Electrical systems part4: resistors make control possible

  Hello, and welcome to the latest installment of electrical system posts. Resistors are basically components that reduce the voltage within a circuit. They are used to control the speed of motors, to allow a computer to read switch position inputs, and in sensors to read temperature, pressure, and fluid level. So circuits for cab climate control blowers, windshield wipers, and gauges, all contain resistors, usually more than one.
  A resistor is a component made of a material that resists the flow of electricity. In reality, all materials are somewhat resistive, but carbon, and alloys of nickel and copper, or nickel and chromium are common resistor materials in the automotive industry. An important property of a resistive material is that, for a given cross section area, resistance is proportionate to length and cross section area. If you have two wires of the same diameter and material, a wire 4" in length will have twice the resistance of one 2" long, but a larger diameter wire will have less resistance than a smaller wire of the same length. This may seem wrong, but think of what would have more resistance to water flow, a large or a small diameter hose? A longer hose or a shorter one?

  You can easily make a resistor and test these principles. The following could also be fun for kids to help with. Take a piece of paper and a soft lead pencil, and draw a long, thin rectangle on the paper. The rectangle should be as dense and consistent as possible for the best results. If you now take a multimeter set to Ohms and touch one probe to each end of the rectangle, you will get a certain resistance reading. If you move one probe to about the middle of the rectangle, the Ohms reading should be about half. Even more interesting is to take that same rectangle and a AA or AAA battery and check the voltage drop through your resistor. First, measure the voltage of the battery with your meter and remember it. Now set one end of the battery on one end of your resistor, put one of the meter leads on the free end of the battery, and touch the other lead to the opposite end of the resistor. The voltage reading should be less than the battery voltage. Now move the meter lead from the end to the middle of the resistor, and the voltage should increase. You have just simulated exactly how things like fuel level senders, pressure sensors, and throttle position sensors function. The computer sends a 5volt reference to the sensors and the amount of voltage that comes back from each tells the computer the position or pressure of that sensor.
 Watch this video to see me do it.     
http://www.youtube.com/watch?v=MB8SNQnNagg
The same idea of a voltage drop through a resistor can also be used to give cab blower motors multiple speeds by using a selector switch to direct power through different resistors and then to the motor. This same method gives wiper motors intermittent settings when different voltages are sent to a circuit board or a computer.
  Thanks for your time, and continue checking this site for the next installment.
Bruce