What is the difference between current limit and thermal protection?
Current limit watches an instantaneous quantity and reacts within milliseconds once the threshold is exceeded; it protects against sudden events such as short circuit, jamming and stall. Thermal protection watches the heat accumulating over time and catches sustained overload that stays below the threshold. The two catch different events; this general principle cannot be attributed to the modules, because no thermal protection declaration exists in the product data.
The two protections work on different time scales.
Current limit: as soon as the measured current exceeds the defined threshold, it cuts or limits the motion. Its response is fast, because in a short circuit there is no time to wait for the temperature to rise. On the PT500 this principle has been made user-adjustable: the A1 pot sets an independent current limit for the clockwise direction and the A2 pot for the reverse direction, and the AntiSwitch function detects the current rise at the end of travel and stops that direction.
Thermal protection: a load that stays just below the threshold but lasts for hours never triggers the current limit; the component temperature nevertheless rises gradually. Thermal protection watches this accumulation.
Why both are needed:
- Current limit only: misses the fatigue caused by slow heating.
- Thermal protection only: is not fast enough in a short circuit.
Note: no thermal protection, operating temperature range or derating curve is declared in the ST42, KS250 and PT500 product data; the comparison above is a general power electronics principle. On these modules the protection settings open to the user are the PT500's A1/A2 current limits and the AntiSwitch function on the KS250/PT500.
The correct approach on the user side is not to make the tripping of protections part of normal operating behaviour. If the protections trip frequently, it means the system is incorrectly sized; the selection should be made again leaving a safety margin against the driver's continuous current rating (KS250 15 A, PT500 30 A).
Full topic: DC Motor Driver Selection Guide