22 October 2026
Why a Diode's Forward Voltage Isn't One Fixed Number
The 0.7 V rule of thumb hides a real spread. Current, temperature, and diode type all shift the forward drop, and a worked example shows how much it matters on the bench.
Why the bench number matters
Most of us learn that a silicon diode drops about 0.7 volts when conducting, and for many jobs that is close enough. Put a diode in series with a resistor to protect an LED or to set a reference, though, and the drop you assumed quietly shapes the current, the voltage at the next node, and sometimes whether a circuit works at all.
A quick multimeter check on the diode-test range usually reads somewhere between about 0.5 and 0.8 volts for a silicon part, and the number you see will depend on the meter's test current and on how warm the junction is. That reading is a useful sanity check, but it is not a specification.
Bench tip The diode-test range on most meters pushes only a small current through the junction, so the reading is typically lower than the drop you would see at a few milliamps of real load.
Current sets the drop
A diode's forward characteristic is exponential. Voltage across the junction grows with the logarithm of current, so a change of current by a factor of ten moves the forward drop by something on the order of 60 millivolts at room temperature for an ordinary silicon junction. Going from 1 mA to 10 mA might move the drop from roughly 0.6 volts to roughly 0.66 volts, while going from 10 mA to 100 mA could push it closer to 0.72 volts. The exact figures depend on the particular part, so treat these as typical ranges rather than datasheet values.
Because the change is logarithmic, the drop looks almost constant over a wide range of currents. That is why the 0.7 volt rule works so often, and also why it fails quietly when your current sits far from whatever the rule was built around.
Bench tip If you need the drop at a specific current, measure it with a resistor and a supply at that current rather than relying on the diode-test function.
Temperature pulls the drop down
Heat makes the forward drop fall. For a typical silicon junction the coefficient is roughly -2 millivolts per degree Celsius, so a diode that reads 0.65 volts at room temperature may read around 0.57 volts after the junction warms by 40 degrees. This is the reason the same diode can behave differently in a warm enclosure than on the bench, and why some older temperature sensing circuits used a diode's forward voltage as the sensing element.
Self-heating matters too. A diode dissipating a fraction of a watt can run noticeably warmer than the air around it, especially in a small package or under continuous load.
Bench tip A finger on the package is a crude but useful check. If the part is too hot to hold comfortably, its forward drop has probably moved from your bench reading.
Worked example: a 330 ohm resistor and a silicon diode
Consider a 3.3 volt supply feeding a silicon diode in series with a 330 ohm resistor, with the diode forward biased. Assume the diode drops 0.65 volts at room temperature. The current is (3.3 - 0.65) / 330, which is 2.65 / 330, or about 8.0 milliamps.
Now assume the diode is warmer, around 65 degrees Celsius, and its drop has fallen by about 80 millivolts to 0.57 volts. The current becomes (3.3 - 0.57) / 330, which is 2.73 / 330, or about 8.3 milliamps. The change is only about 0.3 milliamps, which is small here, but the same reasoning applied to a 5 volt supply with a 1 kilohm resistor or to a circuit that needs a tight reference tells you where the error becomes significant.
The lesson is not that the calculation is hard. It is that the diode's drop is a variable you should consider, and the sensitivity of your result to it depends on how small your headroom is. When the supply is only a volt or two above the drop, a 0.1 volt error becomes a large fraction of the available voltage.
Bench tip Estimate sensitivity before you build: calculate your result with the drop at 0.6 volts and again at 0.7 volts. If the two answers differ in a way that matters, plan to measure.
Diode type changes the baseline
Silicon is not the only option. Schottky diodes, which use a metal to semiconductor junction, typically have a forward drop in the range of about 0.2 to 0.45 volts depending on current and part, which is why they are popular in low voltage rectification and switching supplies. Germanium diodes, now rare in new designs, sit lower still, while LEDs have forward voltages that depend heavily on their color and material, often somewhere between about 1.8 and 3.3 volts.
Because each type has its own curve, a rule that works for silicon will mislead you with a Schottky or an LED. Check the type first, then reason about current and temperature.
Bench tip An LED's forward voltage varies noticeably from one unit to another within the same color, so size the series resistor for the current you want, not for a nominal voltage printed in a table.