Semiconductors
Model class: Exact ideal relationship
Series LED string designer
Calculate the series resistor needed for a multi-LED string at a target current and check voltage headroom.
Interactive engine
Start with the stated conditions.
Values stay in this browser. Choose a representative scenario, then calculate deliberately.
Example ready
Calculate to inspect the result.
The result will identify the direct answer, assumptions, and any warning that changes the next decision.
How much does each input matter?
Each row holds every other input fixed at its current value and moves one input by ±10%, using the same model as the main result. It identifies which input is the binding lever, not a full tolerance stack.
Assumptions to check
- The entered LED-string values represent the stated operating condition.
- This exact ideal relationship is evaluated in the declared lumped or first-pass model.
- A LED-string calculation is not a component qualification or safety approval.
What this LED-string calculation establishes
Calculate the series resistor needed for a multi-LED string at a target current and check voltage headroom. The useful result is the stated electrical quantity and the decision it supports, not an unstated claim about a finished product. This engine keeps the governing relationship visible so an input, unit, condition, or model boundary can be reviewed before a value becomes a component or layout choice.
Remaining voltage headroom is supply voltage minus the total LED forward-voltage drop; required resistance is that headroom divided by target current. Treat the number as a first-pass result for the declared operating point. When a source, load, temperature, frequency, waveform, component tolerance, or measurement condition changes, repeat the calculation at the relevant corner rather than assuming the nominal answer persists.
Worked decision context
12 V with three 2 V LEDs at 20 mA leaves 6 V for a 300 Ω resistor dissipating 120 mW. That example verifies the equation and illustrates the scale of the result, but it does not select a part by itself. Compare the result with available values, ratings, tolerance bands, and the receiving circuit or physical environment before implementation.
Use the primary output to identify the binding constraint. If it leaves little margin, document which input dominates and use selected-part data, a higher-fidelity model, simulation, or measurement. This is especially important when a small numerical difference changes a thermal, timing, noise, or reliability decision.
Limits and validation handoff
It assumes one nominal Vf shared by every LED in the string. Real per-LED Vf spread and temperature change current sharing and may need a current driver instead of a resistor. The calculation does not silently include omitted parasitics, installation conditions, manufacturing variation, or product policy. Those conditions can be decisive even when the arithmetic is exact for the selected model.
Record inputs, units, model assumptions, and the intended decision with the result. Verify the leading risk against the selected component data sheet and a representative measurement when the circuit has consequential energy, high voltage, safety, compliance, or reliability requirements.
Common mistakes
- Treating the LED-string result as a guaranteed operating limit rather than a first-pass exact ideal relationship estimate.
- Mixing a data-sheet value measured under one condition with this LED-string calculation performed at another.
- Selecting a component before checking the boundary this calculation names: it assumes one nominal Vf shared by every LED in the string. Real per-LED Vf spread and temperature change current sharing and may need a current driver instead of a resistor.
Model limit and handoff
Keep the entered LED-string conditions with the calculation, then validate the binding limit using the selected component, physical implementation, and representative operating corner.
FAQs
Is this LED-string result sufficient to approve a design?
No. It applies exact ideal relationship reasoning to the entered LED-string values and names the checks that still need selected-part data, a higher-fidelity model, or measurement. It assumes one nominal Vf shared by every LED in the string. Real per-LED Vf spread and temperature change current sharing and may need a current driver instead of a resistor.
What does this LED-string calculator assume that could make the result wrong?
Remaining voltage headroom is supply voltage minus the total LED forward-voltage drop; required resistance is that headroom divided by target current. If the entered values do not match the real operating condition, the result no longer describes the actual circuit.
Where should this LED-string result go next?
Compare this LED-string result with Preferred values and part selection, then use the stated next decision below the calculator to move from this first-pass number toward an implementation.