OptoMOS relay

OptoMOS relay

An OptoMOS relay is an optically isolated MOSFET solid‑state relay that switches DC or low‑frequency AC loads with very low LED drive current.

An OptoMOS relay is an optically isolated MOSFET solid‑state switch that gives galvanic input‑to‑output isolation, bidirectional blocking for AC/DC paths, millisecond switching and microamp–milliamp drive budgets — making it ideal for low‑power metering, valve actuation and isolated I/O in field nodes. (littelfuse.com)

At a Glance

Choosing between PhotoMOS vs OptoMOS

PhotoMOS (Panasonic) and OptoMOS (Littelfuse/IXYS branding and others) families are functionally equivalent at a systems level — both use an LED input and MOSFET output — but you should choose by: V_P (blocking voltage), Rds(on), required input drive (µA vs mA), package/thermal path and certification (e.g., AEC‑Q101 for automotive). Panasonic PhotoMOS CC‑type devices explicitly target ultra‑low input current (typical ~0.2 mA) for ultra‑low‑power MCU designs. (na.industrial.panasonic.com)

Why OptoMOS relay Matters in Smart Water Management

OptoMOS relays provide silent, spark‑free switching with galvanic isolation between field wiring and electronics, which reduces arcing and contact bounce that can corrupt meter pulse counts and produce radiated/transient interference that degrades LPWAN radios such as LoRaWAN and NB‑IoT. For European deployments this matters because regional parameters (and ETSI/EN rules) limit transmit power and duty cycles — shielding radios from impulsive noise is one low‑risk way to keep a node compliant and reliable. (resources.lora-alliance.org)

In municipal networks, MOSFET SSR parts cut GPIO and coil budgets to a few milliamps per channel, remove contact bounce that confuses pulse counters, and avoid arcing that can radiate into constrained radio backhauls. Compared with reed or mechanical relays, MOSFET SSRs typically exceed 10^8–10^9 cycles and keep timing consistent under temperature and vibration. Representative portfolio coverage spans from low‑voltage signal devices to 1 kV power SSRs. (littelfuse.com)

Standards and Regulatory Context

Municipal controllers and metering equipment that embed OptoMOS/PhotoMOS devices must be designed to meet safety, isolation coordination and EMC expectations for the region and use case.

Data quality note: vendor numbers are typically measured at TA = 25 °C with specific test circuits; always validate derating, leakage vs temperature and switching on your real board. For example, Littelfuse documents portfolio limits and power/heatsink conditions that alter continuous current rating. (littelfuse.com)

Background and Context

An OptoMOS relay (also seen as OptoMOS relé, Optomos SSR or trademarked OptoMOS®) uses an LED input and a photovoltaic generator (PVG) or driver stage to bias one or more MOSFETs arranged back‑to‑back for bidirectional conduction and high isolation. The PVG architecture minimizes drive current (µA–mA), removes mechanical contacts, and defines the electrical tradeoffs you must manage: Rds(on), off‑state leakage, output capacitance (Cout), off‑state capacitance C_DS(OFF), switching times Ton/Toff and input‑output isolation VRMS. Manufacturer datasheets (example: CPC1343G) publish blocking voltage, continuous load, R_ON, isolation and switching times so you can close the thermal and EMC loop on your board. (littelfuse.com)

• Typical use cases: metering pulse isolation, tamper detection, quiet valve actuation, medical signal muxing, high‑cycle ATE matrices and PV generator switching in PVGs.

Practical Implications (what engineers must design for)

Key tradeoffs: on‑resistance (Rds(on)) → I²R losses and thermal budget; isolation VRMS → board clearance/slotting; input drive → MCU GPIO current and available DAC/CC drivers; Cout/C_DS(OFF) → signal integrity when switching analog sensors.

Representative device data you can design from:

• CPC1343G (OptoMOS example): 60 V V_P, 0.9 A continuous load, R_ON max ≈ 0.8 Ω, 5000 VRMS isolation, Ton/Toff ≈ 4/1 ms. (littelfuse.com)
• Panasonic PhotoMOS CC series: CC family examples show voltage‑driven input types needing ≈0.2 mA typical input current and sub‑millisecond to millisecond switching; check R_ON per series. (na.industrial.panasonic.com)
• Toward SiC Opto‑MOS examples: families that extend to multi‑kV blocking (e.g., 3.3 kV parts) and publish 3,750–5,000 Vrms breakdowns for insulation‑critical modules. (relay.com.tw)

Thermal quick math you can show procurement teams:

• P ≈ I_LOAD² × R_ON. At 0.9 A and 0.8 Ω, P ≈ 0.65 W. That heat must be within the package thermal limit — a small SOP or DIP without heatsinking may not dissipate 0.65 W safely at elevated ambient. Use manufacturer thermal derating curves and consider power SIP packages or external heatsink options when P > 0.4–0.6 W.

• Compared to a 2 Ω CMOS analog switch handling the same 0.9 A (rare but illustrative), P ≈ 1.62 W — so the SSR MOSFET path can actually run cooler while providing higher isolation.

Signal & EMI pointers:

• Output / off‑state capacitances create RC paths with high‑impedance sensors — avoid placing high‑impedance analog front ends behind the SSR or use bypass paths for 4–20 mA loops. See 4–20 mA current loop.
• For inductive valves/solenoids, always add RC snubbers or TVS devices at the load; design EMI tests with radios active (e.g., LoRaWAN, NB‑IoT).

Isolation planning:

• Specify parts with 2.5–5.0 kVRMS input–output isolation when field wiring or patient‑side circuits demand large safety margins. Example CPC1343G lists 5000 VRMS for robust installations — still verify board slots/clearance for creepage and altitude. (littelfuse.com)

Automotive & EV:

• Use AEC‑Q101‑qualified opto‑MOS parts for EV/telemetry/BMS nodes and add surge, HTRB/HTGB and temperature cycling testing in your qualification plan.

Key Takeaways (quick callouts)

Key Takeaway from FLOPRES
6 pilot water‑level sensors installed in rural Eastern Slovakia/Poland proved two‑person installation in under 20 minutes per location; the project targeted expansion to 60 villages by February 2025. This highlights the value of compact, IP68, low‑power nodes for rapid scale.

Key Takeaway from Danube River Floodplain Monitoring
A 12‑sensor NB‑IoT deployment delivered millimetre‑level data and reduced manual visits — use high‑isolation, maintenance‑free sensors to enable 5‑year battery‑life operational goals. See MERATCH radar sensor datasheets for typical specs. (meratch.com)

How OptoMOS relay is Installed / Measured / Calculated / Implemented: Step-by-Step

1. Define the load and waveform: identify DC valve, 24 VAC solenoid or pulse source; record V_MAX, I_MAX, ambient T and switching duty.
2. Select V_P with margin: choose V_P ≥ 1.5× V_MAX (DC) or ≥ 2× V_RMS (AC) to allow for spikes.
3. Screen on Rds(on): compute P = I_MAX² × R_ON at worst ambient and consult thermal derating curves; select power SIP or heatsink if required.
4. Verify isolation VRMS and PCB creepage: pick 2.5–5.0 kVRMS parts when field wiring or medical interfaces demand it.
5. Size the input drive: pick LED I_F (e.g., 2 mA typical) or CC voltage (3–5 V) and calculate R = (V_GPIO − V_F)/I_F; confirm turn‑on time in cold/hot corners.
6. Control EMI: add RC snubber / TVS for inductive loads; keep loop area tight and retest with radios active.
7. Layout for reliability: apply IEC 60664 creepage rules, guard traces on high‑impedance nodes and separate wet‑side high voltage areas.
8. Validate switching speed: scope Ton/Toff with your firmware drive and confirm pulse widths.
9. Procure with freeze: lock datasheet revision in BoM and include alternative sourcing (e.g., CPC1008N or similar).
10. Environmental test: run −40 °C to +85/+105 °C soaks to measure leakage vs temperature, R_ON drift and signal integrity on actual PCBs.

Notes: test radio coexistence with active edge gateway and live OTA firmware updates workflows.

References

(Selected project summaries and relevant MERATCH sensor specs — short, verifiable)

• FLOPRES – Flash Flood Prediction System (Malá Poľana, Svidník area; Slovakia / Poland). Initial phase: 6 water‑level sensors + rain gauges; expansion target 60 villages by Feb 2025. Two‑person setup in <20 min/location; validated in earlier DALIA pilot. (Project blog, Meratch).

• Danube River Floodplain Monitoring (Danube floodplain, Slovakia). 12 high‑precision NB‑IoT water‑level sensors deployed; millimetre‑level accuracy, hourly automated transmission, design battery life ~5 years; used for simulated flood management and proactive interventions.

• Bratislava Wastewater Management (Bratislava, Slovakia). MERATCH radar IoT sensors + CORVUS repeaters for underground signal transmission; real‑time monitoring helped move operations from manual estimation to data‑driven alerts (BVS partnership).

• Residential Septic Tank Monitoring (Slovakia). Single radar IoT sensor + LoRaWAN/BTS connectivity; delivered desktop‑visible tank capacity and reduced manual inspections.

• BVS Bratislava – Wastewater Monitoring (Podunajské Biskupice / Lafranconi Bridge). Radar SSR sensors + CORVUS repeaters for shafts; project addresses wastewater flows for a population equivalent of ~4.2 million (national scale benefit).

Technical sensor references (datasheets): MERATCH Radar Level Sensor (0.2–22 m range, ±2 mm precision, IP68, LoRaWAN/NB‑IoT supported) and MERATCH Datanode (OptoMOS relay onboard: opto output 60 V / 300 mA, IP67, autonomy ≥5 years at 1 h interval). (meratch.com)

Frequently Asked Questions

Q1 — Can an SSR MOSFET relay switch both AC and DC?
A1 — Yes. Most OptoMOS/PhotoMOS parts use two MOSFETs back‑to‑back for bidirectional blocking; verify the part’s AC load rating and switching times for your specific 24 VAC solenoid or mains profile. (Check datasheet blocking and I²R limits.)

Q2 — How do I spec isolation VRMS and creepage for a meter PCB that shares space with LoRa/NB‑IoT radios?
A2 — Pick parts with at least 1.5–5.0 kVRMS input‑output isolation and ensure PCB creepage/clearance meets IEC 60664 for your pollution degree and working voltage; provide slots if needed and keep wet‑side traces away from radio feed lines. Use manufacturer isolation test numbers and derate for altitude/humidity. (littelfuse.com)

Q3 — What are the trade‑offs between OptoMOS vs CMOS analog switches for 4–20 mA sensing?
A3 — CMOS analog switches typically show lower leakage but are limited in working voltage and can introduce charge injection. OptoMOS relays handle tens to hundreds of volts with stronger isolation but have finite Rds(on) and output capacitance — design the front end to keep high impedance paths away from SSR switching nodes. See 4–20 mA current loop.

Q4 — How do I drive an OptoMOS from a 3.3 V MCU?
A4 — Calculate series resistor R = (V_GPIO − V_F)/I_F. Example: (3.3 V − 1.2 V) / 2 mA ≈ 1.05 kΩ → use 1 kΩ; verify turn‑on current and Ton/Toff at cold/hot extremes and ensure chosen I_F gives required R_ON at worst case.

Q5 — What thermal precautions are required if the part is carrying 0.9 A?
A5 — Use P = I²R to estimate dissipation. At 0.9 A and R_ON = 0.8 Ω, P ≈ 0.65 W; check package thermal resistance and ambient temperature, and add heatsinking or select a power SIP package if dissipation exceeds the safe steady‑state power for your chosen package.

Q6 — In automotive or EV telemetry, what extra tests are required for OptoMOS nodes?
A6 — Prefer AEC‑Q101‑qualified parts; add HTRB/HTGB, temperature cycling, ESD, surge and combined environmental tests as per automotive qualification flows; derate operating currents and add TVS clamps for inductive surges.

Summary

An OptoMOS relay delivers low‑drive, high‑isolation, long‑life switching that suits metering, valve control and data‑acquisition channels in distributed water infrastructure. With published V_P from ~60 V to 1 kV, family currents from sub‑ampere to multi‑ampere (and power variants that may be heatsinked to ≈22.8 A), and options like 5000 VRMS isolation, OptoMOS devices replace reeds/mechanical relays where silence, endurance and compact footprints matter. Meratch can help benchmark parts, compute I²R losses, validate leakage vs temperature and prepare procurement language for certification. (littelfuse.com)

Author Bio

Ing. Peter Kovács, Technical Freelance writer

Ing. Peter Kovács is a senior technical writer specialising for smart‑city infrastructure. He writes for water management engineers, city IoT integrators and procurement teams evaluating large tenders. Peter combines field test protocols, procurement best practices and datasheet analysis to produce practical glossary articles and vendor evaluation templates.