Power measurement
To tell if an energy conservation plan is working, we have to make accurate measurements of how much electrical power is being used.
Definitions
Power = voltage * current so to get power measurements we have to measure voltage and current.
Power is measured in watts (or milliwatts where 1 watt = 1000 milliwatts)
Voltage is measured in volts (or millivolts where 1 volt = 1000 millivolts)
Current is measured in amps (or milliamps where 1 amp = 1000 milliamps)
AC = alternating current: voltage swings positive and negative 60 times a second
DC = direct current: voltage is constant
Scale of measurements
Whole house: how much power is our whole house using? We want a fast and convenient way to read AC signals from 0 to 100 amps on both the phases coming into our house, 120v and 240v
Appliance level: how much power is our refrigerator using? We want to measure 0 to 20 amps. Ideally also at both 120v and 240v but primarily at 120.
Internal level: how much power is the motherboard using in my computer? We want to measure DC signals up to around 100 watts, low voltage and low current. 0-12v and 0-10 amps
Methods of voltage measurement
Come back later, I am working on the next section right now. Measuring voltages is easy anyway.
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. This makes it a bad choice for high current applications. It's also inconvenient to measure AC current because the voltage is changing.
There is no galvanic isolation as there is with other methods listed here. The sense amplifier has to use one side of the resistor as its ground point. These factors make it a bad choice for high voltage measurements.
SCT sensors
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.
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.
CR Magnetics has both SCT and Hall-effect sensors
Digikey Search catalog for Current Transducer or Current Sensor
Hall effect
Basically you put a hall effect device next to the wire. They can sense AC or DC since hall effect can measure the magnetic field statically.
Since all the hall effect sensors require breaking the circuit to install the transducer I am not interested in them. I don't want to disocnnect the 100A mains circuit. So the SCT's are a better choice for whole-house monitoring. One day maybe we will be able to buy plugin circuit breakers with hall effect transducers built in.
There are two variations on the hall effect sensors, open- and closed- loop.
Open loop sensors pass the current through a bus bar located next to the sensor. The bus bar is either a trace on a PCB or a flat copper plate.
Closed loop sensors put a coil around the conductor. The coil acts like an amplifier making the hall effect sensor more accurate. The coil requires current to charge it. The devices are more expensive. The conductor has to pass through the loop.
Available devices
[Allegro] makes some specifically for monitoring current. Allegro has nice eval kits available for $19.95
LEM makes current sensors too. Their devices are listed in the Digikey catalog but they don't stock them.
Flux gate
Small signals (100 to 400 milliamp)
Magneto-resistive
I did some reading on MR sensing but have not captured info here yet.
Data logging
Once we have a sensing system, we need a way to record the readings. We don't want to leave a desktop computer fired up all the time, so the sensors need to be connected to a data logger.
The data logger temporarily records information which can later be transferred to a desktop computer for analysis or to a display module to get direct readings.
| Onset | Hobo | < $100 | sells matched sensors including split core transformers |
| Pace Scientific | Pocket Logger | $499 | expensive but full range of accessories |
Data transfer
How do we get the data from the sensor to the logger and then on to the computer or display module?
Dialup modem? Connect an external modem to a logger serial port and program it to answer the phone. This is fine if you still use modems and have a phone line available.
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.
Bluetooth
Zigbee
Powerline: HomePNA, X10, LonTalk
AC line monitoring
Whole house
A pair of 100 amp SCT's installed in the main breaker panel and connected to a WattNode is the most common method. The WattNode can be either the pulse or LonTalk variety. One SCT monitors each phase and the WattNode reads voltage directly from a spare circuit in the breaker box.
Appliance level
A common way to get readings (no data logging option) is the Kill-a-Watt.
This is a quick way to get a power reading at the wall outlet. There is a logging alternative to the Kill-a-watt called the Watts Up. It's around $110.
You can use a Wattnode for appliace level monitoring too, but it's expensive.
Internal DC measurements
At this time, I am trying to get readings inside the box on my C3 computer. What I want to do:
- Measure power supply efficiency. (Compare at appliance level 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.
