LED as a Voltage Source: Debunking Myths and Reality
When discussing whether an LED can be used as a voltage source, the response is often divided into two camps: those who mistakenly believe LEDs can act as voltage sources and those who understand the limitations and principles behind this concept.
Myths and Misunderstandings
Myth: An LED can be used as a voltage source because it generates voltage.
While it is true that LEDs can generate voltage due to their diode properties, the idea that they can be used as primary voltage sources is a common misconception. The term "voltage source" implies a device that can provide a consistent and stable voltage output, which is not the case with LEDs. LEDs are not designed to generate voltage perpetually.
Practical Applications and Realities
Despite the misconception, LEDs can be used in certain applications to generate very small amounts of voltage in short pulses. In these contexts, the term "voltage source" is more accurately described as "voltage producer". Let's explore some practical examples:
Light-Sensitive Voltage Producers
One fascinating application involves using LEDs as voltage sources in light-sensitive projects. For example, in a sun tracker, two LEDs are positioned on either side of a partition. The light exposure on each LED creates a difference in voltage. When one LED generates more voltage under the sun's light, this signal can trigger a transistor that powers a motor to adjust the tracker's orientation towards the sun. Another example includes using metal-cased transistors with their metal cases removed to act as voltage sources in certain scenarios.
The Science Behind Light-Dependent Voltage
The principle behind the voltage generation in LEDs is rooted in their diode structure. In a p-type semiconductor brought into contact with an n-type semiconductor, a voltage is generated due to the charge diffusion process. This charge movement creates a voltage differential across the diode's terminals, forming a potential barrier at the PN junction. However, this voltage is not continuous or stable. When the diode is connected to a load like a resistor, the capacitance of the diode quickly discharges, resulting in a voltage drop.
Conditions and Limitations
The effectiveness of LEDs as voltage sources depends on the presence of light. If an LED is encapsulated in black epoxy or placed in a dark box, the absence of light means no voltage can be generated. Additionally, due to the inherent leakage current in the diode, the voltage available is already being depleted by the time it reaches the circuit.
Conclusion
In summary, while it is accurate to say that an LED can generate a small amount of voltage under light exposure, it cannot sustain this voltage for long periods or provide a stable output. Therefore, LEDs are best suited for applications where short, light-induced voltage pulses are required. Understanding the principles of semiconductor voltage generation and the limitations of LEDs can help in designing more effective and efficient circuits.