Electronics > Repair
Lab Technics D2012 Plus mini centrifuge board repair
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psysc0rpi0n:
Hi.

I have the above mentioned equipment and when we turn it on, it stays one for like 2 seconds and then it shuts down the power supply / transformer.
So I disassembled it and one of the control boards, next to the input plug, has a circuit composed by 6 IRF9540N (pictures attached).

I am not sure how the 6 mosfets work together but the middle pin of each one is connected to the middle pin of the mosfet that is above/below it.
Then, one of the rows of 3 mosfets have 3 diodes connected to one of the mosfet pins. The other end of the diode is connected (if I'm not mistaken) to a resistor and a led.

So, I went measure all the 3 pins of all mosfets while the equipment was ON and I got the following results.

Let's say 2 rows, each row with 3 mosfets.


Row 1
Mosfet 1
Pin 1 - 24.4V
Pin 2 - 0V
Pin 3 - 24.4V

Mosfet 2
Pin 1 - 24.4V
Pin 2 - 0V
Pin 3 - 24.4V

Mosfet 3
Pin 1 - 24.4V
Pin 2 - 24.4V
Pin 3 - 24.4V


Then, Row 2
Mosfet 4 (connected to Mosfet 3 by middle pin)
Pin 1 - 0V
Pin 2 - 24.4V
Pin 3 - 0V

Mosfet 5 (connected to Mosfet 2 by middle pin)
Pin 1 - 0V
Pin 2 - 0V
Pin 3 - 0V

Mosfet 6 (connected to Mosfet 1 by middle pin)
Pin 1 - 0V
Pin 2 - 0V
Pin 3 - 0V

Is it likelly that the issue is on Mosfet 3 ?? It's the only one which voltage values are not the same as Mosfet 1 and Mosfet 2. Mosfet 4 middle pin is at 24.4V because is connected to Mosfet 3 middle pin, I guess, otherwise, it would also measure 0V just like Mosfet 5 and 6.

Can you guys tell me anything about this with the provided information?

PS:
I also measure the 3 pins of all mosfets while the equipment was OFF in diode mode and all the mosfets read around 0.4V on 2 of their pins and around 1.5V on the other pin, except for Mosfet 3 that read very low values on its 3 pins! Like close to 0V.
Harry_22:
Hi!

You are right.
The Mosfet #3 is broken and should be changed.
psysc0rpi0n:

--- Quote from: Harry_22 on January 16, 2025, 07:45:12 pm ---Hi!

You are right.
The Mosfet #3 is broken and should be changed.

--- End quote ---

Ok, thank you for your reply. You even figured out that 3 of them are N-channel and the other 3 are P-channel. I didn't even thought about that.
I remember to have worked with a battery charger controller and they also have a similar circuitry where they use like 2 rows of 4 mosfets and they work together. When one row is ON the other is OFF and vice-versa. Probably in this case something similar should be the case, no?
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