Design and development of an open source programmable DC electronic load

Date
2015
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University of Delaware
Abstract
A DC electronic load is a test instrument that is used to simulate loading on an electronic circuit or device. DC electronic loads can be bought as commercial units. These units can be expensive with prices ranging from several hundred to a few thousands of dollars. They tend to be robust and offer more features and power ratings than what is needed by the average embedded electronics design engineer. As a result, there have been several electronics design engineers and hobbyists who have chosen the more cost effective and more rewarding route of designing their own DIY DC electronic loads. One design that started the trend is demonstrated by Dave Jones in episode 102 of his EEVblog on YouTube. In this video, Jones explained the circuitry of the electronic load circuit operating in constant current mode and suggested some improvements that can be made to the circuit. Many electronics hobbyists replicated and improved upon Jones' design in various ways and published their design files over the Internet as part of the open source hardware movement. This thesis documents the design of an open source DIY DC electronic load called the FreeDum Load. The FreeDum Load aims to aggregate some of the good features of the previous DIY designs while making improvements to areas that were lacking. In addition, the FreeDum Load is designed to be a self-sufficient bench top device to have the form factor of commercial electronic loads. This feature is currently lacking in existing DIY designs as the majority of them are bare PCB boards. The FreeDum Load features a streamlined aluminum enclosure and runs off of AC power. It is programmable and the peripherals on board are controlled by a Freescale Kinetis K20 microcontroller with a Cortex M4 core. Peripherals include a 3.2 inch back-lit LCD touchscreen display, a quadrature encoder with RGB LED feedback, and a fan controller with DC fans for cooling. In addition, some firmware has been written for the Kinetis K20 microcontroller to drive the LCD, quadrature encoder, and the fan controller. The firmware proved to be challenging as existing libraries and sample code for the Kinetis K20 microcontroller were non-existent in the open source community. Freescale, the manufacturer of the microcontroller, offered some support in its CodeWarrior IDE, but the IDE itself is proprietary and has a high licensing cost. As a result of this project, the first hardware revision of the FreeDum Load has been completed and a prototype unit has been constructed. A second revision is needed to correct some of the hardware design mistakes made. Major changes would include starting with a completely new microcontroller that would be easier to program with an open source IDE, adding a heatsink to the device, and cutting down costs of the components so the FreeDum load can be made much cheaper than commercial units. Overall, this thesis documents the knowledge gained from the full prototype design life cycle including knowledge of analog circuit design, PCB layout, soldering, PCB fabrication, Cortex M4 firmware development, and mechanical construction.
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