What’s the Difference Between Opto-MOSFET Relays and Solid-State Relays?

Solid-state relays (SSRs) are everywhere—from factory automation to precision test systems—because they switch electrically without moving contacts. Within this family you’ll often see a more specific term: opto-MOSFET relay. This article explains what each term means, how they work, where they differ, and when engineers typically choose one over the other.

Quick Definitions

Solid-State Relay (SSR)

An electronic switch that replaces mechanical contacts with semiconductors. Most SSRs provide galvanic isolation between the control side and the load side.

Opto-MOSFET Relay

A type of SSR that uses an LED on the input (for optical isolation) and MOSFETs on the output (often a back-to-back pair for bidirectional blocking). You may also hear names like "PhotoMOS" or "MOSFET SSR."

Triac/Thyristor SSR

Another common SSR architecture that uses a triac (or SCR pair) on the output. These devices are designed primarily for AC loads.

How They Work (At A Glance)

Input/Isolation:
Both opto-MOSFET and triac SSRs typically use an input LED. When driven with forward current, the LED emits light across an isolation barrier.
Output Stage:
- Opto-MOSFET: Light triggers a driver that turns on MOSFETs. Back-to-back MOSFETs allow current in either direction and block voltage both ways.
- Triac/Thyristor: Light triggers a device that latches on during an AC half-cycle and turns off when current falls below its holding current (for resistive AC loads this generally occurs at the next zero-cross).

Learn more about how Opto-MOSFET Relays work here.

Key Differences

Aspect	Opto-MOSFET Relay	Triac/Thyristor SSR
Output device	MOSFET(s), often back-to-back	Triac or SCR pair
Works with	DC and AC within rated voltage (back-to-back MOSFETs for bidirectional blocking)	AC only (natural turn-off as current drops below holding current, typically at zero-cross)
Off-state leakage	Low (nA–µA range typical)	Higher (can be mA-level due to device and snubber paths)
“On” behavior	Low R_ON (I·R loss)	Conduction characterized by a voltage drop rather than R_ON
Off-state capacitance	Low (reduces feedthrough)	Less emphasized
Switching control	Fast, precise, not tied to waveform phase	Often zero-cross or random-turn-on variants for AC control
Typical strengths	Precision signals, measurement, DC paths	Cost-effective AC mains control

When to Choose Each Relay?

Choose an Opto-MOSFET Relay when you need low leakage, low off-capacitance, clean switching, and the ability to handle DC or AC within the device's rating. These traits matter in instrumentation, data acquisition, semiconductor test, battery systems, and signal multiplexing.
Choose a Triac/Thyristor SSR when you're controlling AC mains for loads like heaters, lamps, and some motors — especially where zero-cross switching reduces EMI and stress.

Key Applications of Solid-State Relays

Precision & Test: Opto-MOSFET relays excel in orce/sense paths, matrix switching, and high-impedance nodes where leakage and feedthrough must be tightly controlled.
Battery & EV BMS: Optical isolation plus nA-to-µA leakage and predictable on-resistance make opto-MOSFET relays well-suited for sensing and protection paths.
Facilities & Industrial AC Loads: Triac SSRs are a practical, economical choice for mains-powered heaters, lighting, and similar AC-only loads, often using zero-cross variants to limit EMI.

Toward Technologies' Solution

At Toward Technologies, we design from the inside out. Our in-house R&D team design both the MOSFETs and the PVGs (photovoltaic generators) used inside our relays, giving us control over the core switching elements. This vertical approach lets us deliver industry-leading turn-on time, load voltage ratings, low on-resistance, and high load currents—not just on paper, but in real production.

We also keep over 90% of the production chain in-house. That depth of manufacturing control safeguards consistency, traceability, and quality at scale. And because we've spent years mass-producing these devices, we can be highly competitive on price without compromising performance.

Highlighted series and key specs:

42 Series - up to 5A continuous load current
53 Series - up to 3,300V load voltage
74 Series - normally-closed configuration
AS Series - 12mm creepage distance for robust isolation
46 Series - ~3 pF output capacitance for minimal feedthrough

Whether you’re optimizing for leakage, capacitance, current, voltage, or isolation geometry, these families give you clear options tailored to real-world design constraints.

Inside of Toward's Opto-MOSFET Relay

Summary

“SSR” is the umbrella.
Opto-MOSFET relays are MOSFET-output SSRs with optical isolation—strong on low leakage, low capacitance, fast and precise control, and DC compatibility (and AC within rating).
Triac/Thyristor SSRs are AC-only specialists—robust and cost-effective for mains loads, often with zero-cross behavior.

Mos Relays: SMD-6, 60V/ 5A, Opto-MOS Relay
The 42 series is a SMD-6, load Voltage at 60V, load current up to 5 Amps, normally open IC Solid State Relay (1 Form A).

SMD6-5, 3300V/ 300mA, Opto-MOS Relay (SiC MOSFET)
The 53 series is a SMD6-5, load Voltage up to 3,300V, load current at 0.3 Amps, normally open IC Solid State Relay (1 Form A).

SMD4, 400V/ 90mA, Opto-MOS Relay, SPST-NC (1 Form B)
The 74 series is a SMD-4 load voltage 400V, load current 90mA IC Solid State Relay is in a normally closed SPST contact arrangement (1 Form B).

SO-16, 3300V/ 350mA, Opto-MOS Relay (SiC MOSFET)
AS53F is a SMD6-5, load Voltage up to 3,300V, load current at 0.3 Amps, normally open IC Solid State Relay (1 Form A) with 12 mm creepage distance for robust isolation.

SMD-4, 80V/ 80mA, Opto-MOS Relay
The 46 series provides 80V load voltage with up to 80mA of load current and ~3 pF output capacitance for minimal feedthrough.