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
Key specs
Input voltage
V DCControl voltage required to activate the SSR (typically 3-32V DC).
Output current
AMaximum current the SSR can handle on the output side (e.g., 10A, 25A, 40A).
Output voltage
V AC/DCMaximum voltage the SSR can switch (e.g., 24-380V AC for AC SSRs).
Isolation voltage
VVoltage 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).
Laws that govern it
The input is an LED into an opto-triac or photovoltaic stack, so it takes a current-limited drive like any other diode.
- Watt's lawP = V × I = I²R = V²/R
Its output is a TRIAC or MOSFET with a real forward drop: about 1.5 V × I of continuous heat, which is why panel SSRs sit on heatsinks.
- Joule's law of heatingH = I² × R × t
Zero-cross switching keeps the I²t stress low, which is why an SSR outlives a mechanical relay on lamp and heater loads.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
How it compares
- Relay vs Solid-State Relay (SSR)
Side by side with Relay (electromechanical) — specs, variants and which one the job calls for.
- Optocoupler vs Solid-State Relay
Side by side with Optocoupler / Opto-isolator — specs, variants and which one the job calls for.
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.