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I just acquired a DeWalt DCD790 cordless brushless drill/driver, and going through the manual, on page 10 under Speed Trigger Switch, I find the following:

NOTE: Continuous use in variable speed range is not recommended. It may damage the switch and should be avoided.

Why and how would this lead to damage to the trigger switch?

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Prabably used a cheap pot as the variable part of the speed control circuit. Rugged ones can get pricey. en.wikipedia.org/wiki/Potentiometer –  Wayfaring Stranger May 19 at 12:09
    
The trigger switch is probably some sort of digipot. It's not stepless; the drill will speed up in steps if you slowly and gently pull the trigger. Perhaps this has something to do with the high current which goes through it (meaning it will fail faster if you don't pull the trigger all the way when the drill is under heavy load)? –  DragonLord May 19 at 13:14
    
On closer examination, it seems the motor and switch are a single, integrated part (which is not unexpected given the brushless motor): servicenet.dewalt.com/Parts/Detail/312269 –  DragonLord May 19 at 13:22
    
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Brushless motors are 3-phase current devices. You have a MOSFET array that needs a rather large heat sink if you've dealt with RC electrics using them for propulsion. Any excess load in variable speed mode ups the current, increases the likelihood of motor stall and if the heat sink isn't big enough, the MOSFETs release their magic smoke. –  Fiasco Labs May 19 at 16:14

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