When developing a custom LED module, customers sometimes begin with a fixed LED quantity.
Others begin with performance requirements such as:
- available operating voltage,
- target luminous output,
- required power,
- PCB dimensions, or
- preferred LED specifications.
In either case, the LED quantity cannot be considered independently from the electrical architecture.
A module that uses 24 LEDs, for example, still needs to answer several questions:
How many LEDs should be connected in each series string?
How many parallel branches are required?
Does the available voltage provide enough headroom?
What total current will the module require?
Can the PCB traces and connectors carry that current reliably?
Will the selected configuration create unnecessary power loss or heat?
For this reason, choosing between series, parallel, and series-parallel LED circuits is not simply a matter of electrical theory.
The circuit architecture needs to work with the LED forward-voltage range, driver type, required output, total current, PCB design, and the operating conditions of the finished product.
1. Series, Parallel, and Series-Parallel LED Circuits: The Practical Difference
The basic electrical behavior is straightforward.
In a series LED string, the forward voltages of the LEDs add together, while the same current flows through every LED in that string.
In a parallel configuration, multiple LED strings operate across approximately the same voltage, while the available current is distributed between the branches.
Many OEM LED modules use a series-parallel architecture, combining several LEDs in each series string with multiple strings connected in parallel.
The important question is not whether series or parallel is theoretically better.
It is:
Which configuration best fits the voltage, current, output, and physical requirements of the actual LED module?
There is no universal answer.
2. Why Supply Voltage Alone Does Not Determine the LED Count
One of the most common mistakes is to treat LED forward voltage as a fixed number.
For example, if an LED is commonly described as having a forward voltage of approximately 3 V, it may be tempting to calculate:
24 V ÷ 3 V = 8 LEDs in series
But an LED's forward voltage is not exactly 3 V under every condition.
The datasheet normally specifies a forward-voltage range, and actual Vf can vary with LED characteristics, operating current, and temperature.
Therefore, the number of LEDs in one series string should be determined by considering:
- LED Vf range,
- available operating-voltage range,
- driver architecture,
- intended current,
- and the voltage margin required by the circuit.
A 24 V system does not automatically mean that eight nominal 3 V LEDs should be connected in series.
The correct series count depends on how the complete electrical system is intended to operate.
This is especially important in OEM projects, where the available voltage may already be defined by the customer's product architecture.
3. Constant-Voltage and Constant-Current LED Modules Require Different Design Logic
The series-parallel architecture also depends heavily on whether the LED module is intended for constant-voltage or constant-current operation.
For a broader explanation of these two approaches, see Constant Current vs. Constant Voltage LED Modules.
The circuit-design implications are different.
3.1 Designing a 24 V Constant-Voltage LED Module
In a 24 V constant-voltage system, the input voltage is fixed.
The design therefore needs additional information such as:
- target power,
- target luminous output,
- LED characteristics,
- and required operating current.
Each LED string normally also requires current-limiting components, such as resistors.
This means the total LED forward voltage should not simply consume the entire 24 V supply.
Some voltage needs to remain available for current limiting.
If too few LEDs are used in each string, more of the supply voltage may need to be dropped across the resistor.
That increases resistor power loss and heat.
On the other hand, if too many LEDs are placed in series and their combined Vf approaches the supply voltage too closely, there may not be enough voltage headroom for stable current limiting across the expected operating range.
The design therefore requires a balance between:
LED series count → LED Vf → resistor voltage drop → operating current → resistor power loss → required light output
The objective is not simply to fit the maximum possible number of LEDs under 24 V.
It is to select a configuration that operates properly across the expected LED Vf range while avoiding unnecessary power loss.
4. Constant-Current LED Modules Are Evaluated Differently
In a constant-current system, the driver controls the total current delivered to the LED module, while the actual operating voltage is determined by the LED configuration within the driver's allowable voltage range.
During module design, NKT reviews:
- the driver's available voltage range,
- LED Vf min/typ/max data,
- number of LEDs per series string,
- number of parallel branches,
- and the required module current.
From this, an expected operating-voltage range can be determined for the module.
The customer then operates the module using the required total current for the complete board.
If multiple branches are connected in parallel, that total current is distributed across those branches.
This is one reason that the LED circuit architecture needs to be considered together with the customer's power system rather than after the PCB layout has already been finalized.
5. More Parallel Branches Are Not Automatically Better
Parallel branches can be useful when the required LED quantity cannot be placed into a single series string.
But increasing the number of branches is not automatically an improvement.
It is important to distinguish between two situations.
If the total constant current is already fixed, simply increasing the number of branches does not automatically increase the total current. Instead, it changes how that fixed current is distributed among the branches.
However, when designing to achieve a specific total optical output or power, a configuration with fewer LEDs per string may require more parallel branches. If each branch needs to operate at a particular current, the total module current may then become relatively high.
Higher current can affect:
- PCB trace width,
- voltage drop,
- copper loss,
- connector current rating,
- power delivery,
- and heat generation.
For this reason, NKT generally prefers, where practical, to use more LEDs in each series string and fewer parallel branches, provided that the voltage range and driver conditions allow it.
This is not an absolute rule.
It is a practical design preference that needs to be checked against the complete electrical requirements of the project.
A mathematically possible series-parallel configuration is not always the preferred practical design.
6. Current Sharing Between Parallel LED Strings Also Matters
When multiple LED strings are connected in parallel, current distribution between the branches needs to be considered.
In the LED modules NKT typically develops, parallel strings are designed using:
- the same LED model,
- the same number of LEDs in each branch,
- and PCB routing that keeps branch conditions as consistent as reasonably possible.
This helps the parallel strings operate under similar electrical conditions.
However, this should not be interpreted as a guarantee that every branch always carries exactly the same current.
LED forward voltage naturally varies within a tolerance range.
Even LEDs with the same part number can have small electrical differences.
The practical objective is therefore to create a balanced architecture rather than assume that parallel branches are electrically identical under every operating condition.
7. Why a Customer-Specified LED Quantity May Need to Change
Some OEM customers begin a project by specifying the approximate number of LEDs they want on the PCB.
That is a valid starting point.
But the requested quantity still needs to be checked against:
- available operating voltage,
- LED Vf range,
- driver type,
- number of LEDs per string,
- number of parallel branches,
- total current,
- target output,
- and PCB constraints.
Sometimes the specified number simply does not divide into a practical electrical architecture.
In that situation, the LED quantity may need to change.
The LED count should be part of the electrical design—not an isolated number decided before the circuit is evaluated.
8. A Real OEM Example: When the Requested LED Quantity Did Not Work
In one industrial OEM project, the customer required a dual-wavelength LED module using 405 nm and 450 nm LEDs.
The two wavelengths were controlled as separate channels and were not intended to operate simultaneously.
Because the 405 nm and 450 nm LEDs had different electrical characteristics, including different forward-voltage behavior, each channel needed to be calculated separately.
The customer initially provided approximate LED quantities.
NKT first reviewed:
- the available operating-voltage conditions,
- the selected LED datasheets,
- the forward-voltage characteristics,
- and the required series-parallel arrangement for each channel.
The issue was not simply that there were “too many LEDs.”
The problem was that the specified LED totals could not be divided into reasonable equal series-parallel groups under the required electrical conditions.
The LED quantities therefore needed to be adjusted before the PCB design could proceed.
This is a useful example of why a customer-specified LED count should be treated as a design input rather than an automatically fixed electrical configuration.
9. Multi-Channel LED Modules Should Be Calculated Channel by Channel
The same principle applies to RGB, RGBW, UV/visible, or other multi-channel LED modules.
Different LED wavelengths or colors can have different forward-voltage characteristics.
If the channels are independently controlled, each channel should therefore be evaluated according to its own:
- LED Vf range,
- LED quantity,
- operating current,
- series count,
- and parallel architecture.
The fact that all LEDs are mounted on the same PCB does not mean that the entire board should be calculated as one electrical string.
In the 405 nm + 450 nm example above, the two channels were physically integrated into the same module but electrically evaluated separately.
This distinction is important in custom multi-channel LED module design.
10. What Information Is Needed Before Choosing the Series-Parallel Architecture?
A customer does not always need to arrive with a completed electrical design.
Some customers already know the exact LED model and approximate LED quantity they want.
Others provide performance targets such as:
- available PCB dimensions,
- mounting requirements,
- supply or driver type,
- operating-voltage range,
- target luminous output,
- required power,
- CCT,
- CRI,
- wavelength,
- and application-specific requirements.
Both types of projects can be evaluated.
For example, a request such as:
“We need a 24 V LED board with approximately 2,000 lumens.”
is a useful starting point.
But voltage and lumens alone are not enough to finalize the series-parallel circuit.
The design still needs to establish:
- whether the system is constant-current or constant-voltage,
- the appropriate LED model,
- LED quantity,
- target current,
- power,
- Vf range,
- and physical PCB constraints.
Once these inputs are understood, the circuit architecture can be developed around the actual project rather than around a predetermined electrical formula.
11. Series-Parallel Design Also Affects the PCB
Circuit architecture is not independent from PCB design.
Higher total current can require wider copper traces.
Longer or narrower current paths can increase voltage drop.
Connector current capacity may become important.
Component positions and routing space can also influence what is practical on the available PCB.
For this reason, electrical architecture and PCB layout should be reviewed together during custom LED module development.
A circuit configuration that looks acceptable in a simplified electrical calculation may become less attractive once current routing, connector capacity, PCB area, and thermal behavior are considered.
This is another reason that NKT generally evaluates the complete module rather than treating LED quantity and circuit architecture as separate decisions.
12. Prototype Validation Confirms the Design Calculations
Once the prototype has been manufactured, actual measurements are used to confirm whether the design behaves as expected.
For a white-light LED module, typical checks may include:
- voltage,
- current,
- luminous flux,
- luminous efficacy,
- CCT,
- and CRI.
For colored LEDs, relevant parameters may include:
- voltage,
- current,
- wavelength,
- and optical output.
For UV LED modules, optical power can be measured according to the requirements of the project.
These prototype measurements help confirm whether the calculated electrical configuration produces the expected module-level performance.
Where necessary, the design can then be adjusted before the customer approves the final configuration.
You can learn more about this stage under Prototype Validation .
13. Operating Limits Should Be Communicated After Prototype Validation
Prototype validation should not end with the statement that the module “works.”
Once the electrical and optical performance has been checked, the customer should also understand the operating limits of the approved LED module configuration.
For NKT projects, this includes informing the customer of the applicable:
- operating-voltage range, and
- maximum current.
These limits are important because the module's electrical performance, power consumption, and thermal load depend on how the customer operates it in the finished product.
A constant-current module, for example, may operate over a certain forward-voltage range, but the customer still needs to keep the drive current within the approved limit.
Operating beyond the confirmed electrical conditions can change both output and thermal behavior.
A validated LED module therefore needs not only an approved circuit configuration, but also clearly defined operating boundaries.
14. Is Series or Parallel Better for an OEM LED Module?
There is no universal answer.
A series-heavy configuration may reduce the number of parallel branches and help keep total current requirements manageable, but it requires sufficient driver voltage.
A configuration with more parallel branches may fit a lower-voltage system, but the resulting current requirements, routing, connector capacity, voltage drop, and thermal behavior need to be evaluated.
For many NKT projects, when the electrical conditions allow it, the preferred direction is:
more LEDs per series string + fewer parallel branches
But this preference is always secondary to the actual project requirements.
The correct architecture depends on:
- supply or driver type,
- available voltage range,
- LED Vf range,
- target output,
- operating current,
- LED quantity,
- PCB size,
- connector capability,
- voltage drop,
- and thermal considerations.
Series vs. parallel is therefore not a choice between “good” and “bad.” It is a system-level design decision.
How NKT Approaches Series-Parallel LED Module Design
NKT develops custom OEM LED modules around the electrical and performance requirements of the customer's product.
The project may begin with a specified LED quantity, or it may begin with target requirements such as voltage, power, lumen output, PCB dimensions, CCT, CRI, or wavelength.
From there, the LED characteristics and available power architecture are reviewed together.
If the requested LED quantity cannot form a practical series-parallel arrangement, the issue is raised before the PCB design proceeds.
The design also considers how the resulting current affects PCB routing, voltage drop, connector requirements, and thermal performance.
After prototypes are produced, relevant electrical and optical parameters are tested, and the customer is informed of the applicable operating-voltage range and maximum current for the approved module configuration.
The objective is not simply to connect a specified number of LEDs. It is to develop an electrical architecture that works with the customer's product requirements and can be reproduced consistently in production.
Conclusion
Series, parallel, and series-parallel LED circuits are basic electrical concepts, but applying them correctly in an OEM LED module requires more than textbook calculations.
Supply voltage alone does not determine how many LEDs should be connected in series.
LED Vf varies.
Constant-current and constant-voltage systems require different design approaches.
A specified LED quantity may not divide into a practical circuit.
Too many parallel branches can create less desirable current and PCB requirements depending on the output target.
And once the module is built, prototype testing is needed to confirm the calculated performance and define the safe operating range.
The most appropriate circuit architecture is therefore the one that balances:
voltage, current, LED characteristics, target output, PCB constraints, power delivery, and the requirements of the final OEM product.
Request a Quote
If you are developing a custom LED module and already know your LED quantity, supply voltage, target power, or optical requirements, send NKT the available project information.
If the series-parallel architecture has not yet been defined, the available voltage, PCB dimensions, performance targets, and LED requirements can be reviewed to determine a practical starting configuration.