The Programmable Power Resistor consists of two large boards: The User Interface and Controller Board and the Relay and Interface Board. Because of the use of relays that isolate the input from the circuits, all circuits can be and therefore are earth-referenced.
The only microcontroller, a STM32G441RBT6, is located on the User Interface and Controller Board. It implements a SCPI interface via USB (VCOM), the user interface and the logic to control the relays. All relays, the relay driver circuits, some external and internal interfaces and the AC/DC converter can be found on the Relay and interface Board.
The Power resistors are located on two heatsinks, each equipped with its own NTC temperature sensor and fan. Initially, the fans were supposed to have a PWM input and tacho output, but due to availability considerations a simpler two-pin fan was selected. To retain the fan control capability two PWM to Voltage Adapter Boards were introduced. These are simple DC/DC buck converters with supporting circuitry on a small PCB that output a voltage proportional to the PWM signal. A second iteration of the design would integrate this functionality on the existing boards, but since there are no major issues with the boards, for now this approach has to suffice.