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A diode conducts in the forward direction after its forward voltage drop (VF) is reached — roughly 0.7 V for silicon, 0.3 V for Schottky, and 1.8–3.6 V for LEDs. To run a fixed current through it from a higher supply, a series resistor drops the remaining voltage.
| Quantity | Formula |
|---|---|
| Series resistor | R = (Vsupply − VF) / IF |
| Forward current | IF = (Vsupply − VF) / R |
| Diode power | Pdiode = VF × IF |
| Resistor power | Presistor = IF² × R |
Silicon signal/rectifier ≈ 0.7 V · Schottky ≈ 0.2–0.4 V · Germanium ≈ 0.3 V · Red LED ≈ 1.8–2.0 V · Blue/White LED ≈ 3.0–3.6 V.
Schottky diodes have a lower forward drop (~0.3 V) and switch faster, so they waste less power — ideal for rectifiers and low-voltage circuits. Silicon (like the 1N4007) is cheaper and handles higher reverse voltage.
Peak Inverse Voltage is the maximum reverse voltage a diode can block without breaking down. Always pick a diode rated well above the highest reverse voltage in your circuit.
It dissipates VF × IF as heat. At high current even a 0.7 V drop adds up — e.g. 3 A gives ~2 W. Use a Schottky or a heat-sinked diode for high-current rectification.
In series their voltage drops add. In parallel they share current unevenly (the one with the lowest VF hogs it), so parallel diodes usually need small balancing resistors.
A forward-biased diode has very low resistance, so without a series resistor the current would rise until the diode overheats and fails. The resistor sets a safe, defined current.
Check the datasheet at your operating current. As a rule of thumb: 0.7 V silicon, 0.3 V Schottky, ~2 V red LED, ~3.2 V blue/white LED.
Use Presistor from the result and pick a resistor rated at least 2× that value for a safe margin.