Chapter 4 : Logical Thinking

EXERCISE TIME

A technician decides to use a potentiometer as a speed control for an electric motor. The potentiometer has a resistance rating of 10 Ω and a power rating of 25 watts:

 

When operating the motor at a voltage of 16 volts and a current of 2 amps, though, smoke begins to pour out of the potentiometer, indicating that its power rating is being exceeded.

The technician is perplexed! According to his calculations, the potentiometer should be dissipating less than 25 watts of power. Why, then, is a potentiometer rated for 25 watts burning up under this condition?

 

 

Connect the necessary wires in this illustration to make the potentiometer act as a variable voltage divider, providing a variable voltage to the voltmeter:

 

With a trough of water and three pieces of wire, you could make a liquid potentiometer:

 

A radio technician is troubleshooting a problem in a simple AM receiver, using a voltage-measuring device called an oscilloscope. An oscilloscope is nothing more than a graphical voltmeter, indicating the “wave shape” of voltages that change rapidly over time. The problem with this radio is that no sound at all is heard in the headphones.
When checking for a voltage signal between points A and B in the circuit, a strong signal is obtained: However, when checking between points C and B in the circuit, no signal is measured:

 

Calculate the voltage between points A (red lead) and B (black lead) (VAB).

 

 

Determine the amount of voltage dropped by each resistor in this circuit, if each resistor has a color code of Brn, Blk, Red, Gld (assume perfectly precise resistance values – 0% error):

Design a voltage divider circuit that splits the power supply voltage into the following percentages: