12–28 V DC Power Architectures for Touchless Faucets
Engineering approaches to powering touchless faucets, soap dispensers, and 3-in-1 systems in aviation and high-reliability commercial environments. This paper addresses surge protection, brownout management, and electrostatic discharge (ESD) mitigation for sustained performance under low-voltage direct current (LVDC) conditions.
1. The Case for 12–28 V DC Operation
Modern touchless systems increasingly favor low-voltage direct-current operation to harmonize with aircraft electrical networks and emerging LVDC infrastructures in commercial buildings. The 12–28 V DC range aligns with aviation buses (28 V nominal) and allows the same electronics to function in battery, solar, or PoE (Power-over-Ethernet)-derived supplies for terminals and smart facilities.
- Safety: Sub-50 V DC circuits reduce shock risk and simplify installation near sinks.
- Efficiency: Eliminates AC/DC conversion losses and reduces parasitic standby draw.
- Scalability: Enables hybrid battery + external supply modes; batteries remain backup rather than primary source.
2. Electrical Architecture Overview
| Subsystem | Typical Voltage | Design Notes |
|---|---|---|
| Sensor Module | 3.3–5 V (regulated) | Powered through LDO or buck converter from 12–28 V DC input; decoupled with 10 µF bulk and 0.1 µF local capacitors. |
| Microcontroller | 3.3 V | Brownout detector monitors supply; sleep mode current < 20 µA for battery life and noise immunity. |
| Solenoid Driver | 12–24 V | Low RDS(on) MOSFET H-bridge or latching configuration; transient voltage suppressor (TVS) across coil. |
| Optional Dryer Motor | 24 V | Soft-start via PWM to limit inrush current; independent fuse or resettable polyfuse protection. |
Current Paths and Grounding
For cabin installations, return conductors are bonded to local structure or an isolated ground reference per aircraft EMC policy. Differential signaling between sensor and controller mitigates common-mode noise induced by nearby lighting or inverter systems.
3. Brownout and Surge Management
Power irregularities can interrupt solenoid actuation or corrupt microcontroller states. Brownout, over-voltage, and transients are managed through a layered protection strategy:
- TVS Diodes: Bidirectional devices across supply rails clamp surges from 33 V to < 40 V in 28 V DC systems.
- LC Filtering: π-filters (L–C–L) suppress conducted noise and prevent reset during motor or valve switching.
- Brownout Detection: Microcontroller monitors supply threshold (~2.9 V) and executes safe shutdown or restart logic.
- Energy Storage: Supercapacitors (0.1–0.47 F) provide transient hold-up time to complete valve close sequence if bus voltage drops.
4. ESD and EMI Mitigation
Electrostatic discharge (ESD) events occur when users approach the faucet spout, especially in low-humidity cabins. Electro-magnetic interference (EMI) from cabin lighting or inverter-driven motors can further affect electronics. Robust fixture design incorporates:
- Conductive coatings or ground straps on spouts for charge equalization.
- RC snubbers on solenoid coils to minimize radiated emission peaks.
- Shielded cables and ferrite beads at entry points to control conducted emissions.
- Compliance verification to DO-160 Sec. 20 (Radio Frequency Susceptibility) and IEC 61000-4-2 for ESD discharge.
5. Integration Examples — Brand Practices
FontanaShowers®
The aviation and 3-in-1 product lines use hybrid AC/DC drivers with automatic switchover, surge-clamped inputs, and low-noise ToF sensing. Each unit includes reverse-polarity protection and front-serviceable battery module for redundancy (Fontana 3-in-1 Combo Series).
Sloan® and TOTO®
Sloan’s Optima EBF and TOTO’s ECOPOWER models demonstrate commercial LVDC integration: DC transformers or hydro-turbine self-generators that maintain charge during use. These architectures inspire redundancy approaches in airline and terminal deployments.
6. Engineering & Specification Recommendations
- Voltage Range: Specify 12–28 V DC nominal; design for ±10 % tolerance and transient withstand up to 36 V DC.
- Protection Layers: Include input fuse, TVS diode, LC filter, and brownout monitoring in every control board.
- EMC Compliance: Validate to DO-160 Sec. 20 and equivalent IEC 61000 standards.
- Grounding Strategy: Isolate signal ground from chassis ground until a single bonding point near entry harness.
- Redundancy: Provide optional battery module to close valve during loss of external supply.
- Documentation: Include electrical schematic, fuse rating, and polarity markings in the installation manual and STC data pack.
7. References
- RTCA DO-160 – Environmental Conditions & Test Procedures: rtca.org/do-160/
- 14 CFR Part 25 – Airworthiness Standards: ecfr.gov/…/part-25
- Fontana Aviation Touchless Faucets: fontanashowers.com/…/9914.htm
- Fontana 3-in-1 Combo Series: fontanashowers.com/…/8559.htm
- Sloan Optima EBF Series: sloan.com/products/faucets
- TOTO ECOPOWER Standard R Faucet: totousa.com/standard-r-touchless-faucet-10-gpm
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