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Diode forward and reverse bias

Diodes and rectifiers · Beginner · about 10 minutes · part of Zen’s Electronics Design Mastery

A diode is a one-way valve. Forward biased (anode positive) it conducts once about 0.6–0.7 V is across it, and that voltage barely rises however much current flows. Reverse biased it blocks almost completely.

Diode forward and reverse bias: the circuit on the simulator's bench

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How it works

Shockley: I = Is(eV/nVT − 1), with Is = 7.20e-8 A and nVT = 60.8 mV for the 1N4007

Solving it together with the resistor: I = 4.331 mA, VF = 669.1 mV

Quick estimate: I ≈ (Vs − 0.7) / R = 4.3 mA

0.000 A0.252.5 mA0.505 mA0.757.5 mA1.0010 mADiode voltage V_F (V)Current

Orange line: theory. Green dots: what you have measured so far.

Try it on the bench

  1. Set the supply to 1, 2, 5, 10 and 20 V. The current rises in proportion but V_F only creeps from about 0.6 to 0.8 V. The dots trace the knee of the curve.
  2. Set it to 0.5 V: almost nothing flows, because the diode has not reached its knee.
  3. Change D1 to a Schottky (1N5819): the knee drops to about 0.3 V.
  4. Open Tools… → DC sweep: sweep V_s from 0 to 5 V and plot A_fwd against V_F. That is the diode’s I–V curve, with the knee near 0.6 V.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Diodes & Power Supplies”.

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