A solid-state relay can often replace a mechanical interface relay, but the change is not a like-for-like substitution. The correct decision depends on the load, switching duty, failure behaviour and electrical characteristics of the complete circuit.
Why engineers consider the change
Mechanical contacts wear and can bounce, arc or weld. A solid-state output has no moving contact, so it is well suited to frequent switching and applications where predictable switching time and long service life matter. It can also reduce maintenance in panels where plug-in relays operate continuously.
The trade-off is that a solid-state output behaves like a semiconductor rather than an ideal contact. It has an on-state voltage drop, off-state leakage, defined polarity or AC/DC limitations, surge-current limits and thermal derating. These values must be compatible with the load.
Seven checks before replacing the relay
- Contact function: confirm whether the circuit needs normally open, normally closed or changeover operation. Many solid-state interface relays provide one normally open function; do not assume a changeover contact is available.
- AC or DC load: select an output technology intended for the actual load circuit. A DC MOSFET output and an AC triac output are not interchangeable.
- Current and inrush: compare continuous, starting and short-duration surge current with the exact datasheet.
- Inductive behaviour: for coils, verify L/R, energy or power-factor conditions and the required suppression.
- Leakage and voltage drop: confirm that the off-state leakage cannot falsely energise the load and that the on-state drop is acceptable.
- Temperature and mounting: apply the manufacturer’s derating curve, grouping restriction and rail orientation instructions.
- Safety and failure mode: a solid-state relay must not be used as a safety isolation device unless the complete circuit and certified function explicitly permit it.
Example: a 24 V DC field load
The SLO24CR is a plug-in DC output relay with a 0–66 V DC load range and no minimum load requirement. Its datasheet specifies 0–3 A continuous load current, 15 A for 10 ms inrush, 1 mA leakage, 0.5 V voltage drop and a maximum inductive-load condition of L/R 5 ms at 24 V / 2 A.
Those figures show why “3 A relay” is not a sufficient replacement specification. A 2 A solenoid with a long L/R time constant may be a more demanding application than a 3 A resistive lamp load. Conversely, a sensitive low-current circuit may be affected by leakage even though its current is far below the rating.
Example: an AC coil
For an AC contactor coil, the important values include nominal voltage, pull-in VA/current, holding VA/current, operation frequency and ambient temperature. A model such as the SLO24TR AC output relay must be checked against those values. The DC data from a MOSFET relay cannot be reused.
Panel retrofit considerations
Check the socket, terminal numbering, wiring polarity, fuse and physical clearance before installation. Delcon plug-in output relays use compatible bases such as the MOS1GN screw-terminal socket; input relays use the corresponding MIS family. A retrofit may therefore require a socket change even when the enclosure width appears similar.
Decision rule
Replace a mechanical relay with a solid-state relay only after the actual load and duty cycle have been matched to a documented solid-state output. The strongest candidates are frequent-switching interface applications with well-defined AC or DC loads. Keep a mechanical solution where true contact isolation, changeover functionality or an application-specific safety architecture requires it.
Use the Delcon product range to shortlist a model, then verify every limit from the current datasheet or ask Delcon to review the load data.