LED array: one resistor or multiple?

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LED array: one resistor or multiple?

Posted Date: 2024-02-02

One of the common but less exciting aspects of electronic design is choosing the right resistor values ​​to limit the current through the ubiquitous little indicator lights called LEDs. The process isn't particularly complicated - we assume a constant voltage drop across the LED and then do some math to determine the resistor that will give us the forward current we need.

The constant voltage assumption is completely inconsistent with reality, but we don't use it because generally we don't mind if the LED current is slightly higher or lower than expected.

However, the constant voltage assumption breaks down when we deal with LED arrays integrated into a single package—for example, a seven-segment display. It fails us because it leads to a conundrum: if we assume constant (and equal) voltage drops across all LEDs in the device, we can drive the entire display with just one current-limiting resistor. However, everyone seems to have decided to use separate resistors for each LED.

Consider the following circuit, which represents a device with three common-cathode LEDs in one package.

Assume that the forward voltage (VF ) of each LED is 1.6 V. If we apply a 5 V drive signal to each pin, the common cathode voltage is 3.4 V and the current through the resistor is 10.3 mA. Because each LED has the same voltage drop, we assume they have the same current, so the forward current (IF ) of each LED is 3.4 mA. You're done - why bother with three resistors?

There are two problems here: First, the pressure drop is not constant. Secondly, we cannot assume that the three LEDs have exactly the same current-voltage characteristics.

The actual current through the LED is determined by an exponential relationship, for example:

Note two points:

Once the LED is in a fully conductive state, VF can be considered approximately constant since even large increases in IF correspond to small changes in VF.

In regions where the slope of the exponential curve increases rapidly, small changes in VF correspond to large changes in IF.

Now let us assume that the current-voltage characteristic of one of the LEDs is shifted to the left relative to the other two LEDs.

When voltage is applied, this troublesome LED will go into full conduction at VF = 1.3 V, and since all LEDs share a cathode, this LED will limit the voltage on the other LEDs to 1.3 V. This is a problem because for the other two LEDs, 1.3 V only corresponds to a small amount of current.

The point here is that you generally don't want to use just one current-limiting resistor because you can't ensure that the LEDs share the current equally; also, one LED may get more current than the other LEDs.

However, LEDs contained in a single package should exhibit fairly consistent current-voltage characteristics (unless they are intentionally inconsistent, such as RGB LED modules). Therefore, a single current-limiting resistor may provide adequate performance in many applications - but remember to consider power dissipation! Power is proportional to the square of the current, so if you need, for example, eight LEDs all operating at significant brightness, the current through a single resistor can become very large.

Aside from the inconveniently high power dissipation of the single resistor approach, the bottom line is that a separate resistor for each LED is the way to go in driving an LED array.

#LED #array #resistor #multiple