7 Segment Display Current Limiting Resistor

A current limiting resistor is required for each segment of a 7-segment display. Each resistor provides a voltage drop for each diode in the segment. An advantage of this circuit is that all the LEDs will light with equal intensity, because each has the same amount of current passing through it.

If the LED had a forward voltage drop of 2.2 V, and the supply was 5 V, then the required voltage drop across the resistor would be (5 - 2.2 = 2.8) 2.8 V.

The current through each LED segment is usually around 20 mA, which is 0.02 A. Therefore using Ohm's Law we can find the value of the current limiting resistor using V/I =R. Hence, (2.8/0.02=140) 140 Ω resistor is required, however 150 Ω is the closest in the E24 series.

Single Resistor - The Wrong Method

If you were to connect a single resistor at the common end, then the current flowing through this resistor will no longer be a constant and will vary depending upon the number of lit segments.

Remember that each segment is driven individually with its own voltage and current, therefore the current through the common resistor will change depending upon the number of lit segments. A disadvantage of this method is that the brightness of the segments will change, which is undesirable.

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