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.
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.
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(content created Jul 30, 2024)