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Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
During a low-speed operation (drilling) suddendly my dremel tool popped and quit working. After a close inspection i realized that stator and rotor coils where ok, as well as brushes. So my attention was focused on driving circuitry. The ceramic board mounts a pretty classic resistror-capacitor-diac-triac circuit with a 330K resistor charging a 100nF cap during low speed and 47K resistor charging the cap when the switch is on the "high-speed" setting. The triac was a BT134W (sot-23 , 1A 600V triac) and had an "explode-hole" in the package. I dissected it and tried to find a replacement. Unfortunately here no shops seems to have it and they gave me a BT137 (8A , 800V TO-220). I tried to connect to the board with the correct pinout but the dremel still doesn't work, it seems it is not firing at all... Maybe the diac is dead too, or the triac needs more gate current to trigger... Bypassing the triac seems to work but the dremel behaves very erratically, with a loud 50hz hum and low speed rotation, so the PWM seems to be fundamental for a correct operation (a too long pulse doesn't drive properly the induction motor). What can be the problem? dead diac or incorrect drive for the triac? And if the triac is wrong where i can find the BT134W?
Registered Member #14
Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383
Also the cap was faulty, tried 100n, 68n and 27nF substitution but it works erratically and with an annoying hum. seems that something is wrong with the motor wiring.
EDIT: In fact i made a mistake when put again it togheter, wrong stator phasing, strong vvibrationssss and slow rotation, this explains why it didn't work with the triac bypassed, now it works at full power (33KRPM).
i substututed the faulty capacitor (measures 1Kohm EPR) with a 0,068uF 100V mylar and it seems to work properly (the 0,027uF cap showed no difference between low speed and high speed) , maybe the "low speed" is a bit faster (before it was very slow, maybe the cap was starting to arc internally, switching erratically and putting stress on the tiny 1A triac). Probable failture cause, cap failture->triac failture, the rest is still intact.
Probably using an 8A triac is a bit an overkill, but it assures me that the next component exploding will not be the triac :D
here there are two simulations at "low speed (103W power)" and "high speed (138W power)" of the drive circuitry
Here are the images. Hope this helps for anyone with the same motor control problem
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Dremel triac failture
