chris@zorin.UUCP (Christopher Nielsen) (06/19/89)
I have an interesting problem...
I have a room, X. It has a 110V power line running to it. I want to detect
if there is any current being used in it. I don't want to place a resistor
inline, but rather, put a coil or something around the 12 guage conductor
leading to it, and some kind of amp driving a meter, or LED. Any
ideas/circuts?
I can probably run an get a clamp on type of meter, but I want this to be a
semi-permant installation, in a wall box. Hence, what I would like to have,
is a coil around the conductor in the wall box, 2 wires leading to an
external detector circuit.
Steve Ciarcia, of Byte fame, some months/years ago (I can dig it up) had
an interesting article about measuring power consumption of household
appliances with your CPU. It involved canabalizing a power strip, and
placing a number 1 paper clip inline with the outlets, and measuring the
power drop. I don't want to affect the current capacity, and don't want to
make a mess in the wall box, where I have to do my thing, though. Anyway,
in Steve's article, his circuits would output a pulse to the CPU, each time
.1 KWH, or so would be consumed. So instead of complicated parallel/data
latch/counter arrangement, you would simply count pulses. Very interesting,
but not quite what I'm looking for...
Thanks for listening...
Christopher A. Nielsen
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< Online Product Information and other services - Call or write for free demo >larry@kitty.UUCP (Larry Lippman) (06/20/89)
In article <184@zorin.UUCP>, chris@zorin.UUCP (Christopher Nielsen) writes: > I have a room, X. It has a 110V power line running to it. I want to detect > if there is any current being used in it. I don't want to place a resistor > inline, but rather, put a coil or something around the 12 guage conductor > leading to it, and some kind of amp driving a meter, or LED. Any > ideas/circuts? > > I can probably run an get a clamp on type of meter, but I want this to be a > semi-permant installation, in a wall box. Hence, what I would like to have, > is a coil around the conductor in the wall box, 2 wires leading to an > external detector circuit. No problem. Since your current draw will probably be 15 amps or less you want a better current ratio than one turn, so just wind about half a dozen turns of _solid_ #20 AWG insulated wire around the "hot" wire. Terminate your current pickup coil leads in some #22 AWG twisted pair to run to the electronics circuit. A good idea would be to use some silicone rubber potting compound to keep the pickup coil and its turns in a stable configuration. The above "pickup coil" MUST be terminated in a resistive load in your electronics circuit (try 50 or 100 ohms as a starting point). You want to use an op amp to measure the voltage across this load; NOT the open circuit voltage of the pickup coil. There are a number of RMS detector circuits available in any op amp application guide which you can use to provide a DC voltage to feed at an A/D converter or to a meter. If you want to measure watt-hours, you will have to integrate the DC voltage from the RMS detector. This is a trivial software task if you are using a microprocessor having realtime capability to monitor this circuit. You can perform the integration in hardware using a V/F converter and a counter circuit, but with inexpensive microprocessors this technique is becoming a thing of the past. If you do not care about great accuracy, you can assume your line voltage to be a constant. If you do care about accuracy, you need a separate line voltage RMS detector, and must multiply the current times the voltage signal. This is trvial in software using a second A/D channel, but a lot tricker (if you are not a good analog designer) in hardware since you will need to use a multiplier chip. Also, if you are going to measure the AC voltage, then use a small plug-mounted transformer for isolation; I do NOT recommend connecting directly across the power line! > Steve Ciarcia, of Byte fame, some months/years ago (I can dig it up) had > an interesting article about measuring power consumption of household > appliances with your CPU. It involved canabalizing a power strip, and > placing a number 1 paper clip inline with the outlets, and measuring the > power drop. This sounds like crap to me, and seems like an invitation to trouble. I would suggest staying with a safe, isolated current pickup coil. > in Steve's article, his circuits would output a pulse to the CPU, each time > .1 KWH, or so would be consumed. So instead of complicated parallel/data > latch/counter arrangement, you would simply count pulses. Very interesting, > but not quite what I'm looking for... You are going to have to do quite a bit in hardware, and do it _accurately_, if you want to count pulses. I would suggest using the A/D approach, which you can also implement using a serial data interface by connecting an A/D directly to a UART. GE/Intersil, among other vendors, has an A/D converter and UART which are compatible in this mode. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"