AC Line Current Detector
This circuit will detect AC line currents of about 250 mA or more without making any electrical connections to the line. Current is detected by passing one of the AC lines through an inductive pickup (L1) made with a 1 inch diameter U-bolt wound with 800 turns of #30 - #35 magnet wire. The pickup could be made from other iron type rings or transformer cores that allows enough space to pass one of the AC lines through the center.
Inductive Current Sensor
Model Train club builds digital controller. Digital systems like DCC also allow to use inductive sensors as with any other "AC" supply. The AC is set in quotes as the digital signal is not sinus shaped as AC usually is, but a square wave shaped signal. This results in the possible problem, that at the edges parasitic capacitors result in current peaks, leading to a false occupancy signal.
I have been inspired by the pages from Rob Paisley. There is an inductive sensor presented together with other sensors. The VT-5 coils used were from Coilcraft but are out of production. They were distributed by Jameco with the part number J9199-A, the only similar device at Jameco is the 164718PS as used by detector 5 at the Teton Short Line.
How to measure current and power - using a hall effect transducer - clamp
The neat thing about using a Hall Effect sensor to measure current, is that if you are not getting the resolution you want, you can loop the wire through the current clamp as many times as you want to double, triple, or quadruple the sensitivity of the device. You just have to remember to calibrate your sensor first with a Digital Multi-meter. You can buy a hall effect transducer clamp off the shelf to measure AC and DC current. Or you can build your own. Digikey sells many types of these sensors.
How to measure current and power using a current sense shunt resistor
This is an example of how a 0.05 Ohm precision resistor can be used to measure / calculate AC or DC current and power produced from your bicycle pedal power generator. If you do not have a precision resistor then you can use just a calibrated section of wire with about one or two feet length. The following table shows resistance per foot of wire by wire size. Typically the wire in your house wall receptacles is 12 AWG. So if for example you are trying to measure current from supplied by a 24V power source, and you had a volt meter that measured down to a 0.001 Volt resolution, you could hook install a 2 foot section of 12 gauge AWG wire into your supply line, and put your voltage meter probes on either end of the 2 foot section of wire. You would then take your reading and divide it by (2 x .00187 ohms) to get your current.
Sungroper (Solar Car Group): Current Sensor Board
The Current Sensor Board contains an Atmel AVR AT90S8535 running at 8MHz which samples six ADC current inputs and uses the remaining two ADC inputs to control an integrating charge counter and packages the results up into data packets which are sent to the Master Board.
Sungroper (Solar Car Group): Extra Sensor Board
The Extra Sensor Board contains an Atmel AVR AT90S8535 running at 8MHz which just samples up to eight ADC inputs and packages them up into data packets which are sent to the Master Board. The ADC samples the battery voltage as well as temperature sensors and is also capable of handling the tacho signal to relive the master board of this task.
Google Answers: Building a Wattmeter (AC Power Calculations)
Since it is AC power, P=VI isn't total correct. S (apparent power) = IV and P=VI cos(phase angle between V and I) or P=VIPf where Pf is the power factor and equal to the cosine of the angle between V and I. P is always less than S and as a general rule the are pretty close in value. If you are looking to ignore this difference and are not looking for real time numbers (which I assume since you are looking to calculate P), all you need is a digital multimeter, then measure V and R separately and P~V/(R^2). Note: measure R without power source disconnected otherwise you would be measuring your Resistance in parrellel with the power source's resistance thus lowering the resistance and increasing your power by a factor of a square.
Mixed-Signal ICs - AC Current Sensor
The Si85xx AC Current Sensor family of products mimics the functionality of a traditional current transformer (CT) circuit with burden resistor, diode and output filter, but offers enhanced performance and added capabilities. These devices use inductive current sensing and onboard signal conditioning electronics to generate a 2 V full-scale output signal proportional to the ac current flowing from the IIN to the IOUT terminals. The Si85xx Evaluation Board demonstrates the Si8512 AC Current Sensor. The kit, available for $19.95, includes an assembled and tested evaluation board and documentation.
I have an idea for a project but I'm not sure where to start. I want to have my own digital power meter which can interface with a computer. Enabling me to see how much power I'm consuming at the time, power usage patterns etc.
Make - Make your own Kill-A-Watt
I actually work on designing these things for a living... 8)
OK - not the particular model on discussion, but the company I work for does domestic and commercial electricity meters of varying complexity.
There are a few choices for the sense mechanism (in terms of the current), these varying in cost and accuracy. By far the cheapest option is to use a shunt resistor and measure the voltage it develops... For the voltage side, a simple voltage divider circuit is all that's needed. I think, looking at the kill-a-watt, this is most likley to use a shunt for current measurements.
For the shunt, going into a vanilla microcontroller, you'd need some form of amplifier, since they maximum voltage you'd want to get from the shunt is in the order of 20mv rms. When you're talking about an ADC with a 5v input, that's a fair amount of gain required.
Once you have the inputs, you need to be able to sample them instantanously (either by running 2 ADCs in parallel (for a single phase meter), or by having enough systematic delay in the circuit so that sequential sampling actually gives instantanous results.
Once you have the samples, you have to multiply them together and repeat at a sufficient rate to give reasonable performance of the measurement for real-world loads - most supplies/loads are non-sinusoidal.
Of course, you've also got the problem of being able to calibrate the thing once it's been made to some degree of accuracy - the manufacturers spend oodles of cash on equipment to do this... you can't really rely on the ratings labels of equipment to calculate the load, as these only (hopefully) represent worst case performace of the appliance, and not what the consumption would be in normal use.
I have actually implemented a three-phase kWh only meter in an 8-bit micro with a sequential 10-bit ADC running at no more than 2.5MIPS. Using the same micro, I also implemented a single-phase kWh/kvarh meter, so the calculations can be done at a sufficiently high rate on inexpensive equipment. Of course, these are for non-US markets, so the accuracy requirements are certainly relaxed.
Both meters are accurate to better than 1% over the range 250mA to 100A. That actually represented about 18 months - 2 years worth of development.
Then, on top of all that, you need to implement the load-profiling so that you can see the demands based on the time of day...
In short - yes, I can be done, but it is certainly non-trivial as a learning excercise. And I would also echo Hiro's comments - when you're dealing with measuring live mains supplies - it isn't very forgiving at all. BEWARE!
DIY Power Meter (Watt Meter) - RC Groups
Some time ago, there used to be a good web site with serveral great projects, one of wich was a Watt Meter. This site is nowhere to be found nowadays. I will attemp to revive that project. Please note that I am NOT the original autor of this project. I don't remeber the original author's name (a frenchmen), but he deserves credit for at least the original version, using a MAX4372H High-Side current sense amp to monitor current. The project was built around a PIC16F876A, wich is convinient for this application. It has a couple of 10bit AD, enough pins left to drive the LCD and Flash program memory for fast software tweaking Another version that I'm currently working on involves using a Hall Effect current sensor from Allegro (ACS750) - that should make the device Brushless compatible!
NQRC - YouKnowWatt open development (Watt meter) - RC Groups
This is the You-Know-Watt development . What does this thing do... fundamentally it watches over voltage and current on a pair of wires, beyond that... well, we're going to see how far we can take this. As a feature that's useful to people like myself, it's also got a PC logging port. It'll have a 16x2 LCD display... and maybe I'll think about putting some buttons on it as well for additional functions (not sure what yes, I guess that's what we're here to toss around).
Feature list (dynamic )
* Input Voltage: 0-20V (could go as high as 30V but there'll need to be some changes)
* Input Current: 0-60A (first estimate, have to see how hot the circuit board gets)
* ESC PWM input: Yes (Digital)
* Prop pulse input: Yes (Digital)
* Temperature sensor input: Yes (0-5V analog)
* LCD display output, initially 1"
Current Sense Amplifiers Information on GlobalSpec
Current sense amplifiers are integrated circuits with operational amplifiers and sense resistors that are used in measuring the amplitude and direction of current in circuits. There are many types of amplifiers. Examples include a differential current sense amplifier, high-accuracy current-sense amplifier, high-side current sense amplifier, and a high voltage current sense amplifier. A differential current sense amplifier is generally used for battery-operated devices that require both charge and discharge currents to be monitored. A high-accuracy current-sense amplifier facilitates current sharing between various DC/DC converter modules in addition to current sensing. As this causes various modules to be paralleled, current sharing ensures reliable operation. A high-side current sense amplifier employs a high-side monitor that is connected directly to the power source. A high-side current sense amplifier can identify any failure downstream and also initiate appropriate corrective measures. A high-side current sense amplifier can measure load current as well as tolerate any level of high voltage within the limitations in the external parts of the equipment. Other current sense amplifiers are also commonly available.
A $5 DIY Power Meter (Resistive Current Sensing) | silentpcreview.com
Basicially it is an ordinary AC cable that has been modified with a 1-Ohm (10W) resister in series with the hot lead. A pair of screw terminals permanently connected to the resistor leads allows multimeter proble ends to be attached easily to measure the voltage drop across the resistor.
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