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The forward voltage of a diode is the nominal (approximate or average) voltage needed to get the diode to conduct while forward biased. It is well known that a diode conducts easily while forward biased and that it does not conduct while reverse biased (within limits of course).
However, it’s not guaranteed that a diode will conduct while forward biased. If the voltage is too low, then the diode won’t conduct. The voltage will probably have to be a little lower than the diode’s nominal forward voltage value before it starts conducting a bit. As current gets up to a fair amount for that diode, then the voltage built up across the diode will probably be the nominal forward voltage.
Remember that diodes do not limit current once their forward voltage is exceeded. You need a resistor or current source circuitry in series with it in order to limit the current based on the remaining voltage.
The voltage build up across a forward biased diode is dropped from series components.
Common forward voltages:
- Schottky diode = 0.3V
- Silicon (rectifier) diode = 0.7V
- Red LED = 2V
- Blue LED = 3V
Forward voltage drop example:
One of the firs things people tend to learn in electronics is how to power an LED.
The LED drops an approx. amount of voltage, 2V for a red LED, and 3V for a blue LED.
The rest of the voltage goes across a series resistor. The voltage across the resistor sets the current through the entire circuit since it is all in series. The Ohms law for current (I) is V/R = I.
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