Programmable precision resistor

Programmable Precision Resistor

A SCPI programmable precision resistor

WARNING: The programmable power resistor presented is mains powered. On the power supply board there will be mains/line voltage. Working with/on such devices can lead to damage, injury or death. Don't do it if you're not qualified or authorized to do so. You use the circuits described at you own risk. You have to make sure that the circuit is safe and adheres to the applicable standards in your location. You might operate the mainboard and ui board more safely with a bench or external power supply.

WARNING: I abandoned the AC/DC converter of the initial design mostly due to EMI issues. I ended up using a EI-core (isolation) transformer, bridge rectifier, capacitor two tiny DC/DC converters. Some of the documentation does not reflect this change.

Background and motivation

In 2021/2022 I designed a DC electronic load that would be more capable, but also much more complex than the usual DIY solutions. However, after building a working breadboard prototype of the analog circuitry with 12 ICs including multiple precision and dual opamps, I thought that it might be better to start with a smaller project that would allow me to gain a lot of experience and write much of the non-application specific code that I could use later on for the digital part. Also, at the time I was looking for a decade resistor box to add to my lab. And this is how I started working on a programmable decade resistor, a pretty specialized tool for niche applications.

Introduction

A programmable resistor is a electronic device whose electrical resistance can be adjusted typically through digital signals. Programmable resistors are available in form of integrated circuits (often referred to as digital potentiometers) as well as stand-alone devices/expansion cards. There are many different applications, each with their own set of requirements e. g.:

programmable gain amplifiers sensor emulation, e. g. for integration tests with hardware-in-the-loop simulators automated testing calibrators prototyping Basic concept The programmable resistor closely follows the main principle of resistor decade boxes: connecting multiple devices (“decades”) in series, with each of the devices having a settable resistance of 0 Ω, 1 ⋅ 10^n Ω, 2 ⋅ 10^n Ω, 9 ⋅ 10^n Ω, where n specifies the decade in question.

Those devices are simple resistor networks complemented by one (multi-throw) or more (single- or multi-throw) switches. While the actual topology and resistor values vary vastly, the switch(es) will always either short out or connect certain resistors or simply tap certain nodes of the resistor network. A simple implementation uses a 10-throw rotary switch to tap the nodes of a series resistor network of nine resistors.

Features

System design

The block diagram provides an overview of the different circuit groups of each functional block, their internal connections and external interfaces. The Programmable Decade Resistor consists of three main functional blocks:

1. Power supply (Power Supply Board), an external EMI filter is required.
2. Programmable decades, control and driver circuits (Mainboard)
3. User Interface (User Interface Board)

The power supply board uses an off-the-shelve AC/DC converter to provide a +15V rail. The +15V rail powers the relays on the mainboard as well as a DC/DC converter. A single DC/DC converter generates a +3.3V supply for both the mainboard and the UI board, including the LED display. The power rails are earth-referenced. (Nevertheless, the inputs of the Programmable Decade Resistor are floating thanks to the isolation of the relays.)

The mainboard’s main controller contains the business logic to control the relays, read the external inputs and run the USB and user interfaces. The main controller communicates with the User Interface Board over I2C which handles the multiplexing of the alphanumeric LED display, scans the switch matrix, decodes the signals of the rotary encoder and drives the buzzer.

Note that these pictures DO NOT represent the final design of the Programmable Precision Resistor and the power supply circuit in particular.

Results

After the final calibration the results are in. The first diagram shows the absolute value of the deviation between the setpoint and the value indicated by the Agilent 34401A multimeter. Connecting lines between data points for visual purposes only.

The second diagram shows the absolute value of the deviation between the calculated resistance (estimate based on calibration values) and the value indicated by the Agilent 34401A multimeter. Connecting lines between data points for visual purposes only

About this repository

This repository contains the PCB design files used in this project. If you'd like to have a look at the interactive bom files you can do so here:

• Mainboard: HTML Preview of interactive BOM
• UI board: HTML Preview of interactive BOM
• Power Supply Board: HTML Preview of interactive BOM

Further information

More details are available at https://hackaday.io/project/191969-programmable-precision-resistor and on my personal site https://sebastianharnisch.de.

Firmware and License

The firmware was created with STM32CubeIDE and uses code auto-generated by or provided with the IDE, including

• the HAL library, code related to the USB device and other code
• FreeRTOS

Further more, this project uses a SCPI parser library that can be found here: https://github.com/j123b567/scpi-parser

Most of the auto-generated code as well as the libraries are not included here. Therefore it is recommended to use STM32CubeIDE to create a project for the target device, with the USB device software and FreeRTOS enabled, add the required SCPI library and adapt the code according to the requirements.

For the code not included in this repository the respective licenses will apply instead of the license specified in this repository.

I DO NOT guarantee that the code provided is free of bugs (see LICENSE file for more details).