70A 690V 3-Phase Solid-State Relay with RS-485 and Overcurrent Protection
Custom 3-phase solid-state relay prototype with MCU-controlled thyristor firing, RS-485 duty-cycle command input, dual-phase current sensing, and hardware overcurrent protection plus status reporting.
Advanced Project
— This is advanced because it combines 690VAC power switching, high-current thyristor drive, fast hardware overcurrent protection, isolated RS-485 communication, and tight real-time MCU timing with safety-critical PCB layout constraints.
Assumptions:
- The 70A, 690VAC load is AC mains industrial power and the SSR is intended for phase-angle or burst-fire control rather than simple on/off switching.
- The three thyristor pairs are back-to-back SCRs or equivalent bidirectional thyristor arrangements, one pair per phase.
- Hardware overcurrent detection means a fast analog comparator path that can force a shutdown independent of firmware.
- The MCU will be powered from an isolated low-voltage supply separate from the mains power stage.
- Prototype means a custom PCB is acceptable, but the user wants readily purchasable parts and not a full production-qualified SSR module.
Bill of Materials
Compatibility Notes
- TMS320F280021 is a 3.3 V-class MCU, so the TMS320F280021 transceiver should support 3.3 V logic; ADM3061E and SN65HVD1782 are compatible choices.
- TMS320F280021 outputs an analog voltage, so the MCU ADC input range and reference must be set so the sensed current stays within safe limits at 70A full scale.
- TMS320F280021 has open-collector outputs, so it will need pull-up resistors to the MCU logic rail or to a fault-latch rail.
- The power stage is 690 VAC, so the control electronics must be isolated from the thyristor gate and current-sense domains; the low-voltage supply and TMS320F280021 interface should not share unsafe ground references with mains.
- If the gate drive is implemented with optotriacs like MOC3023, the gate resistor and trigger current must be sized for the specific thyristor pair chosen; the driver alone is not enough to guarantee reliable turn-on.
You'll Also Need
- Thyristor/SCR power devices for the 70A, 690VAC main switching path were not provided in the database and must be selected separately.
- Gate resistors, snubbers, MOVs, fuses, thermal interface materials, heatsinks, and creepage/clearance-compliant PCB layout are required.
- An isolated current-sense shunt or Hall sensor mechanical arrangement is needed, including busbar or conductor integration.
- Connectors, terminal blocks, enclosure, insulation barriers, and PCB fabrication details are not covered here.
- If the design needs isolated feedback from the power stage to the MCU beyond the current sensors, isolated amplifiers or digital isolators may be needed.
Estimated BOM Cost: $35-80 (based on live distributor pricing)
Design Considerations
Fault Shutdown Path
Do not rely on firmware alone for overcurrent protection. Use the TMS320F280021 output into an TMS320F280021 comparator so a threshold trip can disable firing immediately, then let the MCU log the event afterward. For a 70A system, set the comparator threshold with margin for normal inrush and transformer magnetizing current if applicable.
Gate Drive Strategy
For phase-angle control, the thyristors need repeatable trigger pulses synchronized to the AC waveform, not just a PWM-like average duty cycle. The TMS320F280021 can generate deterministic timing, but the actual gate pulse width and repetition rate should be validated on the bench with the chosen SCRs. If EMI is a concern, burst-fire control is often easier to tame than arbitrary phase-angle firing.
Current Sensing Topology
Two-phase sensing is acceptable for a prototype, but it will not catch every fault equally if the load is unbalanced or one phase is open. The TMS320F280021 is a strong choice because it handles fast common-mode transients well, which matters when the thyristors switch high voltage and high current. Make sure the shunt or sensor placement can survive the thermal dissipation at 70A.
Isolation and Safety
At 690VAC, creepage, clearance, and isolation are first-order design constraints, not layout afterthoughts. Keep the TMS320F280021 side, MCU side, and sensing side physically separated from the mains power path, and use isolated supply and signal boundaries where needed. Prototype boards often fail here first, so plan the PCB stackup and slotting early.
Firmware Architecture
Implement the control as a state machine with states such as idle, armed, firing, faulted, and comms-loss. The MCU should watchdog the TMS320F280021 command stream and force a safe state if duty-cycle updates stop arriving. Also log fault cause, current peaks, and command history so the client can diagnose trips remotely.
Validation and Bring-Up
Bring the design up first with a low-voltage resistive load and an isolated AC source or variac before attempting full 690VAC operation. Verify comparator trip timing, gate pulse timing, and current-sense scaling independently. For a prototype, thermal testing is critical because the thyristors, shunts, and gate networks can drift significantly as temperature rises.
Want to customize this project or build something different?
Try the Project Advisor
