The two circuits demonstrate how a semiconductor diode behaves differently from a standard fixed resistor when a voltage is applied.
To see a graph of the diode’s behaviour, alter the potentiometer form 99% to around 10%. This will produce a classic breakdown curve starting at around 350mV and finishing at 752mV
1. Left Circuit: Resistor Behaviour (Comparison)
The left circuit shows a standard, linear component (a resistor) obeying Ohm's Law.
The Components: It contains a 4 V DC power supply, an ammeter connected in series, a voltmeter connected in parallel across the resistor, and a 1 kOhm resistor.
The Behaviour: With 4 V applied across a 1 kOhm resistor, a steady current of 4 mA flows (I = V / R = 4 V / 1000 ohms = 0.004 A).
Key Characteristic: If you doubled the voltage to 8 V, the current would double to 8 mA. The relationship between voltage and current is perfectly linear (a straight line).
2. Right Circuit: Diode Behaviour
The right circuit uses a variable potential divider to alter the voltage across a diode and observe its non-linear behaviour.
The Setup: A 4.06 V supply passes through a variable potentiometer (495 ohms + 5 ohms). By tapping off between these resistors, a smaller variable voltage (40.6 mV) is supplied to the right-hand branch containing the diode.
The Behaviour: An ammeter measures the current passing through the diode loop, and a voltmeter measures the exact voltage across the diode (40.6 mV).
The Graph (Top): The graph plots Current (I) on the vertical axis against Voltage (V) on the horizontal axis for this diode branch.
Summary of Diode Characteristics: The graph perfectly illustrates the unique properties of a diode.
Forward Bias Threshold: Below roughly 300 mV (0.3 V), the line stays flat at 0 A. The diode behaves like an open switch with an incredibly high resistance, letting virtually no current pass.
Non-Linear Turn-on: Around 320 mV to 350 mV, the graph curves upward.
Rapid Current Increase: Once the voltage exceeds this threshold (climbing to 410 mV in the circuit), the diode's resistance drops drastically. The current shoots up rapidly to 7.07 nA and continues to climb exponentially.
Once the diode is fully conducive, irrespective of the voltage applied to it (within reason) it will produce the same 750 mV (approx) drop across it. This is one of the reasons a diode can be extremely useful in a circuit. With the potentiometer set at 5% try changing the input voltage to see the results.
Unlike a resistor, a diode does not obey Ohm's Law because its resistance changes depending on the voltage applied across it. It acts like a one-way electronic valve that only opens once a specific "threshold voltage" is reached.
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