Build Your Own Total System Power Analyzer

Overall Score

There is a much easier approach, albiet more expensive, to accomplish the same goal and the method we used to build our first prototype. As discussed earlier, all that is needed is the sensors and a data acquisition system (DAQ). Rather than constructing a single form PCB, a sensor can be purchased already mounted on a breakout board and several low cost USB DAQ systems do exist — bingo, off the shelf solutions with no major effort programming microcontrollers, creating PCBs and soldering components.

After obtaining the right number of sensors, a USB base DAQ system, some ATX 24 pin, 8 pin CPU, and PCIe extension cables and some hook up wire/molex connectors, one can complete this project in a just few hours. While cheesy and less elegant than an all-in-one PCB that contains everything, this method also provides a few advantages. As will be discussed below, the DAQ choice has higher resolution than most of the low cost, easy to program MCUs and they come with software for acquiring the data that has already been tested and optimized to collect the data at good sampling rates. Furthermore, there is no tricky soldering surface mounted ICs, which is probably the biggest hurdle for novice electronic hobbyists (such as myself), in fact, no soldering is needed through the entire build — just a heat gun for the heat shrink.

Sensor: Where and What to Buy

AllegroMicro sells evaluation boards (which are essentially break out boards) for the 30 Amp and 20 Amp version of the ACS713 hall effect sensor, in addition they also sell a bidirectional 5 Amp sensor evaluation board allowing for appropriate choice of max rated currents to maximize the sensitivity. An alternative would be a breakout board from a hobbyist robotics shop, for example a 30 Amp version of the ACS715 (essentially the same thing) can be found here though they do not carry a 20 Amp version so some sacrifice for sensitivity will be made on some rails.

For this project we used the AllegroMicro evaluation board. We ordered 5 of the ACS713ELC-20A-T, 2 of the ACS712ELC-30A-T (for the PCIe lines), and 1 ACS712ELC-5B-T. The evaluation boards have a few advantages, for flexibility we can remove one sensor and use it for other projects, in addition each eval board has it’s own voltage regulator so if a +5 VDC supply is not available, any DC source > 6 volts (but less than 17 volts) can be attached and regulated to +5 VDC to drive the sensor. Furthermore, the eval board has several test points which helps in construction and debug. Finally, the eval board is constructed to handle currents to the maximum rated for the sensor, there is no learning curve or research needed to ensure the proper amount of copper nor size of traces, this work has already been done.

Data Acquisition System: Where and What to Buy

The king of DAQ systems is National Instruments, they are to data acquisition and control what Intel and Microsoft are to personal computing. National Instruments makes several products but offers two low cost USB based multifunction DAQs: the 6008 and 6009. Both options were rejected immediately, the 6009 is too expensive, running ~ 280 dollars and both come with LabView Signal Express limited, which does not allow for signal averaging or computation (only logging). There are a few other low cost solutions available, one is the iUSBDAQ but the software is too rigid for this applications.

The best choice is the LabJack U3HV. The U3HV has a few advantages and disadvantages, the strongest advantage is that it comes with DAQFactory Express which has all the functionality necessary to program and acquire the data. Other advantages include 16 flexible IO lines which can be configured as analog inputs, the HV version (recommended) has 4 high voltage inputs which are a capable of measuring > 10 VDC, useful for measuring the +12 VDC rails actual voltage real time (which can help account for droops in the supply and back out much more accurate power measurements). The U3HV also provides a very quiet and stable +5 VDC output (derived from the USB bus) which can be used to power the sensors themselves. The lone disadvantage of the U3HV is that the non-high voltage (high voltage here meaning > 3.6 V) analog inputs can only read max 3.6 V inputs, which limits the dynamic range. For all, but extreme OCing, this limitation is acceptable, otherwise some voltage dividers will be needed to scale the 0-5 VDC output of the Allegro sensors to match the range of the U3HV inputs. For this projects, we used the LabJack U3HV as our data acquisition system.

Miscellaneous Stuff

Besides the sensors and the DAQ system, we also needed a project box to house the sensors, some mounting hardware (extra stand-off’s, acrylic glass to mount the sensors to), hook up wire, an 8 connection terminal block shorted into groups of 4, grommets, some molex (2695 female crimp connectors), threaded rod (6-32), nuts (6-32), assorted crimp connectors, and heat shrink. All can be purchased at local electronics stores or home depot. The total for this was less than 40 bucks, see the below for the layout of all the parts.

 

 

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