Dewalt's "Value Engineering" CYA

[scooterbum46]

I've got lots of Dewalt 20 and 60 volt tools, so when I wanted a 1/2" battery impact, I bought a DCF899 kit. Reading the instructions (like on the third day, you don't think I do that first do ya?) I ran across the following paragraph and warning:

"Variable Speed Trigger Switch (Fig. A)To turn the tool on, squeeze the trigger switch 1 . To turn the tool off, release the trigger switch. Your tool is equipped with a brake. The anvil will stop when the trigger switch is fully released. The variable speed switch enables you to select the best speed for a particular application. The more you squeeze the trigger, the faster the tool will operate. For maximum tool life, use variable speed only for starting holes or fasteners.NOTE: Continuous use in variable speed range is not recommended. It may damage the switch and should be avoided"

I've read that a number of times, all I can assume is that after a bunch of warranty claims due to switch failure, the money people and the product engineering people came up with a solution - put a CYA in the product guide. I mean, it's a variable speed tool, but don't use it that way very much or you may damage it?? Now I really understand AVE's comments about value engineering in consumer level products .

 

[ItsNemo]

I've got lots of Dewalt 20 and 60 volt tools, so when I wanted a 1/2" battery impact, I bought a DCF899 kit. Reading the instructions (like on the third day, you don't think I do that first do ya?) I ran across the following paragraph and warning:

"Variable Speed Trigger Switch (Fig. A)To turn the tool on, squeeze the trigger switch 1 . To turn the tool off, release the trigger switch. Your tool is equipped with a brake. The anvil will stop when the trigger switch is fully released. The variable speed switch enables you to select the best speed for a particular application. The more you squeeze the trigger, the faster the tool will operate. For maximum tool life, use variable speed only for starting holes or fasteners.NOTE: Continuous use in variable speed range is not recommended. It may damage the switch and should be avoided"

I've read that a number of times, all I can assume is that after a bunch of warranty claims due to switch failure, the money people and the product engineering people came up with a solution - put a CYA in the product guide. I mean, it's a variable speed tool, but don't use it that way very much or you may damage it?? Now I really understand AVE's comments about value engineering in consumer level products .

This has been in their literature for ages...I've used the tools without a second thought to this and never experienced a failure. I wouldn't read so much in to it.

 

[scooterbum46]

The funny thing is, I can understand if the warning pertained to motor overheating due to very low speed use (like on the recip saw), but normally the pot in a variable speed switch shouldn't carry enough current to speak of. It just seemed odd to say the switch might fail in what I would consider normal use..

 

[Rabid Badger]

<strike>It's just the nature of the technology. There are mosfets in the switch (or on the circuit board) that control the flow of current. When mosfets run fully on (tool at full power) they have very low resistance and stay cool. When run in an intermediate state their resistance is much higher and they get hot. Eventually they will burn out. There isn't really a way around this with current technology; anything more durable would be too large or prohibitively expensive.</strike>

Edit:

Ignore everything drunk me said above.

Modern, small-package mosfets are more than capable of running a brushless motor in a cordless tool without creating excessive heat.

Also, in an application like a brushless motor the mosfets would not be run in a "partially on" state. They are switched fully on and fully off very rapidly and any heat is generated in the very brief period that the mosfet is switching from one state to the other.

 

[Flyordie]

It's just the nature of the technology. There are mosfets in the switch (or on the circuit board) that control the flow of current. When mosfets run fully on (tool at full power) they have very low resistance and stay cool. When run in an intermediate state their resistance is much higher and they get hot. Eventually they will burn out. There isn't really a way around this with current technology; anything more durable would be too large or prohibitively expensive.

There is, us PC guys know this. We can get a 96% efficiency rating on powersupplies at 30-70% capacity ratings. So the idea is to enable a SMALL amount of airflow over the mosfets as companies like Milwaukee have done by pulling air from down by the battery up through the handle and out through the top of the tool. Remember, you are only needing to cool 8-10W worth of heat. Not 100s of watts.

 

[Rabid Badger]

There is, us PC guys know this. We can get a 96% efficiency rating on powersupplies at 30-70% capacity ratings. So the idea is to enable a SMALL amount of airflow over the mosfets as companies like Milwaukee have done by pulling air from down by the battery up through the handle and out through the top of the tool. Remember, you are only needing to cool 8-10W worth of heat. Not 100s of watts.

Yes, improved cooling can help. I'm not sure I'm fully onboard with the idea of sucking a bunch of potentially conductive dust through the electronics of my tool, though.

Actually, now that the beer is wearing off I'm starting to think that line in the DeWalt manual is just boilerplate language left over from older tools. In a brushless tool the switch really shouldn't be doing any work at all. The power delivery has to be controlled elsewhere since the operation of the motor depends on each coil being driven individually.

 

[Flyordie]

Yes, improved cooling can help. I'm not sure I'm fully onboard with the idea of sucking a bunch of potentially conductive dust through the electronics of my tool, though.

Actually, now that the beer is wearing off I'm starting to think that line in the DeWalt manual is just boilerplate language left over from older tools. In a brushless tool the switch really shouldn't be doing any work at all. The power delivery has to be controlled elsewhere since the operation of the motor depends on each coil being driven individually.

Well, the electronics are actually potted now with just the mosfet surface exposed with a small heatsink attached.

 

[Fly YX]

Like what a few other people have been saying it's probably from their older tools. I had a DeWalt cordless drill years ago and as soon as the warranty went out so did the switch. Haven't bought another DeWalt product until recently.