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Diode Forward Voltage Drop Calculator

Work out diode forward voltage drop instantly with clear inputs, formula shown and shareable results.

About Diode Forward Voltage Drop Calculator

The Shockley equation gives forward voltage as n x VT x ln(I/Is), where the thermal voltage VT is kT/q — about 25.7 mV at room temperature. The logarithm is why forward voltage changes only about 60 mV per decade of current, and dynamic resistance is n VT / I.

Formula

Shockley diode equation

Vf = n x VT x ln(I / Is + 1), VT = kT/q

Worked example

With the default values below, Diode Forward Voltage Drop Calculator returns Forward voltage of 0.9090. Change any input above and every figure here is recalculated in your browser.

Inputs used

Forward current
100 mA
Saturation current
0 A
Ideality factor
1.8
Junction temperature
45 °C

Results

Forward voltage
0.9090 V
Thermal voltage
27.42 mV
Power dissipated
90.90 mW
Dynamic resistance
0.49 Ω

Step by step

  1. 1Values used
  2. 2Shockley diode equation
  3. 3Forward voltage= 0.9090 V
  4. 4Thermal voltage= 27.42 mV
  5. 5Power dissipated= 90.90 mW
  6. 6Dynamic resistance= 0.49 Ω

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Electrical results are planning estimates only. Verify against the applicable wiring rules (IEC 60364, BS 7671, NEC or local code) and have all installation work designed and checked by a qualified electrician.

Frequently Asked Questions

Why does forward voltage fall as temperature rises?

The saturation current grows very rapidly with temperature, and that effect outweighs the rise in VT, giving about -2 mV per degree.

What is dynamic resistance used for?

Small-signal analysis. At 26 mV/I it is 0.26 ohm at 100 mA, which matters when a diode is used as a reference or a signal element.

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