Voltage, Current, Resistance and Power for John Deere Program

Prepared by Charles S. Tritt, Ph.D.
November 2, 1999

Equipment (for each pair of students working together)

12 V (nominal) bulbs and sockets

Each pair of students will also need two banana to banana patch cords from the rack on the wall in the lab.

Introduction

In this exercise you will measure the voltage, current, resistance and power relationship using automotive type lamp and power supply.

Safety Considerations

Dangerous line voltages are present at the outlets on your work bench. Do not insert anything other than approved power plugs into these outlets.

The light bulbs you will be using in this experiment will get hot. Keep them away from combustible materials and handle them with care.

The voltages used in this experiments are low and relatively safe. However, do not intentionally make yourself part of your circuits.

Procedures

Initialize the Power One source of the Elenco Power Supply as follows:
Set Meter 1 to Volts mode.
Turn the Volts knob fully counterclockwise (minimum).
Turn the Current Limit knob fully clockwise (maximum).
Place its Power switch in the ON position.

Initialize the Fluke digital multimeter (DMM) as follows:

Select the mA (current) mode.
Select the 2000 range.
Select the DC mode.
Place its Power switch in the ON position.

Connect one of the 12 V (nominal) bulbs across the Power One output of the power supply via the DMM so that the current through the bulb can be measured. Do this by 1) connecting a patch cord from the 0-20 V 1 A + (red) terminal on the power supply to the mA terminal on the DVM, 2) connecting a patch cord from the Common DMM terminal to either cord on the bulb socket, and 3) connecting the other bulb socket cord to the - (black) terminal on the power supply.

While watching Meter One on the power supply, slowly increase the voltage applied across the bulb in 2.0 volt increment and complete the table below:

V (volts)I (amps)P (VA or Watts)R (V/A or Ohms)
2.0   
4.0   
6.0   
8.0   
10.0   
12.0   
14.0   

Explain why the resistance increased as the voltage (and current and power) was increased.

Return the power supply Volts knob fully counterclockwise. Add a second bulb to your circuit in parallel to the first bulb. Calculate the expected currents and complete the following table. Do not apply more than about 4.0 volts to your bulbs or you'll blow the power supply output fuse.

V (volts)I predicted (amps)I observed (amps)
2.0  
4.0  

Return the power supply Volts knob fully counterclockwise. Reconnect the second bulb so that it is in series with the first bulb. Calculate the expected currents and complete the following table.

V (volts)I predicted (amps)I observed (amps)
4.0  
8.0  
12.0  
16.0  
20.0  

Return the power supply Volts knob fully counterclockwise, turn off the power supply and DMM and disassemble your circuit.