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Drill/Visseuse failure BOSCH PSB 18 LI-2

view of the BOSCH PSB 18 LI-2 ...Once is not customary, here I am confronted with a failure on my equipment, in this case the drill / screwdriver BOSCH PSB 18 LI-2.
Whether with one or the other of my 2 fully charged batteries, when pressing the trigger the engine refuses to start and the 3 green LEDs as well as the white one flash 3 times ...

This kind of mishap always happens at the wrong time, like when you have a job to do ...
view of the printed circuit board ...Never mind, head to the Lab to find out where the problem may come from. Once the machine has been dismantled (9x TORX T10 screws to be removed ... ) we can distinguish on the printed circuit board a PIC16F882 microcontroller and two transistors in DPAK package with the BA6R18 marking. These are two N-channel power MOSFETs that I will hasten to replace because one of the two ( Q1 ) is split just above the S ource pin ... Nothing else seems damaged or destroyed ... it could have been more serious!

My research to find the datasheet of this transistor was not successful but I was able to find on a forum someone who replaced them with P0903BDG ( I D = 50A, R DS (ON) = 9,5mΩ, V DSS = 25V ) but I did not have one ... I mounted instead of NVD6824NL ( models that I have in my drawers ) which hold a current I D of 41A for a voltage V DSS of 100V, and with a Drain-Source R DS (ON) resistance of 20mΩ. That should be enough ...

A tip to remove the transistors ...
I obviously tried with my Pyropen WELLER hot air iron, but impossible to unsolder them ... in fact after the fact, I could see that the whole of the case is soldered on the printed circuit to act as cooler, hence the difficulty ... I therefore used my WELLER WECP-20 station with LR-21 ironfitted with an "ET B" tip with a width of 2.4 mm ( temperature 450 ° C ).
The trick is to cut the legs of the transistor flush with its casing with a small cutting pliers, then heat the remaining pin ( the Drain ) to finally be able to remove it ... all you have to do is unsolder them. remaining tabs on the circuit board then clean it thoroughly. The assembly of the new transistors is then child's play ...

Once the intervention was over, I reassembled and re-fed the machine. This time when the trigger is pressed the engine starts frankly, even if sometimes the phenomenon occurs again but much less frequently than before! As this defect appeared almost from the start and I am not the only one to notice it either ( a search with your favorite browser on both the 18V model and the 14V will show you a number of destitute customers ...), I think this may be a problem with measuring current when trying to start the motor ... and therefore designing the control system, but this is just a simple personal opinion, having neither the electrical diagram of the assembly, nor the content of the software stored in the microcontroller. We should first take the time to reconstruct the diagram in order to be able to study it better ... one day, perhaps ...

For the moment, I will test the machine and see if the phenomenon continues ...

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