A few months back, as of the time of this writing, Xbitlabs published an article called PC Power Consumption: How Many Watts Do We Need which is essentially where this idea originated. In fact, Xbitlab’s Oleg Artamonov deserves full credit for all the information contained in this article since it was his work that drew my attention to a specific component used in his measurements – namely the hall effect IC sensor.
The basic concepts in this case, or many other measurements for that matter, involves 3 major components. First, a sensor or transducer capable of detecting the quantity of interest (in this case, current), a signal conditioner to ensure that the signal for the measurement conforms to an appropriate format (in this case, a voltage porportional to the current that is easily and accurately measured), and a means to record that information for analysis (namely a recorder such as a chart recorder or a computer based data acquisition system).
1) The sensor
The one component that makes this all possible is indeed the Allegro Hall Effect Current sensor, the ACS7XX series ASIC to be more blunt. The concept is rather straightforward, the entire Hall Effect sensor is constructed into a single small footprint where the conduction path for the current and Hall Element are fixed within the same package which turns out to be a simple 8-pin SOIC. While there is one or two other single packaged IC hall effect sensors, the Allegro sensor has been developed to handle the currents of interest and, depending upon the particular sensor chosen, can be utilized with amperage from 5, 20, 30 or even 100 Amps (click here for a complete list of sensors and ranges). More information on this sensor and how it is constructed can also be found in the Allegro Whitepaper.
There are several advantages to this sensor, most of these are discussed in the Xbitlab’s article as well.
- The sensor comes in a very small package, measuring some 5mm x 6 mm total.
- Completely linear over the rated amperage for 0 to 20 or 0 to 30 amps depending on model.
- Very low thermal sensitivity and acceptable accuracy (nominally to within +/-1.5%)
- Very fast rise time (~ 5 microsec)
- High bandwidth, and configurable filter pin to be fitted with choice of capacitor (though not really needed for this application)
- Electrically isolated from the load, with a high breakdown voltage.
- 5 V supply voltage +/- 0.5 volt operating range, max 8.0 V (meaning power to the sensor can be delivered via a USB connection for example).
- Ratiometric output to the supply voltage ( output will scale proportional to the input voltage to the sensor — for example, a 20 Amp sensor will give a voltrage range of 0.5 to ~ 5 volts over a 0 to 20 Amp range).
You can obtain the complete specification at the Allegro web site. In short, the Allegro ACS7XX Hall Effect Sensor contains not only the sensing element for detecting current but also the necessary signal conditioning circuits to produce a stable and accurate voltage which is directly proportional to the current being measured satisfying 2 of our 3 basic components needed to make a measurement.
2) Data Acquisition
The simplest, albeit labor intensive, method for obtaining the measurement would be to hook up a sensor (be it either a shunt or Hall Effect sensor) and measure the output (or voltage drop) directly. This is impractical though for making multiple simultaenous measurements. A more effective, and more attractive approach, is to use some form of data acquisition to digitize the voltage and convert that into a current (and consequently a power) measurement that can be recorded on a personal computer by utilizing analog to digital converters in some form or another.
For the data acquisition, the possibilities are endless and beat, hands down, a multimeter measurement for this kind of application. By constructing and using a digital based acquisition system, real time measurements can be taken from start to finish, sampled at a very high rate. Unqiue details of the time depending draw on power can be easily obtained and yield very interesting results.
The choice of acquisition systems are limitless, in fact. A tedious, but effective, approach is to utilize the A/D found in many cheap microcontrollers (which is exactly what Mr. Artamonov did when selecting the Atmel ATMega 168. However, if you are like me and are one who has little to no experience programming such devices, there are several low cost USB based multipuprose data acquisition systems to be had. We will discuss more about the data acquisition used to build our prototype device a little later in the article.