I've been using my MightyWatt electronic load for the last few months and I got some ideas to make it better. I am introducing the MightyWatt, revision 2. This time, you can buy it as a kit (fear not, most parts are assembled) on Tindie.
|Complete schematic of the revision 2.
MightyWatt has now four input terminals. You can switch between remote (4-wire) and local (2-wire) voltage measurement. The signals are routed by a relay. This gives you the opportunity to exclude the resistance of cables from your measurement.
I have also redesigned the dual-range voltmeter from single-ended to differential.
I have replaced the cheap Microchip's DAC in favour of a higher-end DAC from Analog Devices with internal 2 ppm/°C voltage reference (AD5691RBRMZ). It is more expensive but the accuracy will be improved. The original DAC (MCP4726) is not a bad part, though. It only has somewhat larger inlinearity which cannot be eliminated by simple linear calibration. Feel free to use the MCP4726 if you don't need the improved accuracy.
Better gate driver
I have added additional compensation to the MOSFET gate driver so the current ripple is much reduced.
Because the temperature is monitored in software, I never experienced an overheat. I have decided to drop the red LED that was supposed to indicate overheat.
There were occassional problems when transient voltages were present on the USB line. Some PC power supplies have problems filtering transients in the grid (e.g. motor switching) and it resulted in disconnection of the load. I have added a 5V transient voltage suppressor at the power-entry point.
New software and firmware
Because the load can do 4-wire measurement now, I also updated the software (C#) and the firmware (Arduino sketch). This means that revision 2 software is not backwards compatible with the first version. You can find all the relevant source files on my Google Drive (original revision 2). All the resources for the latest version can be found here: http://kaktuscircuits.blogspot.cz/2015/03/mightywatt-resource-page.html.
- Eagle board and schematic
- Bill of materials
- Arduino sketch for firmware
- Arduino sketch for calibration
- Excel file to calculate calibration values
- Windows software for MightyWatt control, as a C# Visual Studio project