First to say (as you will see in the video comparison) The LED chips from china are not real wattage. So be careful to loose the time and money. Make sure to buy a real 100W chip from china or elsewhere.
Watch the video for more details:
links to the equipments I used:
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Step 1: How It's Done..
Connect the cables from the base to the LED chip and connect them also to the input of the step-down module and make sure of the polarity.
control the output of the step-down so it gives 12v output then connect it's output to the fan and make sure of the polarity.
For more safety, put a diode before both the LED chip and the step-down module.
Any question just drop a comment.