshold for the light.

Thanks again.

Posted: **Tue Apr 17, 2012 1:41 am**

rmarz wrote:bobbickle - You might be right on the threshold of the LED to conduct forward current. Different LED colors (and technologies) require different forward voltage potential (Vf) to conduct forward current. smd-5050 Infrared LED's are usually the lowest Vf devices and will conduct forward current (and therefore emit IR radiation at just over 1 volt). Red LED's are generally the next lowest voltage devices to turn on going up through green, blue and white LED's which require 12 volt led lights the highest Vf, sometimes over 3 volts. If you have access to a multimeter, check the current flow on the 20 or 200 mA scale. LED's tend to operate at high light output at 20 to 30 mA forward current. They might be just visible at currents as low as a few mA in a darkened ambient flexible led strip
room. If you have access to a red LED from your string, try that. It sounds like you are using the 75 watt bulb to illuminate the solar panel. You also might try bright sunlight to see if that raises your output voltage to verify your LED action. Measure the voltage across the LED to understand what the threshold voltage is. You may eventually need a series resistor to limit the current to 20-30 mA, but the use of a capacitor in this circuit will not be helpful.

Rick Marz

Hardwire LED light strip into car?
I bought LED light strips for the exterior of my car. I can get them to work for a bit when i splice them into a power adapter cord that plugs into the cigarette lighter but it keeps blowing it or something cause the adapters wont work anymore. Im supposed to just hardwire them into the power source of my car and dont know how. help please.

Science Buddies: "Ask an Expert"

Light an LED with a 2 volt solar panel

Light an LED with a 2 volt solar panel

Re: Light an LED with a 2 volt solar panel

Re: Light an LED with a 2 volt solar panel

Re: Light an LED with a 2 volt solar panel