Project Specification — SELV Spa-Light Switch Controller
1. Project Overview
Design a SELV control electronics schematic for an IP67-rated 240 VAC spa-light switch controller for use in Australia. The controller receives 240 VAC mains input and drives an on-board, soldered, mains-rated 2-pole relay stage feeding a Spa Electric LV25-12 LED light transformer/driver system.
External switched load requirement: Spa Electric LV25-12 swimming pool LED light system, 240 VAC input, 25 W load. Approximate load current is 25 W / 240 VAC = 0.104 A RMS, but relay/contactor contacts must still be mains-rated with adequate AC switching margin, inrush margin, and compliance review.
This project focuses on the isolated SELV control electronics: MCU, user controls, indicators, optical programming receiver, sensors, mains-present sensing, hold-up power, and relay coil drivers.
2. Environmental Requirement
- Outdoor operating ambient temperature must support up to 60°C.
- Because the electronics are inside a sealed IP67 enclosure, internal air/component temperatures may exceed 60°C; component selection and layout must target industrial/extended temperature ratings and conservative thermal derating.
- Thermal validation is required in the final enclosure under worst-case sun exposure/load conditions.
3. Safety Scope and Non-Negotiable Requirements
- User-accessible buttons, LEDs, optical receiver, sensors, programming/debug, and control logic must remain on the SELV side only.
- SELV must be isolated from 240 VAC mains using a certified isolated AC-DC power module with reinforced or double insulation.
- Do not use capacitive droppers or non-isolated supplies.
- Do not switch protective earth.
- Relay contacts for switched 240 VAC output are now in scope as an on-board soldered relay; the relay footprint/contact routing must be treated as mains circuitry with reinforced SELV separation and qualified compliance review.
- Draw and label the isolation barrier between primary mains circuitry and SELV circuitry.
- Maintain suitable creepage/clearance for 240 VAC mains and require qualified safety review.
4. Key Architecture
- Certified isolated 240 VAC to +5V_ISO AC-DC module.
- +5V_HOLD supercapacitor-backed rail.
- +3V3_MCU low-Iq LDO rail.
- Low-power MCU with watchdog, brownout, ADC, PWM, EEPROM/NVM, and CRC/checksum support.
- SELV-only optical programming receiver and user controls.
- Temperature sensing for sealed enclosure overtemperature lockout.
- Isolated mains-present sensing.
- Single monostable 12 V relay coil driver output for an on-board through-hole relay.
- Mains switching stage: use Omron G2RL-2A-DC12 as the approved low-volume, available-library relay baseline. It is an on-board through-hole 12 VDC coil DPST-NO relay used to switch active and neutral to the Spa Electric LV25-12 load. Final PCB layout must verify contact rating, approvals, thermal margin, and mains creepage/clearance.
- Power architecture: use RECOM RACM06E-12SK/277 as the approved isolated 240 VAC to 12 VDC PSU baseline. +12V_RELAY feeds the relay coil and downstream 12 V-to-5 V buck for hold-up and 3.3 V MCU rails.
5. Required Signals / Net Names
Use these net labels clearly in the schematic: +12V_RELAY, +5V_ISO, +5V_HOLD, +3V3_MCU, 0V_ISO, MAINS_PRESENT, OPTICAL_PROG_IN, RELAY_DRIVE, BTN_ONOFF, BTN_MODE, LED_POWER, LED_STATUS, TEMP_SENSE, LIGHT_SENSE/I2C light sensor, RELAY_FB, LOAD_L_OUT, LOAD_N_OUT.
6. Firmware / Safety Behavior
- Default state after reset, brownout, watchdog reset, overtemperature, invalid configuration, or invalid optical command: relay OFF.
- After programming mode, output remains OFF for 60 seconds before normal operation resumes.
- Non-volatile configuration storage must include checksum/CRC.
- Watchdog and brownout reset support required.
- Temperature firmware must define warning and shutdown thresholds after enclosure thermal characterization.
7. Compliance Notes
Final product must be reviewed by a qualified electrical safety engineer and tested by an accredited lab before connection to mains or sale in Australia. Applicable standards to review include AS/NZS 3100, AS/NZS 3820, AS/NZS 3000, AS/NZS 60529/IP67, AS/NZS 60669 where applicable, and AS/NZS 60335.2.60 if classified as spa/whirlpool equipment.
8. Open Engineering Decisions
- Confirm Omron G2RL-2A-DC12 contact approvals/load category and derating for the LV25-12 switched load.
- Confirm RACM06E-12SK/277 derating in sealed IP67 enclosure at 60°C outdoor ambient.
- Confirm all selected parts meet required internal enclosure temperature margin.
- Define enclosure thermal model and sun-load test conditions.
- Define firmware overtemperature thresholds and hysteresis.