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1 2 NextMay 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 1Ohm 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 $35 total cost:
 1Ohm, 10W resistor
 AC cord
 2screw 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 2contact 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:
 I = V/R
 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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