Reference Resistors

A friend of mine asked me for a quick project (finally it become not so quick, but just because the time it required the parts to arrive).

He is installing pool and other water handling related systems. One of the pool measure system has PT100 temperature sensor input what need calibration. The proper calibration equipment cost a fortune. Actually he only need two reference point. For this he asked me to make some reference resistors in a box what is precise enough and has low tempco.

The values are 100 ohms and 119.4 ohms what is weird a bit. Looked around and find the required resistors in 0603 and 0805 size (100 and 120 ohms 0.01% 2ppm) around $10 each. For the 119.4 ohms I planned 24k in parallel with the 120 ohms (bit less precision as there is not needed better).

I bought some tiny aluminum boxes from the AliExpress. The idea to change the front plate of the boxes to a PCB add the screw-in banana sockets and put the resistors to the back of the boards.

It looks like this:

It was manufactured by PCBWay in the usual excellent quality - Thank you!

I was a bit afraid, if they can follow my request and put the order number to the already crowded component side, but it was managed without problem. It arrived together with my VFD project board, what I'll write about later.

The board soldered (Yes, it is a joke. Only one single resistor in this case)

Some measurements

If I count the cable and the plugs contact resistance is the result get even better. I'm satisfied with the result.

And the final boxes (the request was about two units.

I put two resistance values into a single box. The only thing left is to print some labels on it with the values.