Although the Programmable Power Resistor re-uses some components, many design ideas and a good portion of the software of the Programmable Decade Resistor (Programmable Precision Resistor), there were many changes and also some improvements regarding some aspects of the mechanical construction and the code.

Rear panel
Rear panel with two 60mm fans, USB connector and Line input

Features and specs

So now it’s time to present the feature list/specs:

ParameterValue/Description
Resistance range (nominal, Ω\Omega)short circuit, 1.25 to 80 , open circuit:
0, 1.25, 1.429, 1.538, 1.667, 1.818, 2, 2.143, 2.222, 2.273, 2.308, 2.353, 2.5, 2.727, 2.857, 2.941, 3, 3.077, 3.158, 3.333, 3.448, 3.529, 3.636, 3.750, 4, 4.167, 4.286, 4.444, 4.615, 5, 5.263, 5.455, 5.714, 5.833, 6, 6.5, 6.667, 7.143, 7.333, 7.5, 8, 8.333, 8.571, 9, 9.167, 9.375, 10, 10.667, 10.909, 11.111, 11.667, 12, 12.5, 13.333, 13.636, 14, 15, 15.385, 16.667, 17.143, 17.5, 18.333, 19, 20, 21.667, 22.5, 23.333, 24, 25, 26.667, 27.5, 28.333, 29, 30, 31.667, 32.5, 33.333, 34, 35, 36.667, 40, 42.5, 43.333, 44, 45, 46.667, 50, 55, 60, 65, 70, 80
Setpoint resolutionsettable to 1mΩ1 \text{m}\Omega, selects closest supported resistance value
Short-term setpoint accuracy estimate≤±1% of nominal value+0.2Ω\le \pm 1\% \text{ of nominal value} + 0.2 \Omega
Display resolutionthree decimal places (fixed)
Thermal drift(estimate based on design)50ppm/K50 \text{ppm}/\text{K}
Power/current ratingDepends on resistance setting. Limiting values are calculated and displayed.
Up to ~150 W continuous (400W peak) or 6A, whichever is lower (preliminary)
Bandwidthunspecified
Calibration modes
  • Uncalibrated: Switches are activated according to entered setpoint, display shows setpoint
  • Two-wire (2W), Four-wire (4W): The resistance value closest to the entered setpoint is selected
Operating modes
  • Fixed: On trigger, no change
  • Step: On trigger, step to trigger setpoint value
  • Up: On trigger, increase setpoint (auto, linearly, 1-2-3-4-5-6-7-8-9, 1-2-5, 1-3)
  • Down: On trigger, decrease setpoint (linearly, 1-2-3-4-5-6-7-8-9, 1-2-5, 1-3)
  • List: See list mode
List modeUp to 100 values with individual dwell times:
  • Start: on trigger; or immediately after mode selection
  • Step:
    • Auto: Advance index automatically, solely based on dwell time
    • Trigger: Advance index on each trigger event, ignoring the dwell time
    • Once: Advance index on each trigger event, only after dwell time elapsed
Switching
  • fast (default)
  • break-before-make
ProtectionOver temperature protection (OTP)
TriggerSource:
  • Bus
  • Immediate
  • Manual
  • Timer
Mode:
  • Continuous
  • Single
Parameter:
  • Delay
  • Holdoff
  • Time (0.01 s .. 10^7 s; Source: Timer)
Command and trigger execution10ms loops
User interface16x2 Dot Matrix display:
  • Primary display: Resistance setpoint, (max.) heatsink temp.
  • Secondary display:
    • limiting values (voltage, current, power)
    • trigger state
    • uncalibrated resistance value
User presets0-9 (0 is restored on power-up)
Interface
  • Input and Sense: 4mm safety banana jacks
  • USB (Virtual COM port)
FansTwo temperature controlled 60mm fans (12V, 4500 rpm)
Power supplySwitch mode

ℹ️ Note

I later converted the power supply to an external USB-C supply

A look inside

Top view with cover removed
Eight 10-Ohm 50ppm 50W power resistors mounted on heatsinks are controlled by 15 relays

Looking at the rats-nest of wires, it’s clearly a revision A… Some of it is a product of the small case - with more board space it would have been easier to place the connectors a bit better. Then I decided to use different, more powerful fans than planned initially and those do not have a PWM input. This explains the two small “pwm-to-voltage converter modules”, one on each side of the case.

Also, I could have done a better job managing all the cables, and possibly connecting the resistors in a way that minimizes cable length (or alternatively distributing the heat across the heatsink as evenly as possible depending on the switching state). But “good enough” has to suffice…

bottom view with cover removed
Two large heatsinks fill almost 50% of the case volume. The Kapton tape is meant to direct the airflow

For now, I don’t think I need a second unit. In case that changes, I have some ideas for a second revision with some minor improvements and adaptations (like integrating the pwm-to-voltage circuits), but also one new feature: A simple voltage measurement. This not only allows for a handy display of the voltage, current and power, but also additional fan control options and protection features.