Notes on MightyWatt electronic load

02.02.2019 14:43

MightyWatt is a small, computer controlled electronic DC load made by Jakub Polonský. I recently ordered one for work, since I often need to test power supplies and it was significantly cheaper than a similar professional desktop instrument. I was first looking for a Re:load Pro, since I have been very happy with the old analog Re:load, however it turns out they are out of stock and impossible to get. MightyWatt was the closest alternative that fit the budget. After several months, here is a quick review and some notes on how well it performs in practical use.

MightyWatt electronic load with Olimex USB-ISO isolator.

Compared to Re:load, MightyWatt is not a stand-alone device. It comes in the form of an Arduino shield and you need to mount it on an Arduino Uno (or a compatible) microcontroller board. You also need to compile an open source firmware yourself and program the board with it. The ATmega328 on the Arduino then controls the load and communicates with the computer you connect to its USB port.

It's also worth mentioning that this design already has quite a history. First revision apparently shipped in 2014. I am using revision 3 hardware from 2017 and latest software from GitHub. During these tests I was using an Arduino-knockoff called Smduino. To protect the computer from any mishaps with the load, I'm connecting it over an Olimex USB-ISO USB isolator.

Bottom side of the MightyWatt electronic load PCB.

The initial setup went quite smoothly. The instructions on how to program the firmware using the Arduino IDE were nice and clear. After your order, the author commits the calibration file for your serial number to GitHub, which I thought was a nice approach. The control program for Windows has a pre-compiled build in the GitHub repo, so there is no need to install Visual Studio, unless you want to compile it from the C# source yourself.

In contrast to the straightforward software instructions I could find no illustration showing how to actually mount the device onto the Arduino board and initially I was a bit baffled. The 100 mil male headers on the MightyWatt are missing pins from the standard Arduino shield layout, so it's possible to fit them into the sockets in several possible ways. I'm guessing only one of those doesn't end in disaster. In the end I noticed the (very) tiny pin markings on the MightyWatt silk-screen and matched them with the corresponding pins on the Arduino.

Unfortunately, the headers are also the only thing that keeps MightyWatt mechanically connected to the Arduino. Beefy measurement cables are commonplace when working with large currents and the stiffness of the headers alone just isn't enough to keep the MightyWatt securely connected to the Arduino. On several occasions I found that I have pulled it off when messing with the cabling. I was looking into 3D-printing an enclosure, however this MightyWatt PCB doesn't have any spare holes for additional screws, so it's not easy to work around this issue. It seems that an acrylic case existed at one point, but I couldn't find it for sale and since it doesn't seem to screw onto the PCB I'm not sure it would help.

MightyWatt current measurement compared to VC220 multimeter.

Comparing MightyWatt current readout to VC220. VC220 was in series with MightyWatt current terminals. The load was in constant current mode and was connected to a 5 V power supply.

MightyWatt voltage measurement compared to VC220 multimeter.

Comparing MightyWatt voltage readout to VC220. The load was in 4-point mode with a lab power supply connected to the voltage terminals. Current terminals were not connected.

As far as accuracy and calibration is concerned, I can't be certain since I don't have any good reference to compare it to. After some simple experiments with a VC220 multimeter it seems reasonable, as you can see on the graphs above. The current readout on the MightyWatt is with-in the measurement tolerance of the VC220 for the complete 10 A range. Voltage readout does fall outside of the tolerance of VC220. I don't know whether that is a fault of VC220 or MightyWatt, but in any case, both devices only disagree for about 1% and linearity looks good.

One problem I noticed with constant current setting was that there seems to be a momentary glitch when changing the set point with the load running (i.e. without stopping it). This seems to trigger some fast over-current protections, even when currents should be well below the limit. For example, changing the load from 1 A to 2 A sometimes puts a 3 A supply into foldback mode, but doing the same by stopping the load for the change doesn't.

I really like the fact that MightyWatt supports 4-point Kelvin measurements. The software also supports a mode called Simple ammeter, which puts current inputs into minimum resistance. Combined with the 4-point setting, this converts MightyWatt into a computer-controlled ampere- and voltmeter pair. I have not tried this mode yet, but it sounds like it might be useful as a simple power meter.

Other than that, I haven't looked much into the electronics design. However the author has a blog with many interesting posts on the design of the MightyWatt if you are interested in the details.

Screenshot of the MightyWatt Windows control program.

The Windows software is functional, if somewhat buggy at times. Unfortunately as far as I can see, there is no way to control MightyWatt from Linux at the moment. I would love to automate my measurements with Python, like I've been doing with everything else. Fortunately, the Windows control program allows you do some simple scripting, so that is not that much of a pain. Also, the communications protocol seems well documented and I just might write a Python library for it eventually.

My biggest issue with the software is that the Windows control program seems to often lose connection with the load. This isn't handled gracefully and I often find that it will no longer see the MightyWatt's COM port after that. This then requires some ritual of reconnecting the USB cable and restarting the application to get it working again.

I'm not sure what is the reason for this and I don't know whether this is a software problem on Windows side or whether the Arduino firmware is crashing. First I was blaming electrical interference, since it appeared to often happen when I connected an oscilloscope probe to a supply I was testing. Then I thought the USB isolator was causing it. However after some more testing I found that this still randomly happens even if I just let the MightyWatt run idle, directly connected with a USB cable to a PC.

In conclusion, it's a nice little instrument for its price, especially considering that similar instruments can easily cost an order of a magnitude more. I like that it comes tested and calibrated out of the box and that it's well documented. I really like the open source aspect of it and I always find it hard to criticize such projects without submitting patches. The Windows control software is pretty powerful and can support a lot of different measurements. The ugly part is the flimsy mechanical setup and the connection reliability problem, which means that I can't leave a measurement running without being constantly present to check for errors.

Posted by Tomaž | Categories: Analog

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