On a temperature controller board, relay, SSR drive, and logic outputs each manage load current, switching frequency, isolation, and heat through distinct methods.
When wiring an industrial temperature controller board, engineers commonly encounter a terminal strip containing a pair of relay contacts, a compact SSR drive header, and one or two logic pins positioned side by side. Though the labels are brief, the circuits behind them operate quite differently. One closes metal contacts against a mains load, another delivers a low-voltage command to an external solid-state relay, and the third simply pulls a signal line low for a fan or alarm. Tracing these two switching paths—mechanical contacts and solid-state drive—across the same heating and cooling architecture simplifies reading an output section and aligning each channel with the actuator it will control.
How an Electromechanical Relay Switches a Heater or Compressor Load
The electromechanical relay stands as the most recognizable switching path found on a controller board. When the board's low-voltage control electronics energize the relay coil, the resulting magnetic field draws a movable arm against a fixed contact, closing the circuit. This metal-to-metal contact becomes the current path, presenting a heater, pump, or compressor contactor with a near-zero-resistance connection rather than a semiconductor junction. On a standard industrial temperature controller PCBA, the relay section is rated as one or two SPDT channels at 5A, 250VAC, providing sufficient headroom for resistive heaters on test setups, small ovens, or the coil of a larger external contactor. The SPDT configuration includes both normally-open and normally-closed terminals, useful when a cooling device should default to an off state while a heating channel is wired in the opposite manner. The physical gap between the coil and contacts supplies the relay path's isolation. Control logic remains on one side of that gap, and the switched mains circuit resides on the other, ensuring that a 250VAC load never shares a return path with the board's own supply. This separation explains why relays continue to carry mains-side outputs on many controller boards, even where solid-state switching might be faster. The trade-off becomes evident in the load type. A resistive heater draws steady current and places minimal stress on the contacts; a compressor motor, solenoid valve, or transformer draws a start-up surge and forces current across the gap as contacts part. Contact lifespan follows this behavior, not merely the voltage and current ratings, so a 5A rating describes one load type more accurately than another—hence the importance of considering the load itself in wiring decisions.
What an SSR Driver Output Changes About Switching Behavior
At the terminal strip, the second switching path appears nearly identical yet operates on a fundamentally different principle. An SSR driver output provides a 12VDC or 24VDC control signal to an external solid-state relay, meaning the board issues the command while the switch itself remains outside the controller. Vortixion's industrial temperature controller board includes one or two SSR drive channels alongside one or two relay channels in the same output section, with the solid-state device mounted on a panel or heatsink near the load. This one fact accounts for most of the practical distinctions between the two paths.
1. SSR Drivers Switch Frequently Without Moving Mechanical Contacts
A solid-state relay incorporates a semiconductor output stage paired with an optically isolated input, so the drive signal crosses a light barrier before it reaches the switching element. Inside, nothing moves. Because there are no contacts to pit, stick, or suffer wear, an SSR handles frequent cycling far better than a mechanical relay—this is why heater loops that rapidly toggle around a setpoint are typically assigned to the SSR path. The thermal behavior also differs. A semiconductor experiences a voltage drop while conducting, generating heat at the switching point, and that heat is absorbed by the external device and its heatsink rather than by the controller board. Panel layout thus becomes crucial: the SSR requires airflow or a heatsink surface, while the control board itself remains relatively cool.
2. Relay Contacts Carry Higher Current and React to Load Arcing
Current capacity is one factor that allows the relay path to coexist alongside solid-state drive. A 5A SPDT contact can directly carry mains-level current, enabling a heater to be wired straight through the board's relay terminals without an intermediary device. The trade-off becomes apparent at the moment of opening. As the contacts separate, current continues to flow briefly through an arc, and the energy in that arc depends on the load. A resistive heater produces a modest arc; an inductive load, such as a motor or solenoid, stores energy in its magnetic field and releases it across the gap, accelerating contact surface erosion. For precisely this reason, relay manufacturers categorize contacts by load type, meaning that selecting a relay channel for a compressor requires evaluating the contact rating together with the load rather than relying on the ampere figure alone.
Where Logic and Analog Outputs Sit in the Same Output Architecture
Not every output on a controller board is intended to switch a heater. Logic outputs rely on open-collector or MOSFET stages that pull a small signal line low rather than carrying load current. These are suited for low-power tasks: running a cabinet cooling fan, driving a status lamp or buzzer, or triggering an alarm circuit when a temperature limit is exceeded. Wiring is simpler because these stages operate at the board's own logic level, and their current capacity is well below that of the relay and SSR paths. On the Vortixion board, those logic stages occupy the same output section as the switching channels and support heating-only, cooling-only, and combined heating and cooling modes, enabling a single controller to manage a heating element, a cooling device, and an alarm simultaneously. The combined heating and cooling arrangement is where the output layout becomes fully meaningful. A heating channel typically runs through a relay when it switches only a few times per hour, or through an SSR driver when it pulses frequently to maintain a setpoint. A cooling channel may employ a second switching channel for a compressor contactor or a logic output for a fan. Channel counts—one or two relay channels, one or two SSR drive channels—determine how many actuators the board can drive directly, while a single analog output handles proportional adjustment for a valve when on/off switching is too coarse. The two switching paths handle the on/off loads that define most heating and cooling behavior, and reading terminal labels with consideration of load type, switching frequency, isolation, and heat location indicates which path a given device should be connected to.
Conclusion
The three output types distribute the workload on a temperature controller board in a relatively predictable manner. Moving contacts handle mains-level current and tolerate occasional switching, making them suitable for heaters and compressor contactors. SSR driver channels delegate the switching task to an external solid-state device that absorbs the heat and bears the wear of frequent cycling. Logic stages remain low-power, covering fans, alarms, and status signals. Selecting the correct path for an actuator comes down to four considerations: the current the load draws, the switching frequency, whether the load side requires isolation from control logic, and where the switching heat will accumulate. Vortixion's industrial temperature controller board presents its relay, SSR drive, and logic channels alongside the supported sensor inputs, making it straightforward to see how each path is organized in a custom configuration.
FAQ
Q：What is the difference between a relay output and an SSR output on a temperature controller board?
A：A relay output directly switches the load via mechanical contacts rated 5A at 250VAC in SPDT configuration, so mains current passes through the terminal pair. An SSR driver output provides a low-voltage command of 12VDC or 24VDC that governs an external solid-state relay, which handles the actual switching. The relay path experiences arcing and heat at the point of contact; the SSR path transfers switching heat to the external device.
Q：How does a 5A relay contact rating relate to the heater it switches?
A：The 5A rating indicates the current that the contacts carry when closing and opening at up to 250VAC. A resistive heater drawing steady current fits comfortably within that rating. An inductive or high-inrush load alters the situation, as arcing during contact separation depends on the load's characteristics; thus the same 5A rating yields different contact lifespan across load types. Evaluate the rating alongside the heater's current draw, its inrush surge, and its cycling frequency.
Q：Why do heating and cooling channels on the same board often use different switching devices?
A：Heating channels typically switch a resistive load at mains voltage, making them suitable for either a mechanical relay or a solid-state path. Cooling channels frequently drive a compressor contactor, a fan, or a logic-level alarm device. Switching frequency also varies: a fan or chiller may cycle often, preferring solid-state drive, while a heater might switch only a few times per hour, which a relay manages without issue. Using different output types allows each channel to align with its load's current, switching rate, and thermal requirements.
Sources / References
Overview of General Purpose Relays | OMRON Industrial Automation
Overview of Push Buttons / Indicator Lamps Technical Guide | OMRON Industrial Automation
Temperature Measurement Techniques and Application Examples (AN4449) | STMicroelectronics
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