Classes/2010/VIS147A/Lab2

From Robert-Depot
Jump to: navigation, search

Lab 2 - Voltage, Current, Resistance

Topics

  • Electricity flow.
  • Voltage, Current, Resistance.
  • Ohm's law. V = I * R
  • SI units. (Volts, Amperes, Ohms)

Supplies

  • 4 6V bulbs datasheet "MSCP" stands for "Mean Spherical Candlepower" [1] p. 507 Scherz.
  • 4 1K resistors. (Brown-Black-Red. for reference see the inside of the second page in Scherz)
  • 12 V power supply
  • LM7805 5V regulator datasheet
  • wire clippers, wire strippers, pliers.
  • multimeter
  • red and black hookup wire

Activities

Part 1: Set up your 5V power supply

Set up the voltage regulator on your breadboard. This chip, the LM7805 is a "5 volt regulator", meaning it takes an INPUT voltage ("+12V" from your adapter), and turns it into an OUTPUT voltage, +5V. It also needs to be connected to GROUND. This is a diagram of the regulator:

7805 schematic.png

Notice the regulator has three legs. Each of these legs has to be connected to the right thing. From page 2 of the datasheet for the regulator [2], under "Pin Assignment" we see what each of the legs are. Looking at the front, they are numbered 1, 2, and 3, from left to right, and are labeled "Input", "GND", and "Output":

7805 pinout.png

TIP: use your pliers to insert the 7805 into the breadboard. Its legs are thicker than your typical wire/resistor/LED, and sometimes are hard to get into the board. On the breadboard the circuit will look like this:

7805 on breadboard.jpg

Set up a light bulb to be a power indicator, as shown in the images below:

7805 schematic w bulb.png

7805 with power light.jpg

Connect your power supply. If your board has power, the bulb should turn on.

Be careful with the regulator when the power is on, it may get hot!!!

QUESTIONS. With your multimeter, answer the following:

  • what is the voltage across the bulb?
  • what is the voltage across the input to the voltage regulator?
  • what is the voltage across the output from the voltage regulator?
  • how much current is flowing through the bulb? According to the datasheet for the bulb [3], how much current should the bulb draw? REMEMBER: measuring current with a multimeter is different than measuring voltage. You need to "break the circuit" and re-connect it through the multimeter.

Part 2: Series and Parallel (with bulbs)

For each of the following diagrams, rate the relative brightness of the bulbs. Use a scale of 1 to 5, with 1 being brightest and 5 being most dim. You can sketch them by hand and do this on your own paper, or you can print out the images from this pdf [4].

To get you started, the bulb in diagram 1 will have a brightness of "1"--the brightest a bulb will be in any of the circuits.

Series parallel1.png

Series parallel2.png

For help, see Scherz p 49, "2.11 Electric Circuits".

Part 3: Series and Parallel (with resistors)

Working in pairs, wire up the circuits and verify the voltages and currents you calculated for circuits 1-5 in last week's homework. Get help from your TA in determining which resistors are which using the resistor color code. The color code is described on the back side of the first page in the Scherz book.

Don't build circuit #6!! I will show it to you in class.

...but here is the rationale if you want to think about it:

  • We have a 5 Volt supply and a 10 Ohm Resistor.
  • The resistor is rated at 1/4 of a Watt, or 0.25 Watts.
  • P=I*V. P=I*5V.
  • this amounts to a brief (and cautionary) demo of what a power rating is, and why it is important.