Dane.Kouttron
Inductive.Monitor
Problem: Need to measure how much current goes through a line
Problem 2: Playing with line voltage should be avoided
Solution: Simple inductive measuring device.
Your power line consists of 3 wires, a
ground a neutral and a hot line. Below is the common coloring scheme:
note. it may be different at your house/ place of institution/ country.
check it with a voltmeter. it takes all of 2 seconds. And don't
play with power lines/blame me for your causing of the great Colorado
fire/your dorm burning down. that being said, if you disassemble a
standard unplugged electrical extension cord, you should see this:
If you took a simple physics course, from anyone other than Prof. Timothy Hayes, you would learn that if there is a current flowing through a wire it creates a magnetic field. how can you measure that field? a coil of wire. wrap some thin wire around a torroid which encases an individual wire, in my case i used the isolated neutral wire. Like 200+ turns should do. run them out of the wire:
Step 3:
So you want to measure the current, how?, its ac and my micro controller is only dc?..... ok first we want to snub out any of the alternating magnetic field's effects by simply placing a diode in the path of the input. now you have pulsed dc. so the small cap charges to the level of the pulsed dc over a short time period. the 1Meg resistor is so that the cap will bleed out to zero if no current is flowing. so say you plug in a circular saw to the wire y suring. there will be one hell of a spike, right?. yep... so to thwart this throw on a zenner diode (thanks mike). It acts as a clamp so that only 4.5V max gets through. over that voltage the diode conducts and creates a short. Simple and sweet eh? check the schematic: [Left=input (coil) , right= output (micro controller)
Step 3: (notes)
Note that this method of power measurement is nice for resistive loads (your lamp, your electric heater, your hair dryer), however capacitive and inductive loads will act differently, why? well, a purely capacitive load will phase shift the sinusoidal output by 90 degrees, moreover, imaginary and complex components constitute a portion of the power and are not taken into account via this method. If you take a closer look at your computer power supply (desktop) it might indicate its power factor index. Wikipedia it and find out what i mean.
Step 4: Try it out
Once everything's assembled, electrical taped and heatshrunk, plug your voltmeter up to the adc output and ground line on the output from the inductive meeter. now plug in your hair dryer. If you read a voltage that voltage correlates to the number of watts through that line. so say your 1,200 watt hairdryer = 1.2V than use a proportion to find what value other wattages would have. note that if you get an extremely small value for the voltage output, even while your using a high power device, increase the number of turns of wire. likewise, if your getting more than your micro can handle, decrease the number of turns of wire or make a voltage divider with some resistors.
Step 5: Putting it in The Micro
If your using an Avr and compiling with Bascom, your going to need to read an adc value.
heres a simple example (link)
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Dane.Kouttron
Rensselaer Polytechnic Institute
Electrical & Electrical Power
631.978.1650
