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Fault Symptoms: Dremel works fine except that when you turn the numeric speed control wheel, your Dremel instantly goes to full speed. It has essentially lost its variable speed function.

Fault Diagnosis: In a nutshell there is a chip on the PCB inside that has blown. Luckily this a large 3-pin chip called a MOSFET (Metal oxide Feild Effect Transistor) that can be replaced for a about £2.50 or $3.50 USD. The MOSFET (STP80NF03L-04 MOSFET N-Channel TO-220) has a short to ground caused by overloading or overheating. The heatsink is too small really and inadequate. I have since noticed that Dremel have changed the design in the replacement 8000 series. If you haven't been using a lot, check that the main bearing at the front isn't gritty and that your motor turns freely as these could cause the speed controller to sink more current.

BUY a STP80NF03L POWER MOSFET: You can buy one from here...

Tools Required: 25watt soldering iron, screwdriver, torx set (or small screwdrivers), ideally an extra, small copper heatsink and of course a new STP80NF03L-04 MOSFET N-Channel TO-220. STP08NFL03L Datasheet is here.

Method:

1) The first step is to unscrew the case. It has torx or star screws but you can undo them with an appropriate sized screwdriver if you don't have a torx set.

2) Next pull out the motor, battery connection and bearing. It all comes out together

3) Disconnect the motor - noting which way up the motor was and what coloured lead went to the terminals on the motor.

4) Locate the POWER MOSFET on the dremel speed control PCB.

5) De-solder the old one. This can be tricky because you need to de-solder and prize off the three legs at the same time. These are soldered on both sides so wait for the solder to get really hot and work the legs back and forth a few times.

6) Once you've go it off, solder on the new STP80NF03L MOSFET the same way around as the old one (with the black facing up and the metal against the heatsink). The rivet on the heatsink may come loose to just re-attach it best you can.

7) Ideally, solder on an extra heatsink making sure it fits in the case!

8) Reassemble in reverse and you're are done!

Comments
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Comments:

Dennis Andrews   30/10/2018 at 03:29:07
With reference to the Dremel 800 not operating after the MOSFET was replaced. I think the diode is a suppression diode 3 legs from what I can read it is a DOF4644 P/No. I can not find the part number anywhere I must have read it incorrectly.
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Dennis Andrews   30/10/2018 at 01:28:54
It is not working with the MOSFET replaced? It seems to be the bridge net work on the other side of the board? I can not make out the Part Number of this chip, can you please help? It is connected across the DRAIN of the MOSFET. Much appreciated any light you can shine on this problem. Dennis
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Andrew Green   25/01/2018 at 16:52:46
Excellent tip. Replaced with a similar component, as one specified was not available locally. Works great.
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Steve Jones   25/02/2016 at 03:16:50
Poor design by Dremel initially. Expensive to replace whole switch unit,(same problem afterwards ! ) but this cheap fix is just the job. Many thanks.
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