Can An AC Solid State Relay Be Used For DC?

Two Types of SSRs Based on Input and Output Power
SSRs are categorised by their input and output power levels: AC SSRs and DC SSRs are the main types.

It is important to select the proper type of Solid-State Relay (SSR) for a specific application to achieve the required results of compatibility and efficiency.

What is an AC Solid State Relay?

An AC Solid State Relay (AC SSR) is designed to manage alternating current power circuits through a switching mechanism. An AC SSR integrates a TRIAC or SCR (Silicon Controlled Rectifier) for every load, thus eliminating the need for separately fitted switches. This construction greatly improves the efficiency, speed, and reliability of these relays compared to mechanical ones.

AC SSRs come in several types, with the most popular being the AC to AC SSR, which is used to switch on or off AC loads directly, and the AC to DC SSR, which controls DC power but is mainly used for AC to DC power conversion and switching.

What is a DC Solid State Relay?

Just like the AC SSR, a DC Solid State Relay (DC SSR) does the same work for direct current loads. This type has a special construction with semiconductors like MOSFETs or IGBTs as switches. More often than not, the device is used in solar power systems, DC motors, and telecommunications units.

DC SSRs come in a variety of forms. For example, a DC to DC solid state relay is used to switch on or off direct current circuits, and a DC to AC solid state relay, which performs the reverse operation and converts a DC source to an AC voltage for specific purposes.

What is the Difference Between AC SSR and DC SSR?

Feature	AC SSR	DC SSR
Power Type	Alternating Current (AC)	Direct Current (DC)
Applications	Household appliances, industrial machinery	Batteries, solar systems, DC motors
Current Regulation	Regulates AC current	Regulates DC current
Switching Components	TRIAC or SCR	MOSFETs or IGBTs
Customization	Suited for AC systems with sinusoidal waveforms	Customizable for DC systems

The primary distinction that differentiates AC SSRs from DC SSRs is the type of power each respectively controls. :

• AC SSRs regulate electric currents and are simply suited for supplying power to loads with sinusoidal waveforms or to systems with AC such as household appliances or industrial machinery.
• DC SSRs, on the other hand, regulate electric current from batteries or other renewable sources where circuits with direct current are necessary.

Apart from the type of power, the component structures and switching units also differ from DC to AC SSRs, and thus make it possible for the machines to be customised to their respective power types.

Solid State Relay Schematic

Knowing the circuit diagrams for SSRs enables the user to carry out the installation and troubleshooting function more easily. Below are the two typical circuit diagrams, namely, AC solid state relay circuit diagram and DC solid state relay circuit diagram:

AC Solid State Relay Circuit Diagram

An AC SSR circuit always has an input control unit and a power output unit. The input side is usually fitted with a control device, which may be a microcontroller or a sensor. The output side interfaces the AC load. When the control circuit sends a command, the switching device, which is normally a TRIAC or SCR, is turned ON, power on the control circuit drives current flows through the load.

DC Solid State Relay Circuit Diagram

However, there are some modifications in the components used in the DC solid state relay circuit diagram, like MOSFETS which are more appropriate for the DC current. The trigger control input to the relay causes the MOSFET to switch on to power the load. Features such as optical isolation for safety can be incorporated in the DC SSR circuit.

Can an AC Solid State Relay be Used for DC?

AC solid state relays (SSRs) are not inherently compatible with DC systems. Their design relies on the alternating nature of AC voltage to function safely. Using an AC SSR for DC applications can lead to permanent damage, uncontrolled current flow, or even fire hazards.

Why AC SSRs Fail in DC Systems

Issue	AC SSRs in DC Systems	Reason for Failure
Zero-Crossing Dependency	AC SSRs rely on zero-crossing points to turn off.	DC systems have continuous current, causing SSR to stay “ON”.
Voltage Rating Mismatch	AC SSRs are rated for peak AC voltage (e.g., 220V AC).	DC voltage is constant and can exceed the SSR’s insulation threshold.
Arc Suppression Limitations	AC SSRs lack mechanisms to suppress arcs in DC.	DC arcs are harder to extinguish, and AC SSRs don’t have the necessary snubber circuits.

Zero-Crossing Dependency

• AC SSRs use TRIACs or thyristors that rely on the AC waveform’s natural zero-crossing points to turn off.
• In DC systems, current flows continuously without zero crossings, leaving the SSR latched “ON” indefinitely.

Voltage Rating Mismatch

• AC SSRs are rated for peak AC voltage (e.g., a 220V AC SSR handles ~311V peak).
• DC voltage operates at a constant level, often exceeding the SSR’s effective insulation threshold (typically 50-70% of its AC rating).

Arc Suppression Limitations

• DC arcs during switching are harder to extinguish than AC arcs.
• AC SSRs lack snubber circuits or arc-quenching mechanisms required for DC interruption.

Alternatives to AC SSRs for DC Applications

In some cases, universal AC/DC hybrid SSRs, such as the AC DC solid state relay, can also be used to control both AC and DC loads. However, these hybrid relays come with strict limitations. It’s essential to verify the DC voltage and current ratings in the datasheet, as they are often lower than the AC specifications. Additionally, when using hybrid SSRs for DC, it is advisable to check if the relay includes built-in flyback diodes or DC snubbers to protect against voltage spikes and ensure safe operation.

Shenler – RSD Series AC DC Solid State Relay

The RSD Series AC DC Solid State Relay features a 1 N/O SPST DC output, ensuring a long service life with no contact or sparks. With MOSFET output for fast switching and imported optocoupler isolation, it provides reliable performance. The relay offers a wide control voltage range, LED status indicator, and optional IP20 protective cover for flexible installation.

Commonly used in DC heating, power supplies, valves, and motors, the 12V DC solid state relay in this series is perfect for low-power systems like automotive applications, delivering both performance and versatility.

Conclusion

SSRs are essential components of modern electronics, allowing rapid switching for alternating current and direct current electricity without mechanical movement. Selecting an SSR for a specific application requires knowing the various types, their differences, and their corresponding circuit diagrams. Choosing the correct relay, be it an AC to AC solid state relay or a DC to DC solid state relay, greatly impacts the safety and efficiency of the system.

References

Everything You Need to Know About SSR
How to Wire the Solid State Relay?
What is a DC Solid State Relay? How About Its Feature, Application, and How to Choose a Suitable DC SSR?
How to Choose a Suitable Solid State Relay?

 

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