Topology (1)

A project log for Programmable Power Resistor

A SCPI programmable power resistor

sebastianSebastian 04/18/2024 at 20:150 Comments

In this application the focus is on power handling capability, not so much a high resolution (high number of resistance values) or precision. This is why the topology used in the Programmable Precision Resistor is not as suitable for this application as it has been before. Also my ELMA cases are really small (and I mean really small, so thermal considerations will be challenging), so I can’t simply increase the number of power resistors and relays in the process. That’s why I’ll stick to the original plan of using 8 power resistors.

Three of a total of eight power resistors used in this project

Instead the idea is to find a topology where I can select different resistance values by either connecting the resistors in series or in parallel. Let’s first have a look at a circuit for only two resistors.

A simple circuit allowing to switch between a series and parallel connection of two resistors. It requires one single throw and one dual throw switch.

For this design I assume that both resistors are identical, both in their resistance value R as well as their power rating P. It’s very obvious how this circuit works, but here are the current paths for the different switch states anyway:

Switch states of the presented topology

Since the math behind this is really simple, here are the results without any further explanation.

Switch states and the associated electrical parameters

And now we have our basic building block that we’ll use in the next post of this series.