Detecting small AC currents
Jonathan
Posts: 1,023
Hi All,
I'm working on some smart power saving stuff for my upcoming solar only home. I need to be able to detect small amounts of 120VAC current, on the order of a few milliamps. All of the Hall sensors I see have a much larger range. Any ideas?
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
I'm working on some smart power saving stuff for my upcoming solar only home. I need to be able to detect small amounts of 120VAC current, on the order of a few milliamps. All of the Hall sensors I see have a much larger range. Any ideas?
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
Comments
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- Stephen
The idea is this. Your average wall wart transformer is of course a thief, using power even when not supplying any. One I am playing with on the bench here is using almost 4 watts just sitting here. I want to be able to detect when the current is above the overhead, ie the thief is actually powering something. So in most cases, I would imagine the currents would be on the order of < 1000mA at 120VAC. This thief is pulling ~ 35mA with no load. SO I would like to be able to detect when the load increases a few mA, say 5 or less.
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot
Somewhere I don't follow how you can have it both on for detection purposes and off for conservaton at the same time if that is indeed what you desire.
Here is an alternative. I just divided my wall warts into two piles: ones that needed to be on at all times for standby purposes [noparse][[/noparse]mostly related to my computer systems- the gateway router, the ADSL modem, etc.] and ones that can be off except when needed[noparse][[/noparse]mostly battery chargers].
These are on entirely separate power strips. I turn on the charger strip when the cell phone, the PDA, and the electronic dictionary need to be recharged.
I suppose that I could have another power strip for secondary computer devices - like the audio, the outboard DVD player, etc. and switch those on concurrently with powering up the computer. But I haven't gotten that far. For now, they are switch on/off at the computer power strip when I can remember to be diligent.
I don't think a current sensor alone is logically going to power up on demand. You may need one relay or a series of sequenced relays to do the job.
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
The most responsible thing to do is to unplug (or switch off the source to) the wall wart when its not being used.
Use the available technology to reduce the losses in your inverter, which I assume you'll have in your solar home. Then again, if you have to heat your home, that inverter heat may come in handy in the cooler months.
Cheers,
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Tom Sisk
http://www.siskconsult.com
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Later!
kenjj
Kramer, The plan was (is?) to have the uC powered from the mains, prior to the transformer. The Uc would sleep most of the time, flipping the transformer and checking for a load every 50mS or so. If it finds a load, it stays on until the load is turned off. To power the uC I was thinking of a cheap and dirty 1/2 wave rectifier/zener kind oif thing.
Stamptrol, you are right that just unplugging is the best way. I'm just trying to account for the fact that I am lazy and forgetfull. While it may seem like there is diminishing returns, power bills aren't the issue, its coming up with the actual power that's the issue. Assuming I have 10 wall warts stealing 4 watts, that represents 400us$ worth of panels output for 1 hour on a sunny day.
Kenji, that is a cool product. I could easily make that myself, and for loads that always run at once like a PC that sounds great.
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If you really want to sense current, low side current sensing resistors connected to an op amp are very low power [noparse][[/noparse]but less accurate than high side current sensing resistors]. So I would consider using the low-side configuration. You really don't have to know the actual quiescent threshold as anywhere in the range above it and below the full on would be telling you the same thing.
You might consider setting aside the wall warts and targeting other more productive areas first.
In the average U.S. home, the hot water heater is by and far the greatest waste of energy - most homes have it working 24/7. But we are away from the house for more than 40 hours per week and sleep 8 hours per day. These periods don't need fully heated water if any at all. And if it is an electric hot water heater, it can be easily adapted to having a micro-controller optimize its use with SCRs. That might be a much better project. One can even use a PINK to have a computer manage a complex personal calendar,if that is what you need. With the PINK, one can even turn on the device before one arrives home or shut it down from afar.
Cycling up and down thermal items for energy saving can at times be counter-intuitive.
Living in Taiwan, I am very A/C dependent and my landlady has a watt meter attached to the A/C outlet that I read monthly. When I first got the A/C installed, I ran it only when I was home as a means to saving on the total bill. I would come home to a room that was 100 degrees and 60 percent humidity. As the humidity rose during the summer season, I found that the A/C alone wasn't enough.
So, I plugged in a fan to run 24/7 in the opposite corner of the room and finally I gave up and left the A/C on 24/7. I still leave one small window open to provide fresh air to the room. But the monthly electric bill is LESS than having me periodically run the A/C. I also don't have interior humidity problems that I had before.
It just happens that the concrete walls cool down and stay cool. They provide significant insulation. I also don't have much heat gain from the windows as they are on the north side of the building. I am on the 2nd floor of a 6 floor bulding, so both ceiling and floor seem to stay cool too. The real savings seems to be in running that fan to make sure the air circulates throughout the room properly.
Refrigeration is another big domestic energy user. Self-defrosting refrigerators can go haywire and really run up a bill. Also, I've had friends suffer huge electric bills from acquiring an additional freezer that they thought was going to save them big bucks on grocery bills. They would buy a side of beef, put it in the 2nd freezer and discover that the reason the freezer was cheap is that it cost a lot to run. The smart crowd rents a drawer or locker in a freezer warehouse and pay a set monthly fee that is more reasonable.
Having a computer without a traditional harddisk is a big savings too. I recently got an Asus eeePC with 4gigs of solidstate storage. Most of those 300 watt power supplies are being used to spin the hard disk. This tiny computer can stay on all day for a fraction of what my other conventional PC can.
Of course, if you get rid of the automatic dishwasher and use a compost pile in lieu of a garbage disposal; you might begin to wonder why you have all those appliances. One can grind coffee beans by hand in a few minutes; it was done for centuries and the results are just fine.
I suspect the wall warts are the least wasteful of your home appliances. The modern house today has 150-200 amps of service capacity. Take a good look at where it goes in the fuse box.
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PLEASE CONSIDER the following:
Do you want a quickly operational black box solution or the knowledge included therein?······
All very true. Hot water is indeed the biggest power user. In the last couple of weeks I have been converting my hot tub over to solar. Try keeping 300 gallons of water at 104 degrees if you want a challenge. [noparse]:)[/noparse] My objective is only to have the tub hot between 4:00 and 8:00 PM, not at perfect temp 24/7/365. The system uses solar thermal panels and a woodstove heating coil for the winter. It can still use propane if needed. Right now I still have a grid tie, so now is the time to learn, while I still have all the power I want to pay for. Time to get in all my high power experiments too, like playing with my Tesla coil that can emit 7' sparks. [noparse]:)[/noparse]
Thermal mass = good! My new place will be radiant floor heating. Funny you should mention meat lockers, I raise pigs and rent a locker. I don't have any other high consumption appliances other than washer dryer, and I line dry whenver I can, even waiting for a nice day to do laundry. Right now I have an average daily power use of 3kWH, so I still have a ways to go in terms of saving power.
With the other idea, I am just trying to figure out ways to use a little power as possible. I am putting the idea on hold for now, and will simply try to discipline myself, although I am going to build the PC power supply mentioned by Kenji. Good to know that the harddrive is the biggest power hog in a PC. At least I can set the PC to power down the HD.
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www.madlabs.info - Home of the Hydrogen Fuel Cell Robot