Solid State Relay SSR

Solid State Relays (SSR): A Practical Guide for Selection, Brands, and Popular Models

If your project needs fast, silent, and reliable switching, solid state relays (SSRs) are hard to beat. This post walks through what SSRs are, how they’re classified, where they shine, and which brands/models are most commonly used—so you can choose parts with confidence.

What is an SSR?

A solid state relay is an electronic switch that turns a load on/off when a small control signal is applied. Unlike electromechanical relays (EMRs), SSRs have no moving parts. They use semiconductors (triacs/thyristors for AC, MOSFETs/transistors for DC) to perform the switching.

Why engineers pick SSRs

• Long life & high reliability: No contact wear or bounce.

• Fast & silent: Millisecond response, zero “click.”

• Vibration resistant: Great for harsh environments.

• Lower EMI (with ZC types): Less electrical noise than many EMRs.

Where they’re used
Industrial automation, HVAC and temperature control, reflow/heaters, lighting dimming, packaging lines, home appliances, lab equipment, and more.

How SSRs Are Classified (and why it matters)

By input (control side)

• DC-control (e.g., 3–32 VDC) — most common with PLCs/MCUs

• AC-control — line-driven or special PLC modules

By output / load type

• AC output (triac/thyristor): Heaters, lamps, many motors

• DC output (MOSFET/transistor): DC motors, solenoids, LEDs

By switching mode

• Zero-cross (ZC): Turns on at AC zero crossing → lower EMI and inrush for resistive loads

• Random turn-on (instant-on): Triggers immediately → needed for phase control, some inductive loads

By phase

• Single-phase, two-leg, three-phase (solid-state contactors)

By mounting / form factor

• Panel/chassis (“hockey-puck”), DIN-rail modules, PCB/DIP/SIP board-mount

By thermal design

• Bare module (add your own heatsink)

• Integrated heatsink (sometimes fan-assisted)

By features & protection

• Snubber networks, over-voltage clamps, short-circuit/fault detection, status LEDs, alarm outputs, soft-start/inrush limiting

By compliance

• UL/CSA/CE/UKCA, RoHS/REACH, IEC/EN categories, IP ratings

Tip: Start with load type (AC vs DC), voltage/current, and switching mode (ZC vs random). Then choose the mounting, cooling, and certifications your panel/build requires.

Leading SSR Manufacturers

These brands have broad availability, solid documentation, and long in-market track records:

• Sensata | Crydom — Classic industrial SSRs and solid-state contactors; wide current ranges.

• Omron — G3 series for both panel and PCB; control-panel staple worldwide.

• Carlo Gavazzi — RM1/RM2 panel SSRs; RGC DIN-rail units with integrated heatsinks.

• Schneider Electric (Zelio Harmony) — Slim interface SSRs (SSL), modular (SSM), panel (SSP).

• Siemens (SIRIUS 3RF) — Heavy-duty SSRs/contactors for heating and motion.

• Panasonic — Compact PCB/DIP SSRs (AQ/PhotoMOS families) for appliances and light industrial.

• Phoenix Contact — ELR DIN-rail solid-state contactors and reversing modules.

• Autonics — SR series single/three-phase SSRs with practical current steps.

• FOTEK — Cost-effective DC→AC panel SSRs in common ratings.

• Sharp / Renesas — SIP/DIP opto-triac modules for compact AC switching on PCBs.

Popular & Common Model Families (Quick Reference)

Use these names to search datasheets and line up pin-compatible or spec-compatible alternatives:

• Crydom — D24xx (e.g., D2425/D2450), H12/H24 (high current), CKR/CSW (ruggedized)

• Omron — G3NA (panel 5–90 A), G3NB/G3NE (compact panel), G3MB-202P (PCB 2 A), G3PE (contactors)

• Carlo Gavazzi — RM1A/RM1E (single-phase panel), RGC1/2/3 (DIN with heatsink), RMD (motor direction)

• Schneider Electric — Zelio Harmony SSL (slim), SSM (modular), SSP (panel)

• Siemens — SIRIUS 3RF2 (single/three-phase SSR/contactors)

• Panasonic — AQ-H/AQ-G (DIP), PhotoMOS (low-power DC), AP/AX (application-specific)

• Phoenix Contact — ELR …AC/…DC (single/3-phase, reversing)

• Autonics — SR1 (single-phase), SR2/SR3 (higher current/three-phase)

• FOTEK — SSR-xxDA/-xxAA/-xxDD (10–100 A classes)

• Sharp / Renesas — S202/S216 (SIP opto-triac, zero-cross options)

How to Select the Right SSR (Step-by-Step)

1. Confirm the load

   • AC resistive (heaters): AC output + zero-cross

   • AC inductive (valves/motors): AC output + random turn-on (or add proper snubbers)

   • DC loads: DC output (MOSFET)

2. Size voltage & current correctly

   • Voltage ≥ 1.2× your max line/rail voltage

   • Current: use the continuous RMS rating and derate for temperature (often 20–50%)

   • Check surge/inrush, especially for lamps and nichrome heaters at cold start.

3. Plan thermal management

   • SSRs dissipate heat (I×V drop). For panel/DIN styles, choose a heatsink or integrated-sink model sized to your ambient and enclosure.

4. Match the control side

   • Ensure your PLC/MCU output can deliver the SSR’s input voltage and min turn-on current.

   • For fast PWM or phase control, confirm turn-on/turn-off times.

5. Protection & compliance

   • Prefer models with snubbers, OV clamps, or external RC networks for inductive loads.

   • Verify UL/CE and any industry-specific requirements.

Wiring & Use Notes (Avoid the “gotchas”)

• Heat kills SSRs: Always check thermal curves; a 40 °C cabinet vs. 25 °C lab bench can halve usable current.

• Use proper fusing: Semiconductor fuses or fast-acting fuses protect better than slow-blow types.

• Leakage current is normal: Triac SSRs leak a few mA when “off.” Add bleeder resistors across tiny loads (e.g., small lamps) to prevent ghost glow.

• EMI & dv/dt: For inductive loads, use snubbers/RC networks; verify dv/dt immunity in the datasheet.

• Random vs zero-cross: Use random for phase angle control/dimming or when you must energize mid-cycle.

• Mechanical relays still have a place: For truly zero leakage, ultra-low heat, or very high surge tolerance, EMRs or hybrid relays may be a better fit.

Quick FAQs

Q: Can an SSR replace my mechanical relay one-for-one?
A: Electrically often yes, but account for heat, leakage, and control-side current. Mounting and heatsinking may also differ.

Q: Why does my lamp glow when “off”?
A: That’s off-state leakage. Add a small bleeder resistor or use a relay type with lower leakage.

Q: Zero-cross or random turn-on?
A: Zero-cross for resistive loads and lower EMI; random for phase control or some inductive applications.

TL;DR: A Reliable Starting Shortlist

• Panel AC SSR (general purpose): Crydom D24xx (e.g., D2425), Omron G3NA, Carlo Gavazzi RM1A

• DIN-rail, integrated heatsink: Carlo Gavazzi RGC, Phoenix Contact ELR

• PCB compact AC control: Omron G3MB-202P, Sharp/Renesas S202/S216

• DC load control (board-mount): Panasonic PhotoMOS families