When and Why do LEDs Need Current Limiting Resistors?

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When and Why do LEDs Need Current Limiting Resistors?

If you're working on any circuit involving LEDs, you may have come across warnings or recommendations to always use a current limiting resistor.

We've put together this guide to help anyone from the novice DIY guy to those designing and building LED lighting circuitboards fully understand when, why and how to select the appropriate current limiting resistor.

Understanding the I-V curve of LEDs

As with any passive semiconductor component, understanding the I-V (current vs voltage) curve is critical when designing a circuit around them.

An LED is of course, in essence, a diode, and has a non-linear I-V curve. In other words, the relationship between input voltage and input current does not follow a straight line.

For example, let's take a look at the forward current at 2.7 V - approximately 20 mA. If we increase the voltage by 0.1 V to 2.8 V, forward current increases by approximately 30 mA to 50 mA. If we then increase it by another 0.1 V to 2.9 V, forward current increases by 35 mA to 85 mA.

As voltage increases, the rate of forward current increase also increases. Small changes in forward voltage can result in very large forward current changes.

Therefore, constant current LED drivers are the preferred method of driving LEDs - they operate at a single current and adjust their output voltage correspondingly, ensuring that the forward current remains stable. When constant current input is used, a current limiting resistor is not necessary.

What to do if you're using constant voltage power supplies

Constant current power supplies, however, are generally more expensive and are limited in their flexibility. As a result, almost all LED strip products and other modules use constant voltage input.

Constant voltage power supplies are fixed in their output voltage level, and can produce any level of output current between 0 mA and its rated maximum (which may very well be above the rated maximum for the LEDs and LED system).

But as we saw above, due to the nonlinear relationship between forward current and forward voltage, constant voltage power inputs need additional modification to be used safely with LED systems, for the following reasons:

1) LED forward voltage does not necessarily match that of the power supply voltage level. For example, based on the same LED spec as above, if you have a 3.0 V constant voltage power supply, the forward current will also be confined to 135 mA.

What if we want to run the LED at 20 mA using the same power supply? We will need to provide the LED with 2.7 V only, instead of 3.0 V. However, since most power supply units do not have a variable voltage output option, there is no way to achieve 2.7 V at the LED with the power supply unit alone.

What do we do?

The answer is to place a resistor in series with the LED, and allow the resistor to "drop down" the voltage to the LED by 0.3 V.

How do we calculate the resistor value? We use Ohm's Law, which states that V=IR, and substitute 0.3V (the voltage drop) for V, and 0.02A (desired forward current) for I. Solving for R gets us 15 Ohms.

Similar calculations can be performed regardless of the voltages involved - for example, 12V and 24V LED strips.

In a mass production environment, variations in LED forward voltage are inevitable, and result in multiple voltage bins. Ideally, LEDs from each voltage bin has a different resistor value pairing calculated to ensure the same forward current draw, regardless of the LED voltage bin. Otherwise, wider variations in forward current draw, and therefore brightness, can occur.

Each of the lines above represent a different voltage bin. To achieve 60 mA for all of the LED bins, different resistor specifications must be used to achieve different forward voltages needed to achieve the same 60 mA.

2) Current limiting resistors protect against voltage increases

We saw above the LEDs have a nonlinear relationship between forward current and forward voltage. As a result, a minor increase in voltage can lead to a significant increase in forward current, leading to potential overcurrent and device failure.

Unlike diodes, resistors have a linear relationship between forward current and forward voltage (as shown by Ohm's Law).

Therefore, an increase in the forward voltage will lead to the same, proportional increase in forward current, regardless of voltage level. This property of resistors, when incorporated into an LED circuit, can help moderate the effects of a voltage increase.

Why would voltage increase?

The first possibility is a non-stable power source with significant noise or ripple. If there are issues with the constant voltage power supply providing non stable DC current, forward voltage and intermittently spike - and having current limiting resistors will help moderate a corresponding spike in forward current.

The second, more predictable and pervasive is a property of LED devices themselves.

As an LED heats up, its forward voltage decreases, if we keep forward current constant. This is commonly shown in LED datasheets in the following temperature vs forward voltage change chart:

This is useful information when designing a constant current circuit, since it gives us information about the true range of forward voltages that we might see in a system. But let's rephrase the same principle from a constant voltage perspective:

As an LED heats up, its forward current increases, if we keep forward voltage constant.

Graphically, we can show the same principle in one chart (below). If we use a constant current perspective, we can say the curve shifts left as temperature rises. Or, if we use a constant voltage perspective, we can say the curve shifts up as temperature rises.

An LED's heat generation is dependent first and foremost on its total power dissipation. Therefore, the fact that forward current rises as its temperature rises is potentially catastrophic, because the higher forward current will increase an LED's temperature even more, in turn increasing its forward current even more, in a positive feedback loop. This is called thermal runaway of an LED system, and will lead to catastrophic failures at best, and possibly fire and smoke.

A current limiting resistor helps mitigate the effect of voltage increases by virtue of its linear IV curve. Additionally, resistors behave  oppositely from LEDs in relation to its temperature - as temperature increases, resistance also increases.

This simple but useful feature of resistors have led some to also refer to resistors used in this manner as a ballast resistor.

Bottom Line

LED devices are inherently current-controlled devices, and do not respond well to fluctuations in voltage.

If you are building an LED system using constant voltage power sources, you must absolutely be prepared to use current limiting resistors to ensure stable and safe operation of LED devices.

Need help building an LED circuit? Contact us to discuss today!


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