How long is the PT500's 40A maximum current tolerated?
The 40 A maximum of the PT500 applies within a 15 second window; the continuous rating is 30 A. Always size the system by the 30 A continuous current.
The 40 A maximum current rating of the PT500 is defined for a period of 15 seconds; its continuous running rating is 30 A. This 15 second window is for the starting surge and for short overloads, it is not a continuous operating regime. For sustained loading, size according to the 30 A limit.
The values in the product data
The product data of the PT500 is as follows: 12-24V DC, 30 A continuous, 40 A maximum, duration 15 seconds. These three values have to be read together; on its own the maximum current is not a capacity but a withstand time.
| KS250 | PT500 | |
|---|---|---|
| Continuous current | 15 A | 30 A |
| Peak / maximum | 45 A | 40 A |
| Window duration | 5 seconds | 15 seconds |
The striking point in the table is this: the peak value of the PT500 is lower than that of the KS250, but its window is three times as long. On heavy-duty motors that start slowly and on loads with a high moment of inertia, what is decisive is not the peak value but this duration. The same reading holds on the KS250 side as well: the 45 A figure says not what the board can carry continuously, but that it can take a short start.
How to use the 15 second window
- Measure the starting surge with the real load. Typically it is 5-8 times the rated current and lasts 0.5-3 seconds; that surge stays well inside the window.
- To make the surge smaller, lengthen the ramp time with the RMP pot. A soft start typically brings the surge down to 2-4 times the rated current.
- On applications where starts follow one another, leave time for the board to cool between the peaks; the window does not begin from zero on every start.
A clamp meter with a peak hold feature does the job for measuring the surge; in normal measuring mode the display averages, so you miss the real peak. Take the measurement once with the motor cold and again after it has run for half an hour. Choose the fuse according to this surge as well: a fast fuse chosen according to the continuous current will blow on every start without the board being strained at all.
Where the window is not enough
15 seconds is a period over which the heating can be absorbed by the board, and it rests on two assumptions: that the ambient temperature is reasonable and that the peaks are infrequent. Both can break down. When the ambient temperature rises the same current means a higher body temperature; the situation is the same on a board mounted inside a closed box. If the load jams mechanically and the motor cannot turn at all, the current stays high for the whole window. In that case what will protect the board is not the window but the current limit and thermal protection functions.
Misreading the value
What matters is this: the maximum current rating does not define the capacity of the driver. If the current your motor draws steadily under load exceeds 30 A, choosing the PT500 by looking at the 40 A figure would be wrong. The distinction between continuous current and peak current is useful at exactly this point; confusing the two values is the most common selection error we see on site.
What to do
Run the motor with its real load and measure the two values separately: the peak current at starting and the continuous current in steady running. Always size according to the 30 A continuous value, leaving a safety margin on top. If the measured continuous current comes close to 30 A, the board is not the right choice.
The full topic: DC Motor Driver Selection Guide