| Basic Function | A relay transmitter transfers a control signal from an input circuit to an electrically isolated output circuit. | Signal switching, circuit isolation, alarm transmission, interlocking, and remote control | An input energizes a coil or semiconductor control stage, which changes the state of one or more output contacts or switching channels. | Choose a relay transmitter when isolation, contact multiplication, or switching a different voltage level is required. |
| Operating Technology | The internal method used to switch the output. | Electromechanical relay, solid-state relay, or hybrid relay | Electromechanical types use an energized coil and moving contacts. Solid-state types use semiconductor devices without mechanical contact movement. | Use electromechanical technology for broad load compatibility and galvanic contact switching. Use solid-state technology for high switching frequency, silent operation, and long mechanical life. |
| Input Signal Type | The electrical signal that activates the relay transmitter. | Digital voltage, dry contact, transistor output, pulse signal, or analog threshold input | The input circuit detects a voltage, current, or contact state and drives the relay switching element when the specified threshold is reached. | Match the input type to the controller output. A dry-contact input should not be connected to a powered output unless the device is designed for it. |
| Input Voltage | The nominal voltage required by the input or relay coil. | Common control levels include 5 V DC, 12 V DC, 24 V DC, 48 V DC, 110 V AC, and 230 V AC | The input circuit converts the applied control voltage into magnetic, optical, or electronic actuation. | Confirm the nominal voltage, allowable tolerance, polarity requirements, and whether the input is AC or DC before installation. |
| Input Current and Power | The current or power consumed by the input circuit during activation. | Typically from a few milliamperes for electronic inputs to several hundred milliamperes for larger coils | Coil-based devices draw current to create a magnetic field, while electronic devices use an input circuit to control a semiconductor switch. | Check the controller’s maximum output current. For low-power digital outputs, select a low-consumption input or use an interposing driver. |
| Output Contact Form | The electrical arrangement of the relay contacts. | Normally open (NO), normally closed (NC), changeover (CO/SPDT), DPDT, or multi-channel configurations | When the relay changes state, the common terminal connects to either the NO or NC terminal according to the contact arrangement. | Choose NO for loads that should be off by default, NC for fail-safe monitoring circuits, and changeover contacts when both states are needed. |
| Number of Channels | The number of independent input-output switching circuits in one device. | Single-channel, dual-channel, 4-channel, 8-channel, or higher-density modules | Each channel provides a separate switching path, although some modules may share power, commons, or protective components. | Select the channel count based on system expansion needs, cabinet space, wiring simplicity, and required isolation between channels. |
| Output Load Type | The kind of electrical load controlled by the relay contacts. | Resistive heaters, lamps, solenoid valves, motors, contactor coils, alarms, and electronic inputs | Relay contacts open or close the load circuit; the load’s inrush current and switching behavior determine contact stress. | Evaluate both steady-state current and inrush current. Motors, lamps, transformers, and solenoids may require derating or suppression. |
| Contact Rating | The maximum voltage and current the output contacts can safely switch under specified conditions. | Ratings vary by load type; common control applications use currents from signal level up to several amperes | The contacts must interrupt or carry current without excessive heating, arcing, welding, or loss of insulation. | Use the manufacturer’s rating for the actual AC or DC load. A contact rating for resistive loads may not apply to motors or inductive loads. |
| Switching Voltage | The maximum voltage present across the output circuit during operation. | Low-voltage DC, control-level AC, and higher industrial control voltages, subject to the device rating | Contact spacing, insulation system, and arc suppression determine the voltage the relay can interrupt safely. | Verify the rated voltage for the specific contact configuration and pollution environment. AC and DC interruption capability are not identical. |
| Electrical Isolation | The separation between input, output, and sometimes channel-to-channel circuits. | Galvanic isolation through contacts, transformer coupling, or optocouplers; isolation ratings are device-specific | Isolation prevents the output load voltage from directly reaching the control electronics, provided creepage, clearance, and insulation limits are respected. | Choose a device with separate input-output and channel-to-channel isolation where ground-loop prevention, safety, or different voltage domains are involved. |
| Response Time | The delay between input activation or release and the corresponding output change. | Electromechanical relays commonly operate in milliseconds; solid-state devices are generally faster | Mechanical movement, contact bounce, electronic filtering, and semiconductor turn-on characteristics contribute to the delay. | For high-speed counting, pulse generation, or rapid machine control, use a specified high-speed solid-state or electronic output rather than a general-purpose mechanical relay. |
| Contact Bounce | Brief unwanted transitions that occur when mechanical contacts open or close. | Present in electromechanical contacts; generally absent in solid-state switching | Physical contact surfaces may make and break several times before settling into a stable state. | Use debounce logic, a timer, or a suitable interface when the receiving circuit is sensitive to multiple transitions. |
| Switching Frequency | How often the relay can operate within its electrical and mechanical limits. | Low to moderate frequency for mechanical relays; higher frequency capability for solid-state designs | Each mechanical operation causes contact wear, while semiconductor switching is primarily limited by heat, voltage, and current stress. | Estimate the number of operations per hour and expected service life. Avoid using a mechanical relay for continuously repeated high-frequency switching. |
| Mechanical Life | The number of operations possible without an electrical load. | Often much higher than electrical life because there is little contact arcing under no-load conditions | Mechanical wear affects the armature, spring, bearings, and contact movement even when the load current is absent. | Compare mechanical life with electrical life; the electrical rating and load type usually determine the practical service interval. |
| Electrical Life | The expected number of switching cycles under a specified voltage, current, and load type. | Load-dependent and typically lower than mechanical life | Arcing, contact erosion, contact welding, and thermal stress gradually reduce switching reliability. | Use load-specific life data rather than relying only on the maximum current printed on a product label. |
| Inductive Load Protection | Measures used to limit voltage spikes produced when an inductive load is switched off. | Flyback diode for DC coils, RC snubber, varistor, or dedicated surge suppressor | Inductors resist sudden changes in current and can generate a high reverse-voltage transient at turn-off. | Install suppression close to the inductive load. A diode is suitable for many DC coils but changes the release behavior and must be correctly polarized. |
| Fail-Safe Behavior | The output state adopted after loss of input power, control wiring, or device power. | De-energized-safe, energized-safe, normally open, normally closed, or alarm-on-loss configurations | The contact arrangement and relay energization logic determine whether a fault produces an alarm or an apparently normal state. | For emergency or alarm circuits, select the state that exposes loss of power or wiring integrity instead of masking the fault. |
| Temperature Range | The ambient temperature range in which the relay transmitter operates within specification. | Industrial designs commonly cover sub-zero to elevated cabinet temperatures; the exact range is device-specific | Temperature affects coil resistance, contact capacity, insulation performance, semiconductor leakage, and heat dissipation. | Account for enclosure temperature, nearby heat sources, ventilation, and output derating at high ambient temperatures. |
| Mounting and Wiring | The physical installation method and terminal arrangement. | DIN-rail, panel, socket-mounted, PCB-mounted, screw terminal, spring terminal, or pluggable terminal | The mechanical interface secures the device while the terminals maintain the input, output, and protective connections. | Choose a mounting style compatible with the control cabinet, conductor size, maintenance method, and available clearance. |
| Protection and Environment | Resistance to dust, moisture, vibration, shock, and electrical interference. | Enclosed or sealed relay construction, protective coatings, vibration-resistant terminals, and specified ingress protection | Sealing and enclosure design reduce contamination, while mechanical retention and filtering improve reliability in harsh environments. | Check the required enclosure protection, vibration level, humidity, corrosive atmosphere, and electromagnetic environment. |
| Standards and Compliance | Safety and performance requirements relevant to relay construction and application. | Applicable low-voltage equipment, relay, insulation, EMC, and functional-safety requirements | Standards define conditions such as insulation coordination, dielectric strength, temperature rise, switching endurance, and electromagnetic compatibility. | Specify the standards required by the installation location, industry, voltage level, and safety classification before selecting the device. |
| Best Application Fit | The type of task for which the relay transmitter is most appropriate. | PLC interposing, alarm output, motor contactor control, signal conversion, safety interlocking, and remote status indication | The relay provides a controlled electrical interface between a low-power control system and a separate load or field circuit. | Prioritize electrical compatibility, isolation, load behavior, switching frequency, fault response, and environmental conditions over physical size alone. |
