ComponentsElectromechanical

Solid-State Relay (SSR)

Relay

A semiconductor-based switch that controls AC or DC loads without moving parts, using optocoupler isolation and a triac/MOSFET output.

Schematic symbol

Relay

How solid-state relay (ssr) appears in a circuit diagram.

Types & variants

AC SSR (triac output)DC SSR (MOSFET output)Zero-crossing SSRInstant-on SSRHigh-current SSRLow-voltage control SSR

Key specs

Input voltage

V DC

Control voltage required to activate the SSR (typically 3-32V DC).

Output current

A

Maximum current the SSR can handle on the output side (e.g., 10A, 25A, 40A).

Output voltage

V AC/DC

Maximum voltage the SSR can switch (e.g., 24-380V AC for AC SSRs).

Isolation voltage

V

Voltage rating between input and output circuits (typically 1500-4000V).

On-state resistance

Ohm (Ω)

Resistance when conducting (typically 0.05-0.2Ω for power SSRs).

Markings

Usually labeled with input/output voltage ratings, current rating, and manufacturer logo. May have polarity markings for DC input/output.

Standard values

Common input voltages: 3-32V DC. Common output currents: 5A, 10A, 20A, 30A, 40A. Output voltages: 24-480V AC or 5-60V DC.

How to choose

1) Match input voltage to your control signal (3.3V/5V/12V/24V). 2) Select output current > max load current. 3) Choose output voltage matching your AC/DC supply. 4) Consider zero-crossing vs. instant-on for your application. 5) Check isolation voltage for safety.

Pinout & package

Typically 4 pins: Input+ (anode), Input- (cathode), Output+ (load), Output- (load). Packages: PCB mount (terminal block), socket mount, DIN rail mount.

Example circuits

  • Remote control of AC appliances (lights, heaters) with microcontroller
  • Solid-state switching for motor control
  • Automated lighting systems with zero-crossing switching
  • Battery-powered DC load switching

Common failures

Fails short-circuit (output stuck ON) or open-circuit (output stuck OFF). Visible signs: burn marks, discoloration, or physical damage. Can fail from overcurrent, overvoltage, or voltage spikes.

How to test

1) Check input with multimeter (should show high resistance when off, low when activated with control voltage). 2) Check output with multimeter (should show high resistance when off, low when activated). 3) Test with actual load at low voltage.

Substitutes

Mechanical relays for AC/DC switching, contactors for very high currents, MOSFETs/IGBTs for DC switching, thyristors for AC control.

Where to buy

DigiKeyMouserLCSCRobuAmazon