Programmable resistor decade

Building a 1 W, 0.5 % decade resistance

The seven decade programmable resistor - my variant.

Gerry Sweeney designed and built a really neat little SMD decade resistance board a few years ago. He still offers it for sale, already assembled, check it out! You can find it here:

https://gerrysweeney.com/seven-decade-programmable-resistor-a-low-cost-solution/

Screenshot from Gerry Sweeney's site.

Gerry Sweeney's design is cheap, simple and uses 1 % and 0.5 W resistors in the 1210 size (metric: 3225).

Why build my own?

I decided to build a similar board, but to use at least 0.5 % tolerance, better temperature coefficient and higher power resistors. This was more of an exercise to see what kind of SMD resistors are available and if it is possible to design it for higher power output and better tolerance at the same time.

The answer is that it is, but it comes at a cost. 

What kind of resistors?

I tried to use 1206 size (metric: 3216) resistors where possible. They are a bit smaller and comes nowadays also in 1 W packages. I also wanted to keep the PCB max 100 mm in length, because this is the limit that PCB manufacturers have for their cheapest alternative when you order PCBs online.

I also tried to find resistors with better than the common temperature coefficient of 100 or 50 ppm/°C. I settled with 25 ppm/°C for most of the resistors.

I managed to find 1 W, 0.5 % resistors at reasonable cost for decades 10 Ω to 100 kΩ. These resistors cost between 0.5 - 0.8 €/unit. In the 100 Ω range there were even 0.1 % resistors available. 

Note that some of these 1206 SMD resistors have a long side terminal, meaning that they are soldered on the long sides as opposed to regular SMD resistors that are soldered on the short sides. This requires another footprint on the PCB (more on this later).

For the lowest decade (1 Ω - 9 Ω) I decided to use two parallel 2 Ω, 0.5 W, 0805 size (metric: 2012) resistors. This was the only solution I found to get relatively cheap 1 Ω, 1 W resistances in the first decade. The downside is that the first decade now requires twice the amount of resistors, but 0805 size and 0.5 W power rating are a bit cheaper. Cheaper means also that they are thick film and thus haven't as good temperature coefficient as thin film resistors, only 100 ppm/°C. They still ended up at nearly 10 € for 18 resistors (0.54 €/unit).

For the megaohm decade, no 1 W resistors were available, but you are unlikely to require this kind of power in the MΩ range. I found 0.4 W, 0.1 % resistors.

Note that many SMD resistors also don't withstand as high voltage as through hole resistors. The used resistors have a rating of 150 and 200 V. So if you connect this device to a tube amp (not recommended), make sure to use larger settings (multiple resistors in series).

List of resistors

For reference, these are the resistors used (deviations marked with bold text):

Vishay CRCW08052R00DKEAHP
Thick film, 2 Ω (2 resistors in parallel), 0.5 %, 100 ppm/°C, 0805, 0.5 W,  150 V

Susumu PRG3216P-10R0-D-T5
Thin film, 10 Ω, 0.5 %, 25 ppm/°C, 1206, 1 W, 150 V
Note. Long side terminal.

Susumu PRG3216P-1000-B-T5
Thin film, 100 Ω, 0.1 %, 25 ppm/°C, 1206, 1 W, 150 V
Note. Long side terminal.

Susumu PRG3216P-1001-D-T5
Thin film, 1 kΩ, 0.5 %, 25 ppm/°C, 1206, 1 W, 150 V
Note. Long side terminal.

Susumu HRG3216P-1002-D-T1
Thin film, 10 kΩ, 0.5 %, 25 ppm/°C, 1206, 1 W, 200 V

Susumu HRG3216P-1003-D-T1
Thin film, 100 kΩ, 0.5 %, 25 ppm/°C, 1206, 1 W, 200 V

Vishay TNPW12061M00BEEA
Thin film, 1 MΩ, 0.1 %, 25 ppm/°C, 1206, 0.4 W, 200 V

Result

This is how the design looked in KiCAD:

 

 

I made two revisions, one for regular resistors and a second one for the resistors that had a long side terminal.

Resistors with regular terminals

Resistors with long side terminals

In KiCAD, there was some confusion with the 1206 series long side terminal resistors. The footprints that were available didn't match, so I had to make custom footprints.

Here are some actual measurements of the board, measured with an Agilent 34401A:

1 Ω

This is the 1 Ω setting. 0.09 Ω or 90 mΩ is basically the residual resistance of the board.

 

9 Ω

90 Ω

2537 Ω exactly!

900 kΩ

In the upper ranges there are finally some deviation, but from a quick judgement, it looks like tolerance is better than 0.1 % (even with the 0.5 % resistors). Not bad! 

Summary

In total the price of the resistors was almost 50 €. I only recommend to build it with such resistors if you want a precision decade resistance solution. For normal use, Gerry Sweeney's original design with regular 1% resistors will be fine.

But it depends on your needs. If you want one with higher power rating, it might be possible to find 1 W or higher rated resistors with less tolerance that are cheaper. I didn't look into this alternative this time.