Ohm's Law

Design Challenge 4: Current-Limiting Resistor

Ohm's Law:

Design Challenge 4: Current-limiting resistor


Resistors in electrical circuits are commonly used to provide other components in the circuit with the voltages and currents they require in order to function properly. For example, in this exercise, we will design our circuit (i.e. choose a resistance value) to ensure that an LED receives the voltage necessary for it to light up without allowing excessive current, which could burn out the LED.

This exercise uses concepts introduced in our experiment on Ohm's law. A link to this experiment is provided at the right.

Step 1

The circuit we will build is displayed below. We are using a 5V source to light up an LED; we need to choose the resistor R so that the LED specifications are met. A review of the LED datasheet indicates that the LED requires at least a 2V voltage difference in order to light up. The datasheet also indicates that the absolute maximum forward current through the diode is limited to less than 30mA. Based on the information in the LED datasheet, we will set the following design requirements:

  • Minimum diode voltage difference is 2V.
  • Diode current is between 5mA and 10mA (this is sufficiently below the maximum allowable current to ensure that the diode won't burn out).

Note: Design requirement (a), in conjunction with Kirchhoff's voltage law means that the voltage drop across the resistor must be approximately 3V.

Choose a value for the resistance, R, which meets the above design specifications. Use only fixed resistances from the Digilent® Analog Parts kit.

Step 2

Construct the circuit you designed in Step 1 and measure the diode voltage difference and the current, I. Are the design requirements met?

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