Are electronic devices really in a very low power mode when on stand-by ?
Also, how much energy am I wasting for powering devices when not in use ?
I wanted to check it out and decided to build a device capable to detect stand-by mode of my electronic devices and start metering energy used. All this had to be done automatically without my intervention. Metering would start upon entering stand-by and stopped upon exiting.
To do so I needed an energy meter and a current gauge to monitor the current of mains supplied devices and start/stop metering.
For the meter I first looked for silicon, of course. Energy metering is a hot issue with a lot of chip makers and many of them supply cheap solutions.
The project was proceeding when recently I was lucky enough to find two electromechanical meters at a local special trash collecting center (electrical, white goods, furniture).
Actually there should be a plenty of these scrap meters available here as the local electric company is replacing these meters with remotely controlled electronic meters.
I took them along with two VCRs and a printer. The first meter I opened and dismantled to satisfy my inner primary need. The second meter I decided to use in place of the silicon-based one; also, the ready made electromechanical meter solved the calibration issue.
This PopSci contest made me hurry and change priorities in my to-do-list, so here is my design.
Schematic V0.2 is an improved version of the electronic control box over the previous one. I added a potentiometer to set the hysteresis level. This helps discriminate power on vs. stand-by for noisy power supplies like some switchers are. This also helps get firmer metering on/off states.
The schematic shows in red the differences with respect to previuos version.
Step 1: Caution, Safety First !
Before starting, you must be absolutely aware that this thing is powered from the mains and
as such it could kill you, cause damage or injuries. If you are not really skilled at mains
powered electronics and related safety building practice and are not well aware of the risk related, you are suggested to have a friend help you with this project.
Also, as a general rule, when you are working on dangerous things always have someone next to you instructed on what to do if something goes unexpectedly.
Most parts of the circuit should not be considered safe to touch when the circuit is powered on.
Keep low voltage and high voltage wiring as separate as possible. The relay is the point where the two worlds are closer. Choose a good relay and have the wires soldered firmly. Tape well and possibly use heat-shrink tube.
These notes are not just to scare or bother anyone, but I absolutely want that fun does not turn
Step 2: How It Works. Electromechanical Meter
Electromechanical meters are described very clearly here.
The key point: metering is based on the product of two electrical entities, current I and
voltage V; power is the product of these two entities, V and I. Energy is calculated integrating
over time (that is adding together time after time) the V*I products. It is energy what we are
Within electromechanical meters V and I feed two coils: the voltage coil (which is in parallel to
the line) requiring low power and a current coil (in series with the line) made of thick copper wire.
Metering can be stopped or started interrupting the connection to the voltage coil.
Step 3: How It Works. the Current Monitor
The current required by the electronic devices under test is monitored through a current transformer.
I made it out of a regular wall adapter: I removed the low voltage secondary winding and replaced it with 4 turn of paired 1.5 square mm insulated wire. Current flowing through the new winding produces a voltage across the untouched original 110Vac side of the transformer. The higher the current, the higher the voltage.
The untouched 110Vac primary is then connected to a filter stage, a variable gain amplifier
a full-wave rectifier, a peak detector and a threshold detector feeding a transistor and a low
power relay. The contacts of the relay are in series with the voltage coil of the meter.
The gain of the amplifier stage is regulated through a potentiometer to set the threshold stand-by / operational.
You can find the detailed schematic down here.
All the parts are pretty common and most of them can be salvaged from other electronic devices.
Almost all the electronic components I took from a VCR and an automatic IR night light.
Step 4: Putting It All Together
I had a piece of wood cut to measure to make a stand the size of 25x25x8 cm.
Attached to it the meter, an avory-white plastic box containig the input and output cabling and the
current transformer. Finally attached a gray plastic box with all the electronics. The relay I put
close the wiring points of the meter. This also adds to safety.
The wiring runs at the back of the stand. I dremel-led the paths for the wires in the wood and kept the wires in place with duct tape. Next step will be to add a 3-4 mm thick piece of plywood to cover everything.
Now, check the wiring again, make sure you did not connect in parallel anything supposed to be in series, this applies expecially to the meter.
Rotate both gain and hysteresis potentiometers counterclockwise (minimum resistance, i.e. minimum gain and wider hysteresis window).
Connect the electronics you want to test. Plug the meter.
1.) Turn off your electronics. Rotate the hysteresis potentiometer clockwise until the relay energizes (and the LED turns on) and the meter wheel starts rotating. if nothing happens rotate the hysteresis potentiometer clockwise until the relay energizes.
2.) Turn your electronics on and the relay should de-energize. If nothing happens, turn the gain potentiometer a little further clockwise (higher gain).
Repeat the 2 steps above again to trim.
Should you find difficult or impossible to set the threshold, likely the stand-by mode current is close to the operating mode. This means your equipment consumes almost the same regardless powered on or in stand-by.
Once you know the energy (in Watts per hour) used by your equipment, you may also want to know the average power dissipated by your electronic device. This can be done if time is counted along with the energy. That is, if 10 Wh were metered in 100 hours, an average of 10/100 = 0.1W is the power dissipated by the equipment. For this purpouse a battery operated wall clock timepiece can be connected at the points +/-1.5V in the schematic.
The timepiece will be powered (and then advance) only when the relay is on.
Step 5: Final Notes
If you just want to have an idea of the stand-by current with relation to operational power, you may just want to build the current transformer and connect the filtering resistor and capacitor. A multimeter at the RC filter gives readings in the range of tens of mVolt to a few Volt. Values depend on the transformer used.
You will find that a number of electronic devices do not have a real stand-by mode. Rather they turn a red LED to green and respond to remote control commands, but they are fully powered on when on stand-by as well. This consumes a lot of energy and you can have the feel touching them: they are always warm (quite warm, sometimes !).
I have my home entertaining system connected to a switched outlet I switch on only when we are going to use it, otherwise my 5.1 system would be always hot.
I am not sure that this will cut bills and solve related problems, as shown in the drawing Michele made a few moments ago, but I am absolutely sure that resources we save today will be handy in the future to Michele and his brother.
Ciao a tutti