Research

1. All components operating temperature go down to -40C. (Note that resistor TCR is still a thing)

2. Buck Chip has thermal shutdown feature

3. Raspberry Pi (3B model) draws 1.2A max & 0.4A nominal

Vin min = 4.75V & Vin max = 5.25V

From reading Raspberry Pi 3B’s datasheet, it’s input voltage monitor IC triggers at 4.63±0.07V.

Sources: Raspberry Pi Vin Range, Raspberry Pi 3B current draw

Load Regulation Test

Current Draw	Room Temperature	Freezer
0.4A	Vout @ eload = 4.8553V	Vout @ eload = 4.8510V
1.2A	Vout @ eload = 4.4649V	Vout @ eload = 4.5276V
3A	Vout @ eload = 3.7068V

Comments:

1. The buck converter’s output voltage is not the problem as Vout is measured exactly 5V on PCB at 3A load

2. The wire has resistance. According to Ohm’s Law, there will be a voltage drop in both power and ground wires when current is flowing through the wires. Thus when Raspberry Pi draws maximum current of 1.2A, its input voltage risks being under the Vin_min threshold. This is why raspberry pi reports low voltage during one of the flight test https://uwarg-docs.atlassian.net/wiki/spaces/FT/pages/2351726905/2023-11-19+Flight+Tests#%F0%9F%92%BB-DEBRIEF

3. At 1.2A load, the voltage drop in the wire is 0.5351V.

Action following up (as of 2023-11-25):

1. Assemble a new 24-5 buck with Vout_max = 5.25V (Raspberry Pi max Vin) to mitigate some of the voltage drop in the wire.

2. Build new harness to shorten the cable length to 20cm.

Feedback Resistor Calculation

From Daniel’s feedback resistor optimization script, 0.5% tolerance in E96 series and 0.1% tolerance in E192 series give the highest nominal Vout while not exceeding Raspberry Pi Vin max of 5.25V. Note resistor TCR is possible, but we’re hoping the max temperature span is relatively small.

Vdrop = I * Rwire = I * ( ρ * L / A )

New Harness length is halfed as before therefore the voltage drop in wires is also halfed.

E96 Rfb → Vin @ Raspberry Pi = 5.089 - 0.5351/2 = 4.82V

E192 Rfb → Vin @ Raspberry Pi = 5.138 - 0.5351/2 = 4.87V

As the USB cable wire gauge is higher, the cross-sectional area will be smaller so the actual input voltage at PI is smaller than the calculated values.

(can’t estimate area because Jerry forgot what wire gauge he used in the load regulation test…)

According to this article, Raspberry Pi will start trolling when Vin is less than 4.8V. Since we are using newer Pis, the input voltage threshold will be more critical as there are new sensors/chips on the Pi.

Conclusion: We need to see the voltage drop in the new 20cm harness to make the decision on 0.5% vs 0.1%

Daniel’s Thoughts: Shorter power cable should be prioritized over a resistor based solution because the tolerance adds cost and TCR still has a decent impact though both are pursuable.