OFC Lab: Experiment No.6 Study V-I Characteristics of a Light Emitting Diode (LED)
Object: Study of V-I Characteristics of a Light Emitting Diode (LED).
Apparatus Required:
- Light Emitting Diode (LED): (e.g., Red LED)
- DC Power Supply: (0-15 V, Variable).
- Resistor: Current-limiting resistor
- Ammeter: DC Milliammeter
- Voltmeter: DC Voltmeter
- Breadboard and Connecting Wires.
Theory
An LED is a forward-biased p-n junction diode that emits light through electroluminescence when electrons and holes recombine, provided the forward voltage (Vf) exceeds the Knee Voltage (Vk). The color of the light determines Vk, which is typically lower for red LEDs (approx 1.6v to 1.8v) than for blue LEDs (3v).
A. Forward Bias Characteristics
Connect the circuit as per the diagram (Power Supply, Series Resistor, Series Ammeter, and Parallel Voltmeter across the LED). Increase the voltage gradually.
| S. No. | Supply Voltage (V) (V) | Forward Voltage (VF) (V) | Forward Current (IF) (mA) | Observation |
| 1 | 0.0 | 0.00 | 0.00 | No light |
| 2 | 0.5 | 0.50 | 0.00 | No light |
| 3 | 1.0 | 1.00 | 0.01 | Very faint/No light |
| 4 | 1.5 | 1.50 | 0.15 | LED begins to glow |
| 5 | 1.6 | 1.60 | 0.35 | |
| 6 | 1.7 | 1.70 | 1.00 | |
| 7 | 1.8 | 1.75 | 2.50 | |
| 8 | 2.0 | 1.80 | 4.20 | |
| 9 | 3.0 | 1.90 | 10.50 | Standard operating point |
B. Reverse Bias Characteristics
Reverse the polarity of the DC supply and the Ammeter (use a microammeter, if available). Stop immediately if current increases sharply (breakdown).
| S. No. | Supply Voltage (V) (V) | Reverse Voltage (VR) (V) | Reverse Current (IR) (μA) | Observation |
| 1 | 1.0 | 1.0 | 0 | Negligible leakage |
| 2 | 2.0 | 2.0 | 0 | |
| 3 | 3.0 | 3.0 | 0 | |
| 4 | 4.0 | 4.0 | 0.01 | |
| 5 | 5.0 | 5.0 | 0.05 | Max Reverse Voltage ($\mathbf{V_{R(max)}}$) |
| 6 | 6.0 | 6.0 | $\approx$ Rapid increase | Breakdown occurs (STOP) |
Graph:
Result
The V-I characteristics of the Red LED were successfully studied and plotted.
- Knee Voltage 1.6 V
- The characteristic curve confirms that the Red LED begins to conduct significantly at approx 1.6V and is effectively an open circuit in reverse bias up to its maximum rating approx 5V.
