The executor completed the ESP32 thermostat controller addition, but I cannot perform the required independent reconciliation because no ACUs are available.
Reported as added
ESP32-S3-WROOM-1-N16R8 module
Native USB-C programming
BOOT and RESET controls
Four relay channels: Y1, W1, G, and O/B
HVAC terminals including R/Rh, Rc, and C
Still requires verification
ERC and complete net connectivity
USB-C CC resistors and USB power/backfeed behavior
GPIO selection versus ESP32-S3 boot-strapping pins
Relay flyback protection and safe power-up defaults
Contact routing from R/Rh or Rc to each HVAC output
Updated 3.3 V/5 V power and thermal budgets
Do not manufacture the board until those checks are completed.
The power subsystem accepts a mains-isolated 24 VAC, 50/60 Hz HVAC transformer secondary. The transformer is rated 75 VA, so it can source approximately 3.1 A during a downstream fault. The implemented path is:
J1 24 VAC input → F1 1 A time-lag fuse → BR1 full-wave bridge → C1 reservoir / C2 bypass → U1 65 V synchronous buck → +5V → FB1 ferrite bead → U2 low-noise LDO → +3V3.
DC GND is BR1 negative only. It is not connected to either 24VAC_A or 24VAC_B.
Rectified-voltage assumptions
Nominal unloaded sine peak: 24 V RMS × √2 = 33.94 V.
Allowing about 0.8 V per conducting bridge diode gives approximately 32.34 V on the nominal reservoir bus.
The design requirement treats 42 VDC as a credible high-line/no-load maximum.
VRECT bulk and bypass parts are rated 63 V and 100 V respectively.
U1 is rated for 65 V input, leaving 23 V between the 42 V design maximum and the IC input limit.
BR1 is a GBU4K: 800 V repetitive reverse rating, 4 A average current, and 150 A non-repetitive surge rating.
No TVS is installed across VRECT. With a credible 42 V maximum and a 65 V regulator limit, common 43–48 V standoff TVS devices either risk conduction near the valid bus range or clamp above the regulator rating. A useful coordinated clamp would require a higher-voltage regulator or a separately validated active clamp.
Power and current budget
The +5 V connector is budgeted for 1 A while the 3.3 V LDO branch is also operating.
Table
Case
+5 V external load
+3.3 V load
Buck output current
Buck output power
Estimated buck input power at 90%
Continuous
1.0 A
0.25 A
1.25 A
6.25 W
6.94 W
Short peak
1.0 A
0.70 A
1.70 A
8.50 W
9.44 W
At the nominal 32.34 V bus, estimated DC bus current is about 215 mA continuous and 292 mA at the combined peak. Using a conservative 1.8 multiplier for capacitor-input current waveform RMS gives approximately 0.39 A continuous and 0.53 A peak AC RMS. F1 is therefore a 1 A time-lag fuse: above expected load/inrush behavior but substantially below the transformer's approximately 3.1 A capability.
Reservoir sizing
C1 is 470 µF, 63 V. Estimated reservoir ripple is:
Continuous combined load: approximately 4.57 Vpp at 100 Hz and 3.81 Vpp at 120 Hz.
Combined short peak: approximately 6.21 Vpp at 100 Hz and 5.18 Vpp at 120 Hz.
R1 is a 100 kΩ bleeder. Its discharge time constant with 470 µF is approximately 47 seconds, and dissipation is about 18 mW at 42 V.
5 V buck design
U1 is the Texas Instruments LM65645SRZTRQ1, a 3–65 V, 4.5 A synchronous buck regulator.
Configuration follows the fixed-output datasheet circuit:
FB tied to VCC selects fixed 5 V.
RT tied to VCC selects 400 kHz.
MODE/SYNC tied to GND selects AUTO mode.
BIAS tied to +5V closes the fixed-output feedback path.
C7 = 1 µF from VCC to GND.
C8 = 100 nF from BOOT to SW.
Two 4.7 µF, 100 V input ceramics plus two 100 nF, 100 V high-frequency ceramics are connected across VIN/PGND as required.
L1 = 8.2 µH, the TI table value for 5 V at 400 kHz. The selected XAL7030-822MEC is shielded, rated 5.9 A RMS and 8.4 A saturation, exceeding the LM65645 worst-case 7.25 A high-side current limit.
Five 22 µF, 10 V, 1210 X7R output capacitors are used so that effective capacitance after DC-bias derating can meet the TI target of 70 µF. This must be confirmed against the chosen capacitor manufacturer's bias curves before production.
C14 = 100 nF high-frequency output bypass.
The EN/UVLO divider is 220 kΩ over 20 kΩ. Using the datasheet 1.25 V rising and 1.0 V falling thresholds gives approximately 15 V turn-on and 12 V turn-off.
Layout caveats: keep the VIN capacitor–U1–PGND switching loop compact, keep SW copper small and away from feedback/control nodes, connect the exposed pad to ground, and use at least six thermal vias. TI recommends a four-layer board and substantial ground copper; 2 oz outer layers are preferred for the reference thermal implementation.
Low-noise 3.3 V rail and thermal check
FB1 is a Murata BLM31PG601SN1L ferrite bead (600 Ω at 100 MHz, 1.5 A, approximately 80 mΩ DCR) feeding the LDO branch.
U2 is the Analog Devices ADM7150ACPZ-3.3-R7, fixed 3.3 V, 800 mA. Datasheet support network:
VIN: 10 µF to GND.
VREG: 10 µF to GND.
VOUT: 10 µF to GND.
REF: 1 µF to GND, with REF_SENSE tied directly to REF.
BYP: 1 µF to GND for low noise.
EN tied to filtered 5 V for automatic startup.
Exposed pad and GND tied to the ground plane.
Capacitors on VIN, VREG, VOUT, and REF must remain above the datasheet minimum effective capacitance over bias and temperature, and ESR must remain below 0.2 Ω.
Thermal dissipation:
250 mA continuous: (5.0 − 3.3) × 0.25 = 0.425 W.
700 mA short peak: (5.0 − 3.3) × 0.70 = 1.19 W.
Using ADM7150 datasheet examples, 500 mm² attached copper gives about 68.1 °C/W and 1000 mm² gives about 56.4 °C/W. The continuous rise is therefore approximately 29 °C or 24 °C respectively. A sustained 700 mA load would raise junction temperature approximately 81 °C or 67 °C above ambient, so 700 mA is treated as a short transient only. Use at least 500–1000 mm² effective ground copper, thermal vias to inner/bottom planes, and verify junction temperature at the actual enclosure ambient.
Littelfuse 392 Series TE5 time-lag fuse datasheet.
Coilcraft XAL7030-822MEC product data.
Murata BLM31PG601SN1L product data.
Validation limits
Rectifier/reservoir simulation evidence
A transient model at 24 VAC, 50 Hz used 0.8 Ω source resistance, two 0.8 V conducting bridge drops, C1 = 470 µF, R1 = 100 kΩ, and constant-power downstream loads.
Table
Load
VRECT average
VRECT minimum
VRECT maximum
Ripple
Approx. source RMS current
6.944 W continuous
30.08 V
28.02 V
31.94 V
3.93 Vpp
0.558 A
9.444 W combined peak modeled continuously
29.40 V
26.59 V
31.87 V
5.28 Vpp
0.728 A
The worst simulated VRECT minimum is 26.59 V, which remains 11.59 V above the 15 V turn-on threshold and 14.59 V above the 12 V turn-off threshold. The 1 A time-lag fuse retains margin above the simulated nominal-input RMS current while remaining below transformer fault capability.
24 VAC rectifier and reservoir steady-state simulation
The schematic connectivity and ERC are checked in Flux. Final production approval still requires PCB-level thermal review, verification of effective MLCC capacitance using the exact purchased MPNs, load-transient testing, loop/stability testing, conducted/radiated EMI testing, and worst-case transformer/high-line bench testing.