Electronics > Projects, Designs, and Technical Stuff

A SCPI Programmable (Precision) Resistor

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SebastianH:
You could start with a different topology. (i. e. (1 Ohm)^n, (2 Ohm)^n, (4 Ohm)^n, (8 Ohm)^n per decade) to reduce the number of relays, use cheaper resistors, simplify the UI (the switches/push buttons are expensive...) etc. There is a lot that can be done to make it cheaper. :)

Chipguy:
That is a very good looking design.
I built two programmable resistance decade boxes so far.
However I took a different approach: These are 2 decades in one.
One is paralleling resistors for the range of 1 ohms to a obout 3400 ohms.
Above that a second serial resistance decade is put in serial that contains resistors from 3400 ohms to about 800 K ohm.
So I have a high resolution decade from 1 Ohm to 1.6 M Ohm.
I lives from knowing the exact conductance value of each resistor so it can combine them via succesive approximation to the desired value.
I can use 1% resistors no problem, main thing is: They need to be low tempco.

The unit in the picture is shown in PT100 simulation mode. I use it to simulate all sorts of temperature sensors, conductivity of saline solution and plain resistance of course.
For quick access I added 16 pre-programmable favourite buttons.

I build this in order to improve my embedded C programming skills which were in desperate need of improvement.
Needles to say that the code I wrote is bonkers and eventually got me stuck.

Just wanted to share that with you, so you know you are not alone  ;D

SebastianH:
Very nice! If you don't mind me asking: Are you willing to share the schematics of the resistor networks?

zapta:
Looks great. Initially I thought that you repurposed an old HP case.

What is the topology of the resistors/relays network?  Trying to understand how the resistances of the relays factor in, especially in low settings.

shapirus:
I want to hear how this thing sounds in action. It must be really sweet.