A solid state relay (SSR) is an electronic switch that uses semiconductor components to switch on or off when an external voltage is applied. Unlike traditional electromechanical relays, SSRs have no moving parts, resulting in faster switching times, greater reliability, and longer operational life.
They are commonly used in applications where silent operation, low power consumption, and high durability are required.
Overview
SSR relays can be especially useful for microcontroller projects:
- GPIO Controlled:
Unlike mechanical relays, many SSR relays can be driven directly from a GPIO pin, making them compatible with both 5V and 3.3V microcontrollers. Their power requirements can be as low as 5mA (for 3.3V) and 12mA (for 5V), both well within the current limits of typical GPIO pins. - Low Power Consumption:
The low power consumption can be useful by itself when your project is battery-driven or otherwise sensitive to power consumption.
Careful Selection
Using SSR relays requires careful planning to select the correct type:
- AC or DC?
Since SSR relays operate with solid-state components, they use MOSFETs for DC switching and Thyristors (or Triacs) for AC switching. You need to choose the correct type of SSR relay for your application. Typically:- An SSR relay designed to switch AC household appliances is labeled DA (DC control, AC switching).
- An SSR relay designed to switch DC voltage is labeled DD (DC control, DC switching).
-
Maximum Current:
Pay attention to the maximum sustained current an SSR relay can handle. With budget SSR relays, especially from unverified sources, apply a 400% safety margin. For example, if you plan to switch 2000W (9A at 220V AC), use an SSR relay rated for at least 40A. Exceeding the current rating, even momentarily, can permanently damage the relay. High loads can also cause SSR relays to catch fire or explode. -
Heat Dissipation:
SSR relays generate heat when operating near their maximum current rating. Ensure proper heat dissipation by using an appropriate heat sink or mounting the relay on a metal surface. Choosing a relay with a higher current capacity can also help reduce thermal stress. -
Leakage Current:
Unlike mechanical relays, SSR relays do not provide a perfect open circuit when turned off. They have a small leakage current, which can be enough to power low-wattage devices (e.g., LED bulbs may glow faintly even when “off”). If complete isolation is required, a mechanical relay might be a better choice. - Zero-Crossing vs. Random Turn-On:
When switching AC loads, some SSR relays use zero-crossing detection, meaning they only switch on when the AC voltage crosses zero volts. This reduces electrical noise and extends the relay’s lifespan. Others use random turn-on (phase control), which is useful for applications like dimming. Make sure to select the right type for your use case.
SSR vs. Mechanical Relay – Pros and Cons
| Feature | SSR Relay | Mechanical Relay |
|---|---|---|
| Lifespan | Long (no moving parts) | Shorter (contacts wear out) |
| Switching Speed | Fast | Slow |
| Silent Operation | Yes | No (audible clicking) |
| Heat Generation | Yes (requires heat sink at high currents) | Minimal |
| Leakage Current | Yes | No |
| High Inrush Current Handling | Poor | Good |
Caveats
Affordable DA mini SSR relays, such as the G3MB-202P, are widely available on ready-to-use PCBs and are commonly used in DIY projects.
However, these SSR relays can handle only 2A (440W at 220V). While this may be sufficient for switching simple lamps, it is completely inadequate for most other applications. Exceeding the 2A limit, even momentarily, will immediately destroy the relay.
Industrial SSR relays are often a much better choice. Chinese clones are available at similar prices and claim to handle up to 40A (16A realistically). Even if you only need to switch lighter loads, these higher-rated relays provide a much greater safety margin and improved reliability.
Basic Principle
A classic electromagnetic relais use a magnetic field to electrically separate the trigger circuit from the load circuit. The same electrical isolation is achieved in SSR by using light:
In a SSR, the trigger circuit operates an optocoupler: it turns on an internal LED. The light that is emitted by this LED is picked up by a photodiode, providing the electrical isolation of both circuits.
The photodiode in turn uses a semiconductor to switch the load.
DA, DD, AD, AA
Depending on how a SSR is designed, it can switch AC or DC, and the trigger signal can be either AC or DC. SSRs are marked with two letters to indicate their design:
| Marking | Description |
|---|---|
| DA | a digital control signal (i.e. GPIO) controls an AC load |
| DD | a digital control signal (i.e. GPIO) controls a DC load (i.e. a motor or high-power LED) |
| AA | an AC signal controls an AC load |
| AD | an AC signal controls a DC load |
Semiconductors
SSR use solid-state components to perform the actual switching. Since there are no moving parts involved, SSRs do not wear out easily, and they are completely silent.
The type of semiconductor depends on whether the SSR is designed to switch AC or DC:
- MOSFET: like anywhere else in electronics, SSR use traditional MOSFET to switch DC loads
- Triac, Thyristor: Triacs are most commonly used in SSR for switching AC loads as they can conduct in both directions which makes them ideal for AC. Thyristors are sometimes also used (in pairs back-to-back, since they only conduct in one direction).
Advantages…
There are important advantages of SSR over classic mechanical relais:
- Low Power Consumption: a mechanical relais must maintain a magnetic field for the entire period of time it is switched on (which can require considerable power: mechanical relais typically can not be directly driven by a GPIO). SSRs are controlled electronically and only require a few mA that can be directly supplied by a GPIO.
- Silent: SSRs operate completely noise-free. There is no clack sound that is typically emitted by a switching mechanical relais.
- No Sparks: since there are no moving contacts, there are also no sparks (that can happen with mechanical relais when they switch high loads). In addition, SSR relais often use Zero-Crossing Detection to switch AC loads when the AC waveform crosses 0V. This further minimizes EMR and physical stress since the load is zero at the time where the switch takes place.
- Less EMR: Zero-Crossing Detection mechanical relais can create significant electrical interference: EMR and radio noise. Such electro-magnetic interference is caused by sparks and also the coils used in mechanical relais. SSRs do not produce this kind of interference.
- No Flyback Diode: since classic relais rely on magnetic fields, a flyback diode is needed to protect sensitive electronic parts from being hit by high momentary voltage spikes that can occur when the magnetic field of a classic relais collapses and releases its stored energy. SSRs do not use magnetic fields and thus do not need a flyback diode or can otherwise produce harmful voltage spikes that harm the trigger circuit.
- Wear-Out: like any physical device that uses moving parts, mechanical relais wear out over time. The sparks can corrode the switching contacts, and the relais may eventually fail to reliably switch.
…and Caveats
Despite the many advantages, SSRs have their own important caveats and risks that need to be carefully considered:
-
Heat Sink: The resistance in mechanical switches is almost zero. Solid state components inside a SSR can introduce a slightly higher resistance and/or produce a slight voltage drop, all of which is turned into heat that must be dissipated or else accumulates. Switching small applicances in the range of <400W typically do not produce relevant heat that requires heat sinks or active cooling. When you switch more powerful appliances, i.e. a water heater or powerful motors, the heat dissipation of SSRs must always be taken into consideration: use a proper heat sink, and make sure the housing allows the dissipated heat to vent off. Lack of proper heat dissipation may lead to the destruction of the SSR and can eventually cause fires.
-
Fire Hazard: Generally, SSRs do have an inherent risk of causing house fires when used improperly. Lacking heat dissipation (see above) is just one cause. Another avoidable fire hazard is overloading SSRs. Always keep in mind that SSRs use solid state switches that can explode or catch fire when loads are switched that exceed their specifications. As a rule of thumb, with quality SSRs from known sources, never switch loads that are higher than 80% of the maximum SSR rating. With cheap SSRs from unknown sources, never switch loads that are higher than 30% of the maximum SSR rating
SSRs from renown manufacturers are considerably more expensive than mechanical relais. Cheap generic SSRs (or fake clones) are available for a fraction of that price. These cheap SSRs regularly use inferior semiconductors, and the ratings printed on these SSRs are not trustworthy. Often, a SSR rated for 40A contains semiconductors rated for 12A. Likewise, a SSR rated for 10A can only switch loads up to 3A. Many people around the world successfully use cheap SSRs, however they add generous safety margins. Either do not exceed 30% of SSR ratings with these cheap clones, or disassemble one of them and check out the semiconductor used inside. Never ever trust the printed ratings.
Slow Website?
This website is very fast, and pages should appear instantly. If this site is slow for you, then your routing may be messed up, and this issue does not only affect done.land, but potentially a few other websites and downloads as well. Here are simple steps to speed up your Internet experience and fix issues with slow websites and downloads..
Comments
Please do leave comments below. I am using utteran.ce, an open-source and ad-free light-weight commenting system.
Here is how your comments are stored
Whenever you leave a comment, a new github issue is created on your behalf.
-
All comments become trackable issues in the Github Issues section, and I (and you) can follow up on them.
-
There is no third-party provider, no disrupting ads, and everything remains transparent inside github.
Github Users Yes, Spammers No
To keep spammers out and comments attributable, all you do is log in using your (free) github account and grant utteranc.es the permission to submit issues on your behalf.
If you don’t have a github account yet, go get yourself one - it’s free and simple.
If for any reason you do not feel comfortable with letting the commenting system submit issues for you, then visit Github Issues directly, i.e. by clicking the red button Submit Issue at the bottom of each page, and submit your issue manually. You control everything.
Discussions
For chit-chat and quick questions, feel free to visit and participate in Discussions. They work much like classic forums or bulletin boards. Just keep in mind: your valued input isn’t equally well trackable there.
(content created Jul 30, 2024)