A $5 DIY Power Meter

Power
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May 22, 2002 -- by Mike Chin

Postcript added, June 1, 2002 Please see end of article for a reader's comments.

Measuring PC power consumption is not a task that interests the average computer user. However, if you are interested in minimizing noise in your PC, then you should be concerned about how much heat is dissipated inside the PC. If power dissipation inside a PC can be reduced, the heat generated is reduced proportionately, making it easier to cool with fewer, slower and quieter fans. By measuring the overall AC power consumption, it is possible to obtain a pretty good idea of just how much heat is being dissipated, and how much power your system really requires.

PC hardware engineers have specialized tools by which individual components power consumption and dissipation can be accurately measured. Not here at Silent PC Review. No, we make do with whatever tools can be obtained cheaply, borrowed or in some cases, made. This is one of those cases.

For general purpose consumer use, electricity monitors such as the one pictured above work fine to measure power consumption, but they cost upwards of US$100. It's also more than I need. Our electronics advisor Tommy Yee suggested I could build one really cheaply. With our shoestring budget, that sounded much better. After chatting with Tom for 10 minutes, I decided to get to work. My super simple power meter took half an hour to make.

SUPER SIMPLE POWER METER

Basicially it is an ordinary AC cable that has been modified with a 1-Ohm resister in series with the hot lead. A pair of screw terminals permanently connected to the resistor leads allows multimeter proble ends to be attached easily to measure the voltage drop across the resistor.

Here's the final result:

The parts, about $3-5 total cost:

  • 1-Ohm, 10W resistor
  • AC cord
  • 2-screw terminal
  • 1 plastic tie strap

Tools:

  • soldering iron
  • solder
  • small cutting pliers
  • electrical tape

I had all the parts kicking around except the resistor, which cost under $1. A multimeter is also needed, but I've had one for years, so that was the total cost. The meter does not read power in Watts nor current in Amps directly, but it's pretty close.

The ouside insulator of a standard computer AC power cord was spliced open about 2 inches and cut away. Then about 1.5 inches of the "hot" lead was cut away. The resistor was soldered in place of the missing bit of wire, and the 2-contact screw terminal also soldered in parallel with the resistor. The terminal was strapped to the resistor with the plastic tie strap to keep it secure. The terminals allow measuring probes from a multimeter to be easily attached. Lots of electrical tape was then wrapped around the solder joints.

A bit more detail:

HOW IT WORKS

First some basic Ohm's Law:

  1. I = V/R
  2. P = I*V

I = current in Amperes
V= voltage in Volts
R= resistance in Ohms
P= power in Watts

In a simple closed electrical circuit, the same amount of current runs through every component. If you know the resistance of any single component and the voltage drop across that resistance, then using equation 1 above, you can calculate the current in the circuit. Once you know the current, then you can calculate the power dissipated in the circuit using equation 2.

After the PC is powered up with the modified AC cable, measure the voltage drop across the resistor. I = V/R; therefore, the voltage value = current (I) in the circuit (because of the 1 ohm value of the resistor; i.e., V/1 = V). The current in the circuit is the same through the PSU as through the resistor. Multiply the current by the voltage (in my case, 120VAC) to obtain power in watts. The voltage loss across the resistor is insignificant to the PSU, as it is unlikely to be anything over 2V, even for a fully loaded system. From 120V, it's insignificant.



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