Calculator Inputs
Tip: preset values provide realistic starting assumptions for common diode families. You can still fine-tune saturation current, ideality factor, temperature, and resistance.
Forward I-V Plot
The graph shows forward current versus forward voltage using the same operating assumptions as your calculation.
Formula Used
Main equation
Vf = Ns × n × Vt × ln(I / Is + 1) + I × Rs
Thermal voltage
Vt = (k × T) / q
Dynamic resistance
rd ≈ (Ns × n × Vt) / (I + Is) + Rs
- Vf = total forward voltage.
- Ns = number of similar junctions in series.
- n = ideality factor.
- Vt = thermal voltage in volts.
- I = forward current in amperes.
- Is = saturation current in amperes.
- Rs = lumped series resistance in ohms.
- T = absolute temperature in kelvin.
This practical model combines exponential junction behavior with ohmic loss. It is useful for estimation, graphing, design comparison, and education. Real devices may deviate because of manufacturing spread, self-heating, and datasheet measurement conditions.
How to Use This Calculator
- Select a preset or keep the custom mode.
- Enter the desired forward current and choose its unit.
- Provide saturation current, ideality factor, and temperature.
- Enter estimated series resistance and junction count.
- Set the graph sweep multiplier to control plotting range.
- Press Calculate Forward Voltage.
- Review the total voltage, junction share, resistor share, power, and dynamic resistance.
- Use the CSV or PDF buttons to export the result block.
Example Data Table
| Case | Current | Is (A) | n | Temp (°C) | Rs (Ω) | Junctions | Estimated Vf |
|---|---|---|---|---|---|---|---|
| Silicon Signal Diode | 10 mA | 1.0e-9 | 1.75 | 25 | 1.2 | 1 | ≈ 0.741 V |
| Germanium Diode | 5 mA | 2.0e-6 | 1.20 | 25 | 0.8 | 1 | ≈ 0.245 V |
| Red LED | 20 mA | 1.0e-15 | 2.00 | 25 | 12 | 1 | ≈ 1.814 V |
| Blue LED | 20 mA | 5.0e-18 | 2.20 | 25 | 20 | 1 | ≈ 2.730 V |
These examples are approximate analytical results, not guaranteed datasheet values.
FAQs
1. What is forward voltage?
Forward voltage is the voltage required to push current through a diode in its conducting direction. It depends on current, temperature, diode material, and internal resistance.
2. Why does forward voltage change with current?
As current rises, the exponential junction term increases and the ohmic drop across series resistance also grows. That makes the total forward voltage increase with operating current.
3. Why is temperature included?
Temperature affects thermal voltage and device behavior. Higher temperature changes the relationship between current and junction voltage, so ignoring it can misstate the operating forward drop.
4. What does ideality factor mean?
Ideality factor measures how closely the diode follows the ideal exponential law. Values near 1 suggest more ideal behavior, while larger values indicate stronger recombination or nonideal effects.
5. Why add series resistance?
Series resistance represents bulk semiconductor resistance, leads, contacts, and package losses. It becomes more important at higher current and can noticeably increase the total forward voltage.
6. Is this calculator suitable for LEDs?
Yes. The calculator can estimate LED forward voltage when you use realistic saturation current, ideality factor, and resistance values. LEDs often need higher effective voltage than signal diodes.
7. Can I use multiple junctions?
Yes. Increase the junction count when modeling similar series junctions or stacked diode elements. The calculator multiplies the junction contribution while still adding the selected series resistance term.
8. Are the results exact?
No. They are strong engineering estimates based on a practical diode equation. Real components vary with manufacturing tolerances, self-heating, measurement setup, and datasheet test conditions.