What each device does, when you need a limit, and how the two work together in a heating system.
A temperature controller keeps your process at setpoint. A temperature limit shuts the process down if something goes wrong, and keeps it down until a person resets it. Most electric heating systems that could damage product, equipment or people should have both.
A controller reads a thermocouple or RTD and adjusts heater output to hold a setpoint. PID controllers such as the Watlow PM continuously modulate power, often by driving an SSR or SCR power controller. Controllers can also have alarm outputs, but those alarms can be set to reset automatically once temperature returns to normal.
A limit controller is an independent safety device used alongside the control loop. It can’t control temperature on its own. It watches its own sensor and, when temperature crosses the high (or low) limit, its output latches off. It stays off until the temperature is back in the safe range and an operator presses reset, whether at the front panel or from a remote reset switch.
Because of that latching behavior, a limit can’t be used as an on/off controller. Watlow also notes that a limit’s output must be a mechanical relay rather than an SSR or switched DC output. Some limits offer “auto reset on power loss,” but that only covers how the unit powers up; recovering from an actual limit trip always needs a manual reset.
Every electric heating system follows the same chain. Getting each link right is what makes temperature stable and the process safe.
Holds setpoint with PID or on/off control. Outputs can drive relays, SSRs or SCRs. Alarms can auto-reset. The everyday process loop.
Independent safety shutdown with its own sensor. Always latching and must be manually reset. Mechanical relay output. Cannot control temperature.
If a stuck contactor, failed SSR or bad sensor could overheat equipment, product or people, add a limit.
A latching limit makes sure a runaway condition waits for a person to investigate before heat comes back.
Many ovens, furnaces and heated tanks call for an approved limit. Check your requirements.
If going out of range only means scrap or lost efficiency, a non-latching alarm or a second on/off controller can stop the process and let it recover automatically. That protects product, but it isn’t a safety system. If going out of range could damage equipment or create a hazard, use a latching limit with its own sensor, and follow any requirements from your insurer or local authority.
Seagate’s Watlow line includes FM-approved over- and under-temperature limits. Our engineers can help you pair the right controller, limit, sensors and power control.
Quick answers.
Some controllers offer a limit option, but the safe practice is a separate limit with its own sensor, so a single failure can’t disable both control and protection.
So the heat can’t come back on by itself after an over-temperature event. Someone has to check the problem first.
Per Watlow, a limit’s output must be a mechanical relay, not an SSR or switched DC output. It typically opens a contactor coil.
For product-quality alarms, yes. For safety, no. Alarms can reset automatically and aren’t considered a safety system.
Watlow PM controllers for control, Watlow limit controllers for protection, and DIN-A-MITE SCRs for power switching.
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