Using a resistor to limit the current to an LED is a lot like patching the radiator hose in your car with duct tape. It’s a quick and dirty solution that should work and honestly probably will work for quite a while, but that would cause you to slam the hood and back away if you saw that patch job on something you intended to buy and use for years.
LED with Current-Limiting Resistor
LEDs are constant current devices with a predictable forward voltage drop based on the amount of current flowing through them. These current-voltage (I-V) curves are available in the datasheet for specific LEDs. If you want to drive a green LED with a forward voltage of 3V off a 5V power supply, you need something in that circuit to absorb the extra 2V. If you do not absorb that extra voltage, the LED junction will try to draw enough current to increase that voltage to 5V and burn itself up. The easiest way to absorb extra power in a circuit is to use a simple resistor. You need to know the intended LED to define the resistor using V=IR, but the math is very straightforward once you determine your system parameters. If you want to drive that 3V LED at 10mA, you need to absorb 2V at 10mA in a resistor. 2V = 10mA * R. Solving for R gives a resistor value of 200ohms, a common value that you likely already have in a kit somewhere.
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LED Binning & Resistor Variance
This method is perfectly fine for a power-good indicator LED or any situation where a single low power LED is used. If you do this with LEDs in parallel, the trials begin. LEDs and resistors have their values given within a range, not as absolutes. The cheap, common value resistors you would likely use in a design are typically +/-10%, meaning a 100 ohm resistor could really be any value between 90 ohms and 110 ohms. The forward voltage of a given LED is also a range. LEDs are sorted into groups called “bins” based on their specific color and voltage measurements after production. Depending on how tightly binned you get your LEDs (tighter binning is more expensive), you could see as much as half a volt difference from one LED to the next.
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None of these variances are enough to cause trouble in a single-LED situation. If there is nothing to compare to, our eyes will not notice that an extra few milliamps are flowing through a lone indication LED. However, our eyes will absolutely notice brightness variations in a fixture with several LEDs or strands of LEDs running in parallel. Get a string of lower-voltage LEDs next to one with higher voltages and you will definitely see a difference. It is not impossible to get highly precise resistors and mate the correct resistor to each individual strand of LEDs, but this is not practical on any sort of scale and will be both expensive and time consuming.
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Constant Current Devices: Low Voltage Constraints
This is where designers turn to current regulating devices. When multiple strands must be run in parallel due to voltage constraints, the best solution is often to place a small constant current device in series with each string to ensure all strands are performing equally. This method also avoids thermal race conditions, as discussed in a previous article. These devices can be as small as a power resistor and cheaper than high-precision resistors.
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