Question

Problem 4.0 (20 Points) a Specify the reason to include the current limiting resistor for each segment of a 7-segment display. Draw the circuit diagram with current limiting resistor for common anode and common cathode displays. (10 Points) If the LED has a forward voltage drop of 2.5 volts, and the supply is Sv, and the current b) through each segment is 10mA. Based the specifications compute the current limiting resistor. (10 Points)

Answer 1

1. Why do we use current limiting resistors in each segment of 7 segment display?

A. A current limiting resistor is a resistor that is used to reduce the current in a circuit.

We often use current limiting resistors in 7 segment display to protect LEDs from burning. If high current will flow from led high amount of heat will be generated and since led's are semiconductor device thermal breakdown will occur and led will burn. So to protect led from thermal breakdown we use current limiting resistors in each segment in 7 segment display.

CIRCUIT DIAGRAM WITH CURRENT LIMITING RESISTORS.

w circuit Diagram of 7-segment Display "I current Limiting to common Amade Resistons Tom a Top - For this we use driver circuit

here limiting resistor value is taken as 220 ohms. It varies as we will vary the supply voltage.

In this example, the segments of a common anode display are illuminated using the switches. If switch a is closed, current will flow through the “a” segment of the LED to the current limiting resistor connected to pin a and to 0 volts, making the circuit. Then only segment a will be illuminated. So a LOW condition (switch to ground) is required to activate the LED segments on this common anode display.

But suppose we want the decimal number “4” to illuminate on the display. Then switches b, c, f and g would be closed to light the corresponding LED segments. Likewise for a decimal number “7”, switches a, b, c would be closed. But illuminating 7-segment displays using individual switches is not very practical.

2. GIVEN

Forward voltage drop of led = 2.5V , Supply voltage = 5V , And current through each resistors = 10mA.

# Consider ane of the segment &- I=10 MA SV Now using KVL- weget - - Vg + Ve ED + It Rg=0 -5+254IR=0 -5+254 10X153 & R2=0. R4 = 5-25 10* 10-3 102 Thy = 250.2 , Similarly for all other segments que Rz, kz--... Rg we will get the same value of resistance which is equal to 2502

CONCLUSION - For safe operation of the 7 segment display for specified value of supply voltage and limiting current, atleast 250 ohm value resistance should be used.