LED drive power is generally a constant current source within a certain voltage range, the main power range is 28-41, that is, after the load, in the output 28V-41V this range can maintain 200mA basically unchanged, but beyond the range of power will be out of control, no longer ensure the constant current target value of 200mA.
This power drive LED, as long as the LED multi-chip series, 200mA corresponding Vf within 28V-41V, it is OK.
18V how specific is not clear, the current may be too much, too much of course burned LED; The output may also be turned off, and the LED is not bright. Exactly how to measure. High power resistance of 90 ohm can be used instead of LED connected circuit test.