This device is special because the hardware and software is open source, it is Arduino compatible, and it is a cheap tool all hardware designers should have. It works up to 19.8V, 5A and 16W/21W depending wether you choose the smaller or larger heat sink. The load can be set to constant current, constant power, or constant resistance by simply typing it into the Arduino serial monitor. The constant current circuit is implemented in hardware using an opamp, a mosfet, and a current sense resistor. The input for that circuit is made with a 12 bit DAC. The constant power and constant resistance mode is implemented in software. Load voltage and current are measured using a pair of high precision opamps (MCP6072) and the microcontrollers 12 bit ADC. Battery capacity can be measured by loading the battery with constant current and it will simply display the mAh's in the Arduino Serial Monitor. To protect the battery against undervoltage the load is removed when the voltage falls below a configurable threshold. The device can also be used to simply log a voltage or current over time.

Dave Jones from EEvblog discussed my previous revision device in everyone's favorite segment Mailbag. Check it out.

Specifications

I sell on Tindie

Quick start

Serial Monitor Commands

Storing the data

Arduino Serial Monitor cannot save the serial port data. Well, you can copy what was printed to the screen and store it as a CSV or text file. But I recommend using Realterm because it can store the data in a file and add a timestamp for further processing in other tools, for example in a spreadsheet.

Testing a power supply

Constant voltage power supplies can be tested by applying different loads and see how the voltage is reduced. It may also be used to test a dc-dc converter efficiency under different loads. For example, type cc 100 in the serial monitor and you will see the current being set and the voltage updating at regular interval.

Youtube video with demo of setting the current using R2

And another video where I test a $1.26 1A USB travel adapter

Testing a solar panel

Solar panels generate most power at a certain voltage, the maximum power voltage, Vmp. The Vmp can be found in the IV-curve of a solar panel, which is the current versus the voltage. You can manually apply different loads but the solar radiation may change during testing. To avoid these variations one could make embedded code that quickly tests the solar panel in a second or so.

Testing a thermal electric generator

TEGs generate most power at a certain current, the maximum power current, Imp. The Imp can be found in the IV-curve of a TEG, which is the current versus the voltage. This device is ideal for finding the Imp.

Testing batteries

Battery capacity is usually indicated in mAh (milliamp hours) which is the load current in mA multiplied by the time in hours. This electronic load is ideally suited for battery testing. It can test a battery under constant current, constant power and constant resistance, exactly the three ways capacity is indicated in the battery's datasheet. But be careful. The battery voltage must not drop too low otherwise the battery gets damaged permanently. You first need to type the limit voltage "limit v 1000" (it stops at 1000 mV), "reset" to reset the mAh counter and then "cc 1000" to start the measurement under a load of 1000mA.

Youtube video with demo of measuring battery capacity

Testing a supercapacitor

The capacitance of a supercapacitor can be verified using the electronic load. You need to charge it to let's say 2.0V, then load it with 1A constant current and simply count the seconds for the voltage to drop 1V. The time in seconds is the capacitance in Farad.

This youtube video shows how to test super capacitor capacitance

Pulsed load

The constant current circuit settles in just 20us, that is so fast it can be used for pulsed loads. For example GSM module current pulses can be simulated. If you type "pulse 500 5000 500 50 500 2500 500 50"  you get a repeating pulsed load of 500us@5000mA, 500us@50mA,  500us@2500mA, and 500us@50mA. And it looks like the picture below.

Typing "pulse 0 5000 0 0” basically means 0us at 5000mA, 0us at 0mA. Then the signal looks like this. 

As you can see setting duration to 0us actually gives 156us pulse witdh, this is caused by the time it takes to send a new DAC setting from the MCU to the DAC. 

Software

The software is open source and published by my brother Bertrik.

Software description here

Actual software is here

Hardware

Hardware is open source and it is published here.

What you get

You will get a fully assembled, pre-programmed, and tested device with the above specifications, it looks like the picture above. The USB cable is not included.

USB driver installation

On a Windows 10 machine (without any drivers or Arduino IDE installed) the device was immediately detected and a serial COM port appeared. On a Windows 7 machine, I had to Download this zip file. Run drivers/win/install_drivers.bat and install_STM_COM_drivers.bat. If that doesn't work I recommend to open Windows device manager, right click on Maple DFU, select update driver, and make Windows search for driver in the folder drivers/win/.

Re-programming the Electronic Load

If you want to update embedded code to the latest version or write your own Arduino program for the STM32 microcontroller, you need to follow the installation steps on this page. It's easiest to program the board over USB because I have already put a bootloader on the STM32 MCU. The embedded code you need to clone or download from this Github page. Once you have installed the STM32 board files you need to select the board "Generic STM32F103C series". If programming fails even though the STM32 shows up as a COM port on your computer you may need to install the USB driver. See USB driver installation above.

Calibration

Using a benchtop power supply and a multimeter you can make your device most accurate at a certain voltage/current.

Calibration the voltage

Let's say you want the voltage reading to be most accurate at around 10V follow these steps

Calibrating the current

You need to put your multimeter in series with the power supply

Let's say you want the current setting/reading to be most accurate at around 1A follow these steps

To read the calibration factors you need to type "cal". If you want to reset the calibration factors you need to type "cal r".

Thank you for reading all the way to the end.