Engineering Guide
LED Series String Design: Voltage Headroom, Resistors, and Power
A series LED string is not sized from nominal forward voltage alone. The design must operate across supply tolerance, LED forward-voltage spread, temperature, preferred resistor values, and component power limits. This guide turns those constraints into a repeatable first-pass workflow.
Series String Design Workflow
Use the LED Series Calculator after establishing credible supply and LED limits. The calculator solves string voltage, headroom, resistance, preferred E24 resistance, current, and resistor power; the engineering decision still depends on worst-case corners and thermal margin.
| Step | Input | Check |
|---|---|---|
| 1 | Supply range | Use minimum and maximum operating supply, not only nominal voltage. |
| 2 | LED VF range | Use forward voltage at the intended current and temperature; include production spread. |
| 3 | String count | Require VS,min > N x VF,max plus enough control headroom. |
| 4 | Resistance | Calculate R = (VS - N x VF) / IF at the chosen design point. |
| 5 | Standard value | For passive limiting, selecting the next higher preferred resistance reduces current. |
| 6 | Power and heat | Check resistor and LED dissipation at the worst credible voltage and VF corner. |
Core Equations
Formula reference
Series LED string
V_string = N x V_FV_headroom = V_S - V_stringR_required = V_headroom / I_FI_actual = V_headroom / R_standardP_R = I^2 R = V_headroom x IVariable definitions
- N
- number of LEDs in the series path
- VF
- forward voltage of one LED at the evaluated current and temperature
- VS
- supply voltage at the evaluated corner
- Rstandard
- selected preferred resistor value
- Positive voltage headroom is required for a passive resistor solution
Maximum Count Is Not the Design Count
The floor of VS/VF is only a mathematical upper bound at one operating point. If the calculated count leaves almost no resistor voltage, normal supply and VF variation creates a large current change or eliminates conduction margin entirely.
Evaluate minimum supply with maximum VF before accepting the count. For a constant-current driver, include its required compliance or dropout voltage. For a resistor, preserve enough headroom that tolerances do not dominate current.
Worst-Case Current and Power
A resistor does not regulate current against supply and forward-voltage variation. Calculate the same circuit at each relevant corner rather than applying one percentage to the nominal answer.
| Check | Typical corner | Why |
|---|---|---|
| Highest LED current | VS,max and VF,min | Largest voltage across the resistor. |
| Lowest LED current | VS,min and VF,max | Smallest positive voltage headroom. |
| No regulation margin | VS,min <= N x VF,max | The passive design cannot guarantee the target current. |
| Highest resistor power | Usually highest current/headroom corner | Calculate directly instead of assuming nominal power is worst. |
Preferred Values and Thermal Margin
An exact calculated resistance may not be stocked. Selecting the next higher E24 value is a conservative passive-design convention because it lowers current at the same voltage corner. Recalculate current and optical output after rounding; never use the displayed rounded current as an input to later power calculations.
Choose resistor wattage from worst-case dissipation, ambient temperature, enclosure conditions, PCB copper, and manufacturer derating. A nominal 2x wattage margin is a useful first check, not a universal thermal guarantee.
Worked Examples
Three red LEDs from 12 V
- Given VS = 12 V, N = 3, VF = 2.0 V, and IF = 20 mA.
- Vstring = 3 x 2.0 = 6.0 V; Vheadroom = 12 - 6 = 6.0 V.
- Rrequired = 6 / 0.020 = 300 ohms. The next higher E24 value is 300 ohms.
- Presistor = 0.020^2 x 300 = 0.12 W. A 0.25 W part gives a 2x nominal dissipation margin.
Blue LED string with inadequate supply
- Given VS,min = 9 V, N = 3, and VF,max = 3.2 V.
- Worst-case string voltage is 9.6 V, which exceeds the available supply.
- No positive series-resistor solution exists at this corner. Reduce LED count, raise the supply, or use a suitable driver topology.
Current at a preferred resistor value
- Given VS = 5 V, two LEDs at 1.8 V each, and Rstandard = 75 ohms.
- Headroom = 5 - 2 x 1.8 = 1.4 V.
- Iactual = 1.4 / 75 = 18.67 mA, lower than the 20 mA result from a 70 ohm exact resistance.
Supply and VF corner check
- For two nominal 2.0 V LEDs, VS ranges from 4.75 V to 5.25 V and each VF ranges from 1.8 V to 2.2 V.
- At VS,max and VF,min, resistor voltage is 1.65 V. At VS,min and VF,max, it is only 0.35 V.
- This wide current spread shows why a resistor is a poor regulator when voltage headroom is small or tightly controlled current is required.
Common Mistakes
Related Calculators
Related Engineering Guides
Frequently Asked Questions
Support reference
FAQ
How many LEDs can I connect in series?
The theoretical nominal count is floor(VS/VF), but a practical design must also preserve positive headroom at minimum supply and maximum LED forward voltage. A driver may require additional compliance voltage.
How do I calculate the resistor for multiple series LEDs?
Use R = (VS - N x VF) / IF, where N is the number of series LEDs. Then select a practical preferred value and recalculate actual current.
Why should I select the next higher resistor value?
For simple passive current limiting, the next higher value keeps current at or below the exact nominal target, assuming the same supply and forward voltage.
How do I calculate LED string resistor power?
Use P = I^2R or P = Vresistor x I at the same operating corner. Check worst-case supply and LED forward-voltage limits before choosing a power rating.
Why does LED forward voltage vary?
Forward voltage changes with current, junction temperature, manufacturing spread, color, material system, and device construction. Use datasheet curves and limits rather than one universal value.
Can series LEDs share one resistor?
Yes. LEDs in one series path carry the same current, so one resistor can limit the string. Separate parallel strings should generally have independent current limiting.
What happens when supply voltage is too low?
If the supply cannot exceed the total LED forward voltage plus required headroom, the target current cannot be maintained and the LEDs may be dim or fail to turn on consistently.
When should I use a constant-current driver?
Use one when current accuracy, efficiency, dimming, high LED power, wide supply range, small voltage headroom, or thermal stability matters more than minimum cost.
Does the resistor protect against every LED fault?
No. It limits normal-path current but does not replace transient protection, reverse-voltage protection, thermal design, open-string handling, or shorted-LED fault analysis.
Should all LEDs in a string be identical?
Matched type and operating conditions simplify optical and electrical behavior. The same current flows through every series LED, but brightness and forward voltage can still vary.
Summary
A robust series LED design selects string count from worst-case voltage headroom, calculates resistance at a stated operating point, rounds to a preferred value deliberately, and verifies current and power at supply, VF, and temperature corners. Use a regulated driver when passive current variation or dissipation is unacceptable.