The current limiting resistor is a universal method used for driving 5mm LED circuits. This primitive method is extremely simple and very cost efficient. However, the current limiting resistor is not suitable for all simple LED circuits. For example, high power LED circuits always require a constant current source. Employing a simple LED driver circuit such as the current limiting resistor in a high power LED application results in an unstable and potentially dangerous circuit design. This is primary because the forward junction voltage will continuously vary, and can lead to thermal runaway.
Employing the current limiting resistor is usually safe under the conditions that the circuit contains traditional 5mm LEDs, and the source voltage will not dramatically vary. Start by measuring the source voltage using a calibrated voltmeter. If the LED lighting design is automotive, remember to measure voltage with the engine running. If your LED driver circuit is utilizing a small power supply or battery, do not forget to account for voltage variations induced by any external current loading. For example, some power supplies or small batteries can actually decrease their voltage output as current draw increases. After the source voltage is determined, obtain the LED manufacture's datasheet before proceeding to the next step designing your simple LED circuits.
Select a drive current for your LED driver circuit by first referring to the LED manufacture's datasheet. Most often, people choose the typical forward current rating as their specified LED drive current. Refer to the luminous output vs. forward current graph to determine how much light output this will produce. Keep in mind, under driving an LED light is more efficient and allows for a safety margin if ambient temperature should increase. Next, refer to the forward current vs. forward voltage graph in the manufacture's datasheet. This graph will indicate the forward voltage drop according to the drive current you have selected for your LED electric circuits. This information is critical during the process of determining which resistor to utilize for current limiting.
In the world of custom LED lighting, it is extremely important to understand the relationship between circuit stability and circuit efficiency. A stable LED circuit is less responsive to variations in source voltage. However, the circuit dissipates more heat from the resistor and is therefore less efficient. An unstable circuit is more efficient, but variations in source voltage will cause dramatic fluctuations in current. This can lead to overdriven light emitting diodes causing damage, or if under driven, the LED lights may not light at all. When designing an LED circuit with current liming resistor circuit, the goal is to find the best compromise between stability and efficiency. To accomplish this, it is best to consider a few different variations of the same LED driver circuit in an attempt to determine the most feasible solution. This becomes extremely critical when designing for LED tail lights.
We have already determined the LED forward voltage drop by matching our selected drive current with a voltage value from the LED datasheet. Continue by dividing the measured source voltage by the LED forward drop, then round down to the nearest whole number. This calculation determines the maximum number of LEDs suitable per single LED cluster. For example, suppose our measured source voltage is 12 volts and our selected drive current is 15mA. According to the LED datasheets corresponding with this example problem, the LED forward voltage drop should equal 3.1 volts. We divide the 12-volt source by 3.1 volts to determine the LED forward voltage drop goes into the source voltage 3.8 times. Rounding down to the nearest whole number, we can determine that an LED cluster of three will be the maximum number of LEDs suitable per a single resistor.
In previous steps, we have determined how many LEDs will fit into our circuit based on the source voltage and LED forward voltage drop. Continue to determine the proper resistor value for the simple LED circuit. Start by multiplying the total number of LEDs by the LED forward voltage drop to calculate for total LED voltage drop. Subtract the total LED voltage drop from the source voltage to calculate for the resistor voltage drop. Finally, divide the resistor voltage drop by your selected drive current to determine the exact resistor value. In our example problem, the maximum limit was three LEDs per resistor. Multiply 3 by the LED forward voltage drop, 3.1 volts, for a total LED forward voltage drop of 9.3 volts. Subtract this from the source voltage, 12 volts, to determine a voltage drop of 2.7 volts across the current limiting resistor. Finally, when we divide the resistor voltage drop of 2.7 volts by the selected drive current of 15mA (0.015 amps), we discover that a 180 ohm resistor is needed.
We have just calculated for the most efficient resistor driven circuit based on given values for source voltage, drive current, and LED forward voltage drop. However, this circuit design is susceptible to becoming unstable under the condition of a varying source voltage. If the circuit's power supply is not voltage regulated, the source voltage can change and cause the drive current to vary. An example is an automotive application in which the source voltage can vary between 12 volts and 14 volts when the alternator is charging the battery. In an operative circuit, adding the voltage drops across each component will always total the source voltage. To stabilize the circuit, we must increase the resistor voltage drop by removing one or more LEDs from the circuit. Of course, this also means that we will need to recalculate for the resistor value in order to maintain our selected drive current. By removing one LED from the circuit, the resistor voltage drop increases to 5.8 volts. To maintain our selected drive current of 15mA, we now require a 387-ohm resistor. This overall circuit design is less susceptible to a deviating source voltage, but is also less efficient. Although the circuit is very stable, it produces only two-thirds the luminous output of our previous unstable circuit design.
After completing the circuit design, it is important not to forget about calculating for resistor power dissipation. Failing to do so may result in overheating and possibly catastrophic failure. Simply multiply the drive current by the resistor voltage drop to calculate for resistor power. Resistor power should not exceed the resistor's maximum power rating, in watts. In our previous circuit example, we determined a resistor voltage drop of 5.8 volts. Multiply this by our drive current of 15mA for the resistor power rating, 87mW (0.087 watts). In this example, a resistor rated at one-quarter watt is more than sufficient to dissipate the power.
The LED forward voltage drop will vary between manufacturing lots. Each time the LED manufacture produces an LED, the forward voltage drop can be slightly different from the last. Refer to the LED manufacture datasheets to determine the minimum and maximum possible forward voltage drops. Using these values along with your calculated resistor value, recalculate for circuit current draw. You may find that your current has drastically changed due to the voltage variation across the LED junction. The best solution is usually to reduce the total number of LEDs per series circuit. This will minimize total variations in forward voltage across the junctions. A resistor driven circuit may not be feasible in your application if a reduction in LED per series circuit is not possible, or is possible but total current remains unstable. In this case, you must revert to the recommended constant current source circuit configuration, as a substitute for the resistor driven circuit.