Introduction: Relay, SSR drive, and logic outputs on a temperature controller board each handle load current, switching frequency, isolation, and heat in different ways.
Anyone wiring an industrial temperature controller board eventually faces the same terminal strip: a pair of relay contacts, a small SSR drive header, and one or two logic pins, all sitting side by side. The labels are short, but the paths behind them behave very differently. One closes a metal contact against a mains load, one sends a low-voltage command to a solid-state device mounted elsewhere, and one simply pulls a small signal low for a fan or an alarm. Following those two switching paths — moving contacts and solid-state drive — through the same heating and cooling architecture makes it much easier to read an output section and match each channel to the actuator it will actually drive.
How an Electromechanical Relay Switches a Heater or Compressor Load
An electromechanical relay is the most familiar switching path on a controller board. A coil inside the relay energizes from the board's low-voltage control electronics, and the magnetic field pulls a movable arm until it presses against a fixed contact. That closure completes the load circuit. The metal-to-metal contact becomes the current path, so a heater, pump, or compressor contactor sees a hard, near-zero-resistance connection rather than a semiconductor junction. On a typical industrial temperature controller PCBA, the relay side is specified as one or two SPDT channels rated 5A at 250VAC, enough headroom for a resistive heater on a test rig, a small oven, or the coil of a larger external contactor. The SPDT arrangement also provides a normally-open and a normally-closed terminal, which helps when a cooling device should fail into an off state while a heating channel is wired the other way around. The mechanical gap between coil and contact gives the relay path its isolation. Control-side logic stays on one side of that gap and the switched mains circuit stays on the other, so a 250VAC load never shares a return path with the board's own supply. That separation is why relays still carry the mains-side output on so many controller boards, even where solid-state switching would be faster. The trade-off shows up in the load type. A resistive heater draws steady current and treats the contacts gently; a compressor motor, solenoid valve, or transformer pulls a surge at start-up and pushes current across the gap as the contacts separate. Contact life tracks that behavior, not the voltage and current numbers alone, so a 5A rating describes one load type better than another — which is why the load itself belongs in the wiring decision.
What an SSR Driver Output Changes About Switching Behavior
The second switching path looks almost identical at the terminal strip but works on a different principle. An SSR driver output supplies a 12VDC or 24VDC control signal to an external solid-state relay, so the board issues the command while the switch itself sits outside the controller. Vortixion's industrial temperature controller board offers one or two SSR drive channels and one or two relay channels on the same output section, with the actual solid-state device mounted on a panel or heatsink near the load. That single fact explains most of the practical differences between the two paths.
1. SSR Drivers Switch Frequently Without Moving Mechanical Contacts
A solid-state relay contains a semiconductor output stage with an optically isolated input, so the drive signal crosses a light barrier before reaching the switching element. Nothing moves inside. With no contacts to pit, stick, or wear, an SSR tolerates frequent cycling far better than a mechanical relay, which is why heater loops that pulse on and off around a setpoint often end up on the SSR path. The heat story changes too. A semiconductor drops voltage while conducting, so it generates heat at the point of switching, and that heat belongs to the external device and its heatsink rather than to the controller board. Panel layout then matters: the SSR needs airflow or a heatsink surface, while the control board itself stays comparatively cool.
2. Relay Contacts Carry Higher Current and React to Load Arcing
Contact current is one reason the relay path survives alongside solid-state drive. A 5A SPDT contact can carry mains-level current directly, so a heater wires straight through the board's relay terminals without an intermediary device. The cost appears at the moment of opening. As contacts separate, current keeps flowing briefly through an arc, and the arc's energy 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, eroding the contact surface faster. Relay makers categorize contacts by load type for exactly this reason, so choosing a relay channel for a compressor means reading the contact rating together with the load rather than the ampere figure alone.
Where Logic and Analog Outputs Sit in the Same Output Architecture
Not every output on a controller board is meant to switch a heater. Logic outputs use open-collector or MOSFET stages that pull a small signal line low instead of carrying load current. They suit low-power jobs: running a cabinet cooling fan, driving a status lamp or buzzer, or triggering an alarm circuit when a temperature limit is crossed. Wiring is simpler because these stages work at the board's own logic level, and their current handling sits far below the relay and SSR paths. On the Vortixion board, those logic stages share the output section with the switching channels and support heating-only, cooling-only, and combined heating and cooling modes, so one controller can manage a heating element, a cooling device, and an alarm at the same time. Combined heating and cooling is where the output layout starts to make sense as a whole. A heating channel usually runs through a relay when it switches a few times an hour, or through an SSR driver when it pulses often to hold a setpoint. A cooling channel may use 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 — set how many actuators the board can drive directly, while a single analog output covers proportional adjustment for a valve when on/off switching is too coarse. The two switching paths carry the on/off loads that define most heating and cooling behavior, and reading terminal labels with load type, switching frequency, isolation, and heat location in mind tells you which path a given device belongs on.
Conclusion
The three output types divide the work on a temperature controller board in a fairly predictable way. Moving contacts handle mains-level current and tolerate occasional switching, which suits heaters and compressor contactors. SSR driver channels hand the switching job to an external solid-state device that absorbs the heat and takes the wear of frequent cycling. Logic stages stay low-power and cover fans, alarms, and status signals. Matching an actuator to the right path comes down to four points: how much current the load draws, how often it switches, whether the load side needs isolation from control logic, and where the switching heat will land. Vortixion's industrial temperature controller board lists its relay, SSR drive, and logic channels together with the supported sensor inputs, which makes it easy to see how each path is arranged in a custom build.
FAQ
Q：What is the difference between a relay output and an SSR output on a temperature controller board?
A：A relay output switches the load directly through mechanical contacts rated 5A at 250VAC in SPDT form, so mains current flows through the terminal pair. An SSR driver output is a low-voltage command of 12VDC or 24VDC that controls an external solid-state relay, which performs the actual switching. The relay path carries arcing and heat at the contact; the SSR path moves switching heat to the external device.
Q：How does a 5A relay contact rating relate to the heater it switches?
A：The 5A figure describes the current the contacts carry as they close and open at up to 250VAC. A resistive heater drawing steady current sits comfortably inside that rating. An inductive or high-inrush load changes the picture, because arcing during contact separation depends on the load's behavior, so the same 5A rating supports different contact life across load types. Read the rating together with the heater current, its start-up surge, and how often it cycles.
Q：Why do heating and cooling channels on the same board often use different switching devices?
A：Heating channels usually switch a resistive load at mains voltage, which suits a mechanical relay or a solid-state path. Cooling channels often drive a compressor contactor, a fan, or a logic-level alarm device. Frequency differs too: a fan or chiller may cycle often, favoring solid-state drive, while a heater may switch a few times an hour, which a relay handles easily. Splitting output types lets each channel match its load's current, switching rate, and heat needs.
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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