Directly Connecting LEDs to Power Supplies: Understanding Resistance Values

When an LED is directly connected to a power supply without a series resistor, it can lead to severe consequences. Unlike the popular belief that solid-state devices will 'burn up,' LEDs may unexpectedly turn off or operate at diminished brightness before failing. To ensure the proper functioning and longevity of LEDs, it's crucial to understand the role of a current-limiting resistor and the appropriate resistance values based on your specific power supply.

Understanding the Risks of Direct Connection

Directly connecting an LED to a power supply without a series resistor introduces significant risks. Under an undetermined voltage, the LED fails to provide current limitation, leading to either excessive current or an instant failure. The abuse of such a connection can cause permanent damage to the LED, making it unsuitable for continued use.

Calculating Resistor Values Using Ohm's Law

To determine the suitable resistance value for a series resistor, you need to apply Ohm's Law (R U/I) and carefully consider your power supply and LED specifications. Here is a step-by-step guide:

Example with a 9-Volt Battery and a Desired Current of 20 mA

Let's use a 9-volt battery as the power supply and assume a desired current draw of 20 mA:

Determine the voltage drop across the resistor: 9 volts - 1 volt (LED forward voltage) 8 volts.

Calculate the resistor value using Ohm's Law: R U/I, where U is the voltage drop (8 volts) and I is the current (0.02 Amps).

Plug in the values: R 8 / 0.02 400 ohms.

In cases where a 400-ohm resistor is not readily available, a 390-ohm resistor can be used.

Ensure you use at least a 1/4 watt resistor to dissipate the power: Power (P) Voltage (V) x Current (I), so 8 x 0.02 0.16 watts.

Alternative Calculation for High-Brightness LEDs

For high-brightness LEDs, a simple heuristic can be applied:

For high-brightness LEDs, a 1K resistor can provide sufficient brightness for low-to-mid voltage ranges.

If the power supply voltage is higher, you can calculate the resistor value directly using the LED's forward voltage and the desired current.

Specific Voltage and Current Adjustments for Different LED Types

LEDs are current-driven devices, so the voltage and current requirements can vary depending on the color and application. Here are some general guidelines:

Example with a 12-Volt Power Supply, 10 mA, and a 2.2V LED

Determine the voltage drop across the resistor: 12 volts - 2.2 volts (LED forward voltage) 9.8 volts.

Calculate the resistor value: R U/I, where U is the voltage drop (9.8 volts) and I is the current (0.01 Amps).

Plug in the values: R 9.8 / 0.01 980 ohms.

A 1 kohm resistor is a standard value that can be used.

Keep in mind that different LED colors have varying forward voltages, and the current requirements also affect brightness. For instance, blue, white, or UV LEDs may require a higher voltage, while red LEDs might need a lower voltage. Additionally, the current should be adjusted based on the brightness desired from the LED.