What Causes a Voltage in a Circuit with an LED and Filament Lamp?

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Discussion Overview

The discussion centers around the phenomenon of generating voltage in a circuit containing an LED and a filament lamp. Participants explore the underlying mechanisms, particularly whether this effect is related to the photo-electric effect, and the conditions under which light can produce current in an LED.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions why illuminating an LED with a filament lamp produces voltage, suggesting it may relate to the photo-electric effect.
  • Another participant explains that shining light on an LED can indeed produce current, as the LED functions as a p-n junction where light can free electrons in the depletion region.
  • A subsequent reply seeks clarification on whether white light will definitely free electrons from stable atoms in the depletion region of the LED.
  • Further, it is noted that the frequency sensitivity of an LED is complex, but a good thermal white light source should provide sufficient energy across frequencies to stimulate measurable current in the LED.
  • One participant expresses gratitude for the clarification regarding the energy of visible and infrared light in relation to freeing electrons.

Areas of Agreement / Disagreement

Participants generally agree on the connection between light and current generation in LEDs, particularly through the photo-electric effect, but there remains some uncertainty regarding the specifics of how different light frequencies interact with the LED.

Contextual Notes

The discussion includes assumptions about the behavior of light and electrons in the context of LEDs, and the complexity of frequency sensitivity is acknowledged without resolution of specific conditions or definitions.

Who May Find This Useful

This discussion may be useful for individuals interested in electronics, particularly those exploring the interaction between light and semiconductor devices like LEDs.

Mrs Moggins
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Does anybody know why illuminating an LED with a filament lamp would produce a voltage in a simple circuit with just an LED and voltmeter? I'm guessing it's related to the photo-electric effect, but I have no idea what is actually going on.

If any of you guys could help me out, or point me in the right direction, it would be a great help!

Thank you in advance!
 
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It would only make sense -- if passing current through an LED produces light, why wouldn't shining light on an LED produce current?

It is, in fact, the photo-electric effect. An LED is basically just a p-n junction, comprised of two kinds of semiconductors joined together. The region around the junction is normally depleted of charge carriers, because free electrons on the n-side of the junction want to rush over into the electron-starved p-side.

When you shine light on the depletion region, the photo-electric effect kicks electrons out of atoms. These free electrons then immediately slide down the potential hill toward the p-side of the device, and a current flows.

- Warren
 
Yeh, that seems to make sense. It was the point about the photoelectric effect freeing electrons in the depletion region that I was originally unsure about. Will white light incident on the depletion region definitely free electrons from these otherwise stable atoms?
 
The frequency-sensitivity of an LED can be pretty complicated. A good thermal white light source should have enough light of every frequency to stimulate any LED to produce a measurable current.

- Warren
 
Ah ok, so visible or even infrared light has enough energy to free these electrons. Thank you very much for your help, it really is appreciated!
 

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