Diodes and LEDs

Diode One-Way Model

forward direction matters

This diode-one-way-model lesson treats the diode as an ideal one-way branch: forward orientation closes a path, while reverse orientation blocks current. The 12 V source, 2 V fixed drop, 5 ohm limiter, and 0 A reverse rows are exact only inside the stated training model. Real diodes and LEDs have curved current-voltage behavior, temperature dependence, dynamic resistance, source limits, tolerances, finite precision, and measurement uncertainty.

An ideal diode conducts only when its forward direction matches the source polarity in this circuit model. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

A diode has a forward direction

With this source polarity, the shown diode direction gives a conducting path in the ideal fixed-drop model. Real diodes have curved current-voltage behavior; this model keeps one fixed forward drop.

forward direction⇒conducting path\text{forward direction} \Rightarrow \text{conducting path}
Forward diodeThe diode orientation agrees with the source.12 V2 V5 ohm2 A+-2 V+-10 V

The forward budget closes

The source is 12 volts. The LED drop is 2 volts, so the resistor gets 10 volts.

Vsource=VLED+VR=2 V+10 V=12 VV_{\text{source}} = V_{\text{LED}} + V_R = 2\ \text{V} + 10\ \text{V} = 12\ \text{V}
Forward diodeThe diode drop and resistor drop close the KVL budget.12 V2 V5 ohm2 A+-2 V+-10 V

Reverse orientation blocks

Reverse the same diode with the same source and limiter. The ideal model draws no branch current arrow.

Ireverse=0 AI_{\text{reverse}} = 0\ \text{A}
Reverse diodeThe reversed diode is not a conducting edge.12 V2 V5 ohm

Forward current comes from the leftover voltage

For a forward-biased ideal LED, subtract the fixed drop first. The diagram shows the middle row.

VsVfRVRI7 V2 V5 ohm5 V1 A12 V2 V5 ohm10 V2 A17 V2 V5 ohm15 V3 A\begin{array}{c|c|c|c|c}V_s&V_f&R&V_R&I\\7\ \text{V}&2\ \text{V}&5\ \text{ohm}&5\ \text{V}&1\ \text{A}\\12\ \text{V}&2\ \text{V}&5\ \text{ohm}&10\ \text{V}&2\ \text{A}\\17\ \text{V}&2\ \text{V}&5\ \text{ohm}&15\ \text{V}&3\ \text{A}\\\end{array}
Forward diodeThe middle table row is the checked forward diagram.12 V2 V5 ohm2 A+-2 V+-10 V

Reverse orientation keeps current at zero

Reverse-biased rows do not claim a KVL loop through the diode. Changing source voltage does not create a conducting branch in this ideal model.

VsVfRVRI6 V2 V5 ohm0 V0 A12 V2 V5 ohm0 V0 A18 V2 V5 ohm0 V0 A\begin{array}{c|c|c|c|c}V_s&V_f&R&V_R&I\\6\ \text{V}&2\ \text{V}&5\ \text{ohm}&0\ \text{V}&0\ \text{A}\\12\ \text{V}&2\ \text{V}&5\ \text{ohm}&0\ \text{V}&0\ \text{A}\\18\ \text{V}&2\ \text{V}&5\ \text{ohm}&0\ \text{V}&0\ \text{A}\\\end{array}
Reverse diodeThe middle table row is the checked reverse diagram.12 V2 V5 ohm