When LEDs are wired in parallel, each branch sees the same voltage, but the current divides between branches. Because LEDs can have slightly different forward voltages, they may not share current evenly: one can draw more current, heat up, and become brighter or fail sooner. For a voltage supply, the usual way to limit this imbalance is to put a separate resistor in series with each LED branch.
How does a parallel LED circuit work?
In a parallel circuit, each LED branch connects across the same two supply points, so the voltage across every branch is the same. The supply current is the sum of the current flowing through the branches. That does not mean each LED automatically receives an equal share: the current in an individual branch depends on the LED and any current-control components in that branch.
For a practical comparison of series and parallel LED connections, see Analog Devices’ discussion of powering white LEDs in series or parallel.
Do LEDs in parallel share current equally?
Not necessarily. LEDs have forward-voltage variation, even among parts of the same type. A branch with a lower forward voltage can draw more current than one with a higher forward voltage. Texas Instruments explains this mismatch risk in its application report, “Pitfalls of Parallel LED Arrays”.
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Temperature can make the imbalance worse: as an LED heats, its forward voltage can fall, which may allow that branch to draw still more current. The result can be uneven brightness, excess heat, shortened service life, or a failed LED. These outcomes are risks, not certainties; actual behavior depends on the LEDs, current-control arrangement, thermal design, and driver.
Do parallel LEDs need a resistor?
With a basic voltage supply, each LED branch needs its own current-limiting method. The simplest common approach is a resistor in series with each LED branch. A single resistor shared by multiple unmatched parallel LEDs can leave current sharing poorly controlled; separate branch resistors reduce how much a forward-voltage difference changes each branch’s current.
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Calculate a resistor for each branch
For a simple resistor-limited branch, estimate resistance with:
R = (Vsupply − Vforward) / Ibranch
Use the supply voltage, the LED’s forward voltage at the intended operating point, and the target branch current. Then check the resistor’s power dissipation and rating, as well as the LED’s current rating, against the component datasheets. There is no one resistor value that suits every supply and LED.
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DigiKey’s 2019 example considers ten white LEDs at approximately 3.0 V and 20 mA each with a 5 V supply, and calculates a 10 Ω shared bias resistor for that illustrated configuration. Those are example assumptions, not a general recommendation; the article is “Precisely Control LED Luminance Using a Current-Output DAC and TIA”.
What are the pros and cons of parallel versus series wiring?
| Arrangement | Advantages | Tradeoffs |
|---|---|---|
| LEDs in parallel | Does not require the supply voltage to cover the sum of all LED forward voltages in a long string; branches can be controlled individually. | Total supply current rises as branch currents add. Forward-voltage variation can cause unequal current and brightness unless branches are controlled. |
| LEDs in series | The same current flows through every LED in the string, supporting consistent current through those LEDs. | The supply must provide enough voltage for the sum of LED forward voltages plus current-control headroom. An open fault can interrupt the whole string. |
| Parallel branches with individual resistors | A straightforward way to limit current branch by branch from a voltage supply. | Each resistor must be calculated for its branch; resistors dissipate power and do not provide precision current matching. |
| Current-regulated driver | Can control LED current more accurately and support more consistent illumination. | The driver’s topology, voltage compliance, thermal limits, and channel arrangement must suit the array. |
What happens if one LED fails?
The result depends on the failure and on how the circuit controls current. In a shared-current arrangement, an open LED can leave more current available to the remaining LEDs. A shorted LED can cause its branch to draw more current. Separate branch resistors, the driver design, and any protection affect the consequences; neither fault has one universal outcome.
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How can you make a parallel LED array more reliable?
- Use a separate calculated resistor for each branch when powering LEDs from a voltage source.
- For arrays where brightness consistency or reliability matters, consider matched or binned LEDs and a suitable current-regulated driver.
- Check driver voltage compliance, current limits, component ratings, and thermal conditions against the actual datasheets.
- Consider how the circuit responds to open and short faults, and add appropriate monitoring or protection for the application.
Texas Instruments’ report recommends binning or matching forward voltage to support safety and reliability in parallel LED systems. The amount of current mismatch and the best control method depend on the specific parts and circuit. Analog Devices discusses current-regulation approaches and resistor-fed branches in its series and parallel LED power article.
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