Discrete light-emitting diodes (LEDs) are indispensable tools for visual communication and illumination.  And yet, LEDs can be damaged beyond use by excessive electric currents.  For this reason, LEDs may need to be paired with a current-limiting resistance.

This walkthrough offers basic tips on pairing an LED with a voltage source and a current-limiting resistor.  This project will summarize the process as 3 design decisions:

• Step 1: Define important LED properties
• Step 2: Select a specific part number based on step 1
• Step 3: Pair the LED with a resistor

Scope

Optimizing an LED circuit requires a balance of circuit design and heat management.  For brevity, this guide will ignore the heat management aspects.

Today’s project will use low-voltage components (below 50 volts), and low-intensity LEDs (below 1 candela).  These low-intensity “indicator” style LEDs are typically used for visual communication rather than illumination.

Step 1: Define important LED properties

A good starting point is to define the intended color, brightness, and mechanical design of the LED.  Then use a search engine to narrow down the list of candidates:

LED color is expressed by search engines and datasheets in a variety of ways, including:

• Plain language (red, orange, yellow, etc.)
• Peak emission wavelength (λ_P), which describes the strongest wavelength emitted
• Dominant wavelength (λ_D), which describes the strongest color as seen by human eyes
• Tables and charts

LED brightness is more complicated because the light’s spread, LED size, and distance affect how bright the LED will appear.  A simple search criterion is the millicandela (mcd) rating of the part.  By definition, 1000 mcd has roughly the same luminous intensity as a burning candle.

Mechanical designs for LEDs describe the physical shape of the LED assembly.  Two important factors to decide early are:

• Mounting type(surface mount, through-hole, panel mount, etc.)
• Standardized package type

Step 2: Narrow down a specific part number

After narrowing the field to the right color, brightness, and mechanical requirements; the next step is to estimate the LED current needed to produce the intended brightness.  The LED datasheet and the decisions from step 1 above will guide this process.

Always Operate Below Absolute Maximum Ratings

The “absolute maximum ratings” section of an LED datasheet lists the conditions that can damage the LED. 

• Absolute maximum power dissipation
• Absolute maximum forward current
• Operating temperature and storage temperature

Estimate the Forward Current

The lumineous intensity of the LED depends on the forward current (I_F) passing through it. Their relationship will be specified in the LED datasheet. Often, this is expressed as a single test current, or as a look-up table.

Estimate the Forward Voltage

Any given forward current will produce a forward voltage (V_F) across the 2 terminals of the LED. The LED datasheet will provide the estimated (i.e. expected) I_F to V_F relationship on a chart.

As a rule of thumb: longer wavelength LEDs (reds and infrareds) tend to have smaller V_F than short wavelengths (blues and violets).  Also, many white LEDs are actually blue LEDs with built-in phosphor conversion.  The exact V_F depends on the semiconductor material and its temperature.

Optional: Stringing LEDs Together

If the supply voltage (V_CC) is more than N times greater than the LED voltage, then the circuit can be optionally connected to N many LEDs in series.

Step 3: Pair the LED with a resistor

Finally, the circuit will need to place a current-limiting resistor (R1) in series with the LED(s).

The resistor’s own voltage drop (V_R1) must make up the difference between the supply voltage (V_CC) and the combined forward voltages of all LEDs in series.

For a single LED:

V_{R1}=V_{CC}-V_F

For multiple LEDs connected in series:

V_{R1}=V_{CC\ }-\ \sum_{i=1}^{N}V_{Fi}

Where: V_R1 is the resistor voltage drop, V_CC is the supply voltage, V_F is the LED forward voltage, N is the number of LEDs in series, and V_FN is the forward voltage of the Nth resistor.

Estimating Resistor Value

Next, estimate the R1 resistor value using the results above.

R_1=\frac{V_{R1}}{I_F}

Where: I_F is the forward current intended for the LEDs. 

If R₁ is not commercially available, its value may be increased to a value that is.

Calculate Resistor Heat Dissipation

The amount of heat power the resistor will dissipate (in watts) is:

P_{R1}=I_{F}^2R_1

If P_R1 exceeds the wattage rating of the resistor, the resistor will likely overheat and fail.  At a minimum, choose a resistor with a power rating greater than 1.25(P_R1), or follow the resistor manufacturer’s guidelines.

Closing Remarks

Sometimes, the desired LED forward voltage will be higher than the supply voltage.  For example: driving a 3.8 V white LED from a single 1.5 V alkaline cell.

In this case, an LED driver with built-in boost conversion can supply the required voltage.  A Joule Thief is one such kind of driver that can be made from discrete components and a transformer.