Objective

The goal of this project is to build a circuit for testing power consumption by low-power (< 25 W) transformers and switching power supplies (SPS). You will then use your circuit to measure power consumption for several of these devices in the both the idle state (i.e., plugged in to house current, but not powering a device) and under load (i.e., connected to a 20 ohm, 25 watt power resistor).

Introduction

Note: It has come to our attention that the "Vampire Watt Meter" described in this project does not accurately measure power consumption, since it does not measure the relative phase of the voltage and current, which is necessary when measuring AC power. We're working to come up with an alternative device, and when we do, we'll post a revised project. In the meantime, Science Buddies recommends that you not pursue this project as written, since the results you obtain are likely to be inaccurate.

In 2004, the California Energy Commission passed new regulations on so-called "energy vampires" that will take effect this year (2006). Many small household appliances, such as answering machines, phone chargers, VCRs, battery chargers, etc., have transformer-type power supplies that consume between 2 and 10 watts of electricity even when the appliance is not in use. In addition, many of these power supplies are not very efficient when they are in use. The new regulations will require these external power supplies to use between 1 and 3 watts when in use, and 0.5 watts or less when idle. California regulators calculate that the regulations will save California's businesses and households more than $3 billion over 15 years. (AP, 2004)

How many of these power supplies can you find in your house? How much electrical energy are they wasting? How much money could the new regulations save you?

In this project, you'll build a "Vampire Watt Meter," a relatively simple circuit that you can use, with a digital voltmeter, to measure the power consumption of low-power external power supplies. To do this project, you need to have an understanding of basic electrical circuits and Ohm's law.

Safety note: this project involves building a device that will carry household current. It should be safe if built and used as instructed. As with any project that involves high-voltage electricity, proper precautions must be taken.

• Use a household circuit tester to verify that your wiring is correct before making any measurements with the Vampire Watt Meter.
• Pay attention to the wattage ratings of the resistors specified.
• Use the specified 1 A fuse.
• The Vampire Watt Meter should only be used with low-power external power supplies (15 V DC or less).
• Do not touch the metal parts of the voltmeter probes when taking measurements from the Vampire Watt Meter.
• Adult supervision is required for this project.

Terms, Concepts and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• voltage,
• resistance,
• current,
• Ohm's law,
• calculating electric power.

Questions

• If you know the voltage and the current in a circuit, how do you calculate the power consumed?

Bibliography

• Henderson, T., 2004. "Electric Circuits," The Physics Classroom, Glenbrook South High School, Glenview, IL [accessed April 7, 2006] http://www.glenbrook.k12.il.us/gbssci/phys/class/circuits/u9l1a.html.
• AP, 2004. "California Regulates 'Energy Vampire' Appliances," Associated Press, published in USA Today [accessed April 7, 2006] http://www.usatoday.com/tech/news/techpolicy/business/2004-12-16-power-vampires_x.htm.

Materials and Equipment

• two double duplex electrical outlets,
• a metal or plastic electrical box for the outlets,
• clamp for knock-out hole in electrical box,
• a 4-outlet cover,
• wire nuts (3 for 14 gauge, 1 for 20 gauge),
• electrical tape,
• a pair of 2 ohm, 0.5 watt resistors,
• a 20 ohm, 25 watt power resistor (resistors are available from Mouser Electronics, http://www.mouser.com, 1-800-346-6873; the 2 ohm, 0.5 watt is part number 293-2.0-RC, and the 20 ohm, 25-watt is part number 280-CR25-20-RC),
• selection of DC power jacks for connecting power supplies to power resistor (jacks are female, the male part is the plug; available at Radio Shack, size K 274-1565, size M 274-1563, size N 274-1576),
• three-wire 120 V AC power cord and plug,
• solder (rosin core),
• soldering iron,
• screwdrivers,
• wire stripper/cutter,
• digital voltmeter with insulated probes,
• a few short pieces of insulated 14 gauge and 20 gauge solid copper wire,
• heat shrink tubing or electrical tape,
• a fuse holder,
• 1 A fuses,
• house current circuit tester,
• a selection of "Vampires" (low power transformers and switching power supplies) to test, for example:
  • transformers for answering machines, portable CD players, phone chargers, battery chargers, etc.;
  • switching power supplies for laptop computers.

Experimental Procedure

Safety note: this project involves building a device that will carry household current. It should be safe if built and used as instructed. As with any project that involves high-voltage electricity, proper precautions must be taken.

• Use a household circuit tester to verify that your wiring is correct before making any measurements with the Vampire Watt Meter.
• Pay attention to the wattage ratings of the resistors specified.
• Use the specified 1 A fuse.
• The Vampire Watt Meter should only be used with low-power external power supplies (15 V DC or less).
• Do not touch the metal parts of the voltmeter probes when taking measurements from the Vampire Watt Meter.
• Adult supervision is required for this project.

Building The Vampire Watt Meter

Figure 1, below, shows the schematic of the Vampire Watt Meter circuit. It is a relatively simple device, built from two (grounded) duplex outlets, an electrical box, cover plate, grounded electrical cord, fuse, resistors and connecting wires.

Figure 1. Schematic of "Vampire" watt meter circuit.

The narrow socket of the outlet is "hot," (120 V AC), the wide socket is neutral and the round socket is ground. The 1 A fuse is connected in series with the "hot" side. A 1 ohm, 1 watt resistor (OK to 1 A) is connected between the neutral sockets of the two outlets. The 1 A fuse protects the resistor from overstress if the load on the right-hand outlet exceeds 120 watts (1 A of current). The left outlet is at nominal 120 V AC, and the right outlet will be at a slightly lower voltage when under load.

Notice that by measuring the voltage drop between the two neutral sockets (i.e., the voltage drop across the 1 ohm resistor), you can calulate both the current being drawn and the power dissipated by the transformer or switching power supply plugged into the right-hand outlet. A difference of 1 volt between the two outlets would correspond to a current of 1 A (or a load of 120 watts) on the right-hand outlet (verify this for yourself using Ohm's law and the formula for calculating electrical power). Thus, for every 1 mV measured between the neutral sockets of the two outlets, the device plugged into the right-hand outlet is consuming 0.12 watts of power.

Your grounded power cord will have three wires. The green wire should be connected to the ground terminals (usually green) on the duplex outlets. The white wire should be connected to the neutral socket (bright screw terminal, corresponds to wide spade on plug, wide socket on outlet). The third wire (either red or black), should be connected to the fuse socket, which should then be connected to the "hot" socket (brass screw terminal, corresponds to narrow spade on plug, narrow socket on outlet). Use insulated 14 gauge wire and wire nuts to make connections as shown in Figure 2, below (bare 14 gauge wire is OK for ground connection, as shown). Trim the leads and use solder to connect the two 2 ohm, 0.5 watt resistors in parallel. One pair of resistor leads can be connected to the neutral terminal of one duplex outlet. Solder a 14 gauge connecting wire to the other pair of resistor leads, cover all exposed metal with heat-shrink tubing (or electrical tape), and connect the other end of the wire to the neutral terminal of the second duplex outlet (refer to Figure 2, below).

Figure 2. Front and back views of actual circuit.

Before using your Vampire Watt Meter, double-check all of your wiring and make sure that it is correct. Attach the cover plate over the outlets. It would be a good idea to label the cover plate so that someone doesn't think that you've made a handy extension cord. Remember that this circuit is designed for testing low-power DC power supplies, not for general use! Plug the cord into a grounded outlet, and use the circuit tester to check your wiring. If you wish, you can also check with the digital voltmeter. We highly recommend that you unplug the Vampire Watt Meter before inserting the voltmeter probes. Make sure the voltmeter is set to AC volts, and that the scale is set to 150 V AC or greater. Never touch the metal portion of the voltmeter probes when connecting to or disconnecting from the Vampire Watt Meter! When the probes are securely inserted, then plug in the Vampire Watt Meter and take your reading from the digital voltmeter (see Figure 3, below).

Figure 3. Testing the circuit. Use the circuit tester (plugged in to top left outlet) to do the initial check-out of your Vampire Watt Meter. Then, use the voltmeter. Unplug the Vampire Watt Meter before inserting the voltmeter probes. Never touch the metal portion of the voltmeter probes when connecting to or disconnecting from the Vampire Watt meter! At left, the voltmeter is showing 122.9 V AC between hot and neutral. At center, the voltmeter is showing 122.3 V AC between hot and ground. At right, the voltmeter is showing 25 mV AC between neutral and ground. All of these values are normal.

Connectors to Attach Low-Voltage Power Supplies to Power Resistor

Figure 4, below, shows an array of different-sized jacks connected in parallel to the 20 ohm, 25 watt power resistor (white "slab" component in photo). This is a handy and safe way to connect your test power supplies to a load of known wattage. Use insulated 20 gauge wire to make the soldered connections. The jacks should accommodate all different sizes of power-supply plugs.

Figure 4. Array of jacks connected in parallel to power resistor (white "slab" component). Shown are (clockwise) the blue wire nut, a series of DC power jacks, and the 20 ohm-25 watt power resistor to simulate a load on the SPS or transformer.

Measuring 'Vampire' Power Dissipation

You will be measuring the voltage drop between the neutral sockets (wide socket, corresponds to wide spade of a plug) of the left- and right-hand outlets. Use the top outlet, and reserve the lower outlet for the power supply to be tested. Figure 5, below, shows that there is no voltage drop between the neutral sockets when nothing is plugged in.

Figure 5. As expected, there is no voltage between the neutral sockets when nothing is plugged in.

Since the Vampire Watt Meter is designed to draw a maximum of 1 A, you should only test power supplies rated at 15 V DC or less to stay comfortably within the safe range.

1. As shown on the left side of Figure 6, test the power supply with no load (not connected to an appliance or the power resistor). Write down the voltage reading in your lab notebook.
2. Connect the power supply to the 20 ohm, 25 watt power resistor, using the appropriate jack. Write down the voltage reading in your lab notebook.

   Figure 6. At left, the voltmeter measures 9 mV AC between neutral sockets when idle SPS is plugged in. At right, the voltmeter measures 123 mV AC between neutral sockets when SPS is plugged in under load.

3. Use Ohm's Law to relate voltage, resistance and current:
   I (current in amperes) = V (voltage in volts)/ R (resistance in ohms)
   
   Thus, a voltage drop (I×R) across a 1 ohm resistor is equivalent to the current in amperes.
4. Power is the product of current and voltage. Thus, if you see a 20 mV AC signal across the 1 ohm measuring resistor while idle, that would correspond to a current of 20 mA AC, and a power dissipation of 2.4 watts (multiplying by nominal voltage of 120 V ac).
5. As an example, a 123 mV voltage drop corresponds to a current of 123 mA or a power dissipation of 15.1 watts as input to the SPS.
6. To calculate efficiency, compare the power consumption calculated from the Vampire Watt Meter (the input to the power supply), to the power consumption calculated across the load resistor (the output of the power supply under load). If the DC voltage delivered to the 21 ohm load resistor were 15.8 V, then the current would be 0.75 A and the power dissipation would be 11.9 watts. Thus, this particular SPS is 79% efficient under load (output power/input power = 11.9 watts/15.1 watts). The author found an efficiency of about 40% under load for a small, light-duty transformer with a nominal output of 6 V DC!

Variations

• Do background research to find out how much extra cost (if any) is added to make power supplies efficient enough to meet the new California standards. If the cost is higher, how long will it take for power savings to pay for the increased cost?

Credits

Richard Blish, Ph.D.,  

Edited by Andrew Olson, Ph.D., Science Buddies

Last edit date: 2006-06-08 12:00:00