Power measurement

Methods of current measurement

Series resistor - put a small resistance in series with the load and measure voltage drop across it. Simple, but wastes lots of power thereby defeating my whole purpose.

Splitcore Current Transducer (SCT) - a sensing coil around the conductor Only works for AC measurements since the coil can only sense a changing magnetic field.

Hall-effect - basically you put a hall effect device next to the wire. Allegro makes some specifically for monitoring current. So does LEM. They can sense AC or DC since hall effect can measure the magnetic field statically.

Current taps

I happen to have a couple sct's left over from my halcyon days at Coactive Networks. They are Magnelab Model File:Sct-0750.pdf-100 and they say "100A to .333v Full Range". that means that when 100 amps runs through the cable, it puts out 1/3 volt DC.

From the datasheet, it puts out .066 volts at 20% or 20A in this case.

This is good for whole house. You put one around each cable coming into the breaker panel. 100A is a bit much for other applications though. I would like something in the 15 amp range (one circuit breaker)

You can buy direct from the factory; they are $46 each. If you don't need the split core feature you can get sensors in the $30 range.

Other sources

CR Magnetics has both SCT and Hall-effect sensors

Digikey Search catalog for Current Transducer or Current Sensor

Data loggers

What do I connect my sensors to? A logger records information which can later be transferred to a desktop computer typically via serial port for analysis. Using a logger to buffer data means you don't have to have your computer turned on all the time to log data in real-time. You can leave it plugged in all the time, and just grab data when the computer is powered on.

Onset	Hobo	< $100	sells matched sensors including split core transformers
Pace Scientific	Pocket Logger	$499	expensive but full range of accessories

Remote locations

This won't work if your sensors are located up the street in another building. How can we move the data over a longer distance?

Dialup modem? Sure. Connect an external modem to the serial port and program it to answer the phone. This is fine if you still use modems :-) and have a phone line available at the remote site.

Network connection or WiFi connection - Well now, this could be where the WRT54GS project comes in handy. For around $60 and a little hacking I can connect a WRT54GS access point to the data logger via RS232 serial port, and it can then act as the bridge to the network via either its 5 ethernet ports or its Wifi port. In fact for this tiny app I could probably also just use one of my old USR2450 WiFi modules. They already have serial ports built in.

AC line monitoring

Appliance level

This is a quick way to get a power reading at the wall outlet. There is a logging alternative to the Kill-a-watt. There is the Wattnode. Both of these are good options for monitoring at the wall outlet.At this point though, I want to be able to measure internal components too.

Internal DC measurements

• Measure power supply efficiency. (Compare at wall and at power supply outputs)
• Check effectiveness of cpufreq and powernowd.
• Compare power used by different hard drives and other components.
• Compare power use with and without WiFi board installed.
• Figure out how to power down unused components and see if it works.

Ideally I want to be able to measure power over long periods of time, so connecting my trusty Fluke 77 and watching the display is not an option.