Power and wiring harness

Example 1 Current budget of the rails

The team’s rule is that peak load must not exceed 80% of the rating. Device currents are peak values from hypothetical datasheets.

DERATE = 0.80
rails = {   # rail: (rating A, {device: peak current A})
    "FC TELEM1 5V": (1.5, {"telemetry radio": 0.5}),
    "FC other ports 5V": (1.5, {"GNSS + compass": 0.15, "RC receiver": 0.10, "ground display radio": 0.50, "LED + buzzer": 0.10}),
    "BEC A 5V": (3.0, {"companion computer": 2.5, "camera": 0.8}),
    "BEC B 12V": (2.0, {"gimbal": 1.2}),
}
for rail, (rated, loads) in rails.items():
    total = sum(loads.values())
    u = total / rated
    flag = "ok" if u <= DERATE else ("OVER RATING" if u > 1 else "above derating")
    print(f"{rail:<18} {total:.2f} / {rated:.1f} A = {u:5.1%} -> {flag}")

pm_out = 5.3                                   # V from the power module
print(f"power module {pm_out} V inside 4.9-5.5 V: {4.9 <= pm_out <= 5.5}")

FC TELEM1 5V       0.50 / 1.5 A = 33.3% -> ok
FC other ports 5V  0.85 / 1.5 A = 56.7% -> ok
BEC A 5V           3.30 / 3.0 A = 110.0% -> OVER RATING
BEC B 12V          1.20 / 2.0 A = 60.0% -> ok
power module 5.3 V inside 4.9-5.5 V: True

The BEC feeding the companion computer and camera has a peak load above its rating. In the lab it may look fine because the computer is not working hard, but when it processes images while the camera records, the voltage sags until it reboots in the air. Replace it with a 5 A BEC or move the camera to another rail.

Example 2 Checking the wire list

The companion computer needs at least 4.9 V at its terminals. The BEC supplies 5.1 V, and each connector has about 20 mΩ of resistance (hypothetical).

OHM_PER_KM = {20: 33.31, 22: 52.96, 24: 84.22, 26: 133.9}   # NBS Handbook 100, 20 °C
R_CONN = 0.020                                 # ohm per connector
wires = [   # id, from, to, type, AWG, round-trip length m, current A, source-side volts, load-side volts
    ("W01", "BEC A", "companion 5V", "power", 24, 0.6, 2.5, None, None),
    ("W02", "BEC A", "companion 5V", "power", 22, 0.6, 2.5, None, None),
    ("W03", "TELEM2", "companion UART", "signal", 28, 0.3, 0.0, 3.3, 3.3),
    ("W04", "TELEM3", "rangefinder UART", "signal", 28, 0.3, 0.0, 3.3, 5.0),
]
V_SRC, V_MIN = 5.1, 4.9
for wid, a, b, kind, awg, length, amps, va, vb in wires:
    if kind == "power":
        r = OHM_PER_KM[awg] * length / 1000 + 2 * R_CONN
        v = V_SRC - amps * r
        note = f"{awg} AWG: {v:.3f} V at load -> {'ok' if v >= V_MIN else 'too low'}"
    else:
        note = "logic levels match" if va == vb else f"LOGIC MISMATCH {va} V vs {vb} V, needs a level shifter"
    print(f"{wid} {a} -> {b}: {note}")

W01 BEC A -> companion 5V: 24 AWG: 4.874 V at load -> too low
W02 BEC A -> companion 5V: 22 AWG: 4.921 V at load -> ok
W03 TELEM2 -> companion UART: logic levels match
W04 TELEM3 -> rangefinder UART: LOGIC MISMATCH 3.3 V vs 5.0 V, needs a level shifter

The 24 AWG wire leaves the computer below its minimum voltage even though the wire does not get warm at all. Connector resistance matters a lot in short, high-current runs. A device with 5 V signals connected to a 3.3 V port needs a level shifter, or it may damage the flight controller’s port.