Skip to content 
For anyone working in automation, solid state relays (SSRs) are the unsung heroes of stable equipment operation. But let’s be real with all the jargon like “DC to AC,” “photo-coupled,” and “normally open or closed,” picking the right one can feel like solving a puzzle. Today, we’re breaking down every type of SSR in plain language. By the end, you’ll know exactly which one fits your setup—no more guesswork, no more costly mistakes.
SSR by Input and Output Voltage Type
Let’s talk about the “language” your SSR speaks is input (the signal that triggers it) and output (the load it controls). Get this right, and you’re halfway there.
DC to AC SSR
Takes a low-voltage DC signal (usually 3–32V) and controls AC loads like industrial heaters or three-phase motors. Perfect for pairing with PLC systems,industrial workhorses, basically.
DC to DC SSR
Uses a DC signal to switch DC loads, think robotic arms or small DC fans. Fast, precise, and a must-have for low-power, high-accuracy setups.
AC to AC SSR
AC signal in, AC load out. Great for everyday stuff like building lighting or fan controls—simple, reliable, and easy to integrate.
AC to DC SSR
A bit of a specialist—takes an AC signal and switches DC loads, like charging batteries from an AC source. Not as common, but a lifesaver in specific setups.
AC/DC to AC SSR
The flexible one works with either AC or DC input to control AC loads. Ideal for systems where the input signal might vary, no need to swap out relays.
SSR by Output Power / Load Type
Now, let’s match the SSR to what it’s actually powering(AC and DC loads have very different needs.）.
AC SSR
Built for AC loads, using triacs to handle the back-and-forth flow of alternating current. Whether it’s a big three-phase motor or a bank of industrial lights, this one’s up to the task.
DC SSR
Designed for DC loads, with MOSFETs or transistors that handle one-way current. Faster switching than AC SSRs, making it perfect for things like DC motors in robotics or LED displays.
SSR by Isolation Method
Isolation keeps the control side and load side separate（safety first, and it cuts down on annoying electrical noise.）. Here are the main players:
Photo-Coupled SSR
Using light to isolate the input and output, which means great resistance to electromagnetic interference. Perfect for sensitive gear like medical equipment or precision temperature controllers.
Transformer-Isolated SSR
Uses a transformer for isolation, so it can handle way higher voltages . If you’re dealing with heavy industrial setups or high-voltage motors, this is your go-to.
Reed-Isolated SSR
Mixes a mechanical reed switch with solid-state parts. Offers top-notch isolation and low resistance, making it a favorite in aerospace or high-end lab equipment where reliability is non-negotiable.
SSR by Switching Method
When the SSR turns on relative to the AC voltage cycle matters a lot. It can save your equipment from damage.
Zero-Crossing Switching
Waits until the AC voltage hits zero to turn on. This cuts down on electrical noise and inrush current, which is great for resistive loads like heaters or incandescent lights. Your equipment will thank you for the longer lifespan.
Random Switching
Turns on as soon as it gets the signal, no waiting for the voltage cycle. Inductive loads like motors or transformers need this to delay can cause dangerous voltage spikes.
Peak Switching
Turns on at the peak of the AC voltage. It’s a niche option, mostly used in specific power regulation setups. Chances are, you won’t need this for everyday applications.
SSR by Operating Mode
What does the SSR do when there’s no signal? That’s its operating mode, and it’s all about safety.
Normally Open (NO) SSR
The default mode for most setups. It stays off until it gets a signal, then turns on. Think of it like a light switch(off until you flip it.). Great for heating, lighting, and most standard controls.
Normally Closed (NC) SSR
Stays on until it gets a signal, then turns off. This is all about safety—like emergency shutoffs. If something goes wrong and the signal is lost, it keeps the load running, which can prevent disasters.
SSR by Mounting Method
Where you put the SSR depends on your space and power needs. Pick the wrong mounting, and you’ll be dealing with headaches.
PCB Mounted SSR
Tiny and soldered right onto circuit boards. Perfect for low-current stuff (under 10A) like sensors or small gadgets. Saves space and keeps things neat.
Panel Mounted SSR
Screwed onto a panel, often with a heatsink. Handles high currents (50A and up) for big loads like industrial ovens or large motors. Needs good airflow to stay cool.
Bracket Mounted SSR
Attached to a bracket for better cooling. Good for medium currents (10–50A) in control panels where space is tight but you still need reliable performance.
DIN Rail Mounted SSR
Clips onto a standard 35mm DIN rail, just like breakers or fuses. Super easy to install in electrical cabinets, and makes maintenance a breeze. Great for industrial control systems.
SSR by Construction
How the SSR is built affects what it can do. There are two main types:
Discrete SSR
Made with separate components (like an optocoupler and a switch). You can customize it for specific voltages or speeds, which is handy if you have unique needs.
Hybrid SSR
Mixes solid-state parts with mechanical relay bits. Gets the best of both worlds—fast switching and the ability to handle huge currents. Perfect for EV charging stations or big power distribution setups.
Conclusion
Choosing an SSR comes down to three steps:first, check if the load is AC or DC; second, see if there’s enough space for installation; and finally, consider if isolation is needed and when to switch. Just follow this guide to find the right match, and your equipment will run smoothly for sure!If you haven’t found a suitable product after reading the article, you might as well try C-Lin‘s SSR.
