MOSFET question

[nydirac]

Hi all! I recently began troubleshooting a TV (SHARP LC-32GD8E) and i think i found the cause of the trouble, there is a MOSFET (SSM6J51TU) that i think is not working but having very little knowledge about mosfet i thought to ask: the configuration is in the attached image (from the service manual of the TV) and the pins 1-2-5-6 are shorted (reading 0v), the pin 3 read 0v and the pin 4 read 5V... why using this configuration? what is the reason? protection of the 5v bus? I'm right in my assumption of a bad MOSFET? (no output in pin 1-2-5-6)

PS: Sorry for the horrible articulation of the question... and maybe bad english?

[stj]

Re: MOSFET question

check if q1703 is shorted.

q1703 is acting as an invertor, it grounds the gate pin to turn off the fet that i assume is getting gate drive through r1728

i cant tell more without more of the schematic.

[Longbow]

Re: MOSFET question

If you will look at the device data sheet, you will see that 1-2-5-6 are all internally connected. First things first, right?


The voltages you give do not indicate a short in the device. Turn the power off and test it with your ohmmeter. Pins 4 to 6 shorted (i.e. 0 ohms)?

Time to review: MOSFET basic operation. No conduction from Source to Drain until the Gate threshold voltage is reached.

[budm]

Re: MOSFET question

Hi all! I recently began troubleshooting a TV (SHARP LC-32GD8E) and i think i found the cause of the trouble, there is a MOSFET (SSM6J51TU) that i think is not working but having very little knowledge about mosfet i thought to ask: the configuration is in the attached image (from the service manual of the TV) and the pins 1-2-5-6 are shorted (reading 0v), the pin 3 read 0v and the pin 4 read 5V... why using this configuration? what is the reason? protection of the 5v bus? I'm right in my assumption of a bad MOSFET? (no output in pin 1-2-5-6)

PS: Sorry for the horrible articulation of the question... and maybe bad english?

It is simple electronics switch. The MOSFET is P-CH being used as a switch for sourcing the Voltage, so for the MOSFET to be turned on and supply the 5V out put on the Drain pin 1,2,5, and 6. the Gate (pin 3) Voltage must be 1.5~1.8V lower than the Source pin 4 (input).
As shown in the circuit the Transistor has to be Bias on to turn on the MOSFET, if transistor is shorted out then you will have 5V on D pins.
You need to find out if the Base of the transistor is getting the command on signal or not.
You can manually turn on the MOSFET by connect 100 Ohms resistor between Gate pin 3 and Ground then you will see the 5V on the Drain output pin 1, 2, 5, 6.


You should post this in TV section with all the info you have including the service manual and what the TV is not doing.

[nydirac]

Re: MOSFET question

Thanks to all for the replies! the NPN seem ok, and the pin 4-6 of the mosfet aren't shorted. Sorry for the unclear question! I simply don't understand why using a mosfet in this case (without bothering to put up the entire schematic... i'm stupid )...
To semplify: I measured the voltages of all the pins and i have 0v on the gate and 5v on the source... so i have a gate-source voltage > 1,5V... why i don't have 5v on the drain? bad mosfet?
I will create a thread on the TV section when i can't diagnose anything else, just started yesterday and i'm learning new and interesting things! I don't want to bother you guys with huge schematics while i can!

Sorry for the ramblings!

[budm]

Re: MOSFET question

If the Gate is 0V then it should be on, that will indicate bad MOSFET if you do not have 5V at the Drain pins, but you should check the resistance between the Drain pin and Ground to see if it shows low resistance.

[nydirac]

Re: MOSFET question

Thank you! Already done and the resistance seem compatible while on-circuit (around 2,8kOhm). I wanted a confirmation before starting to unsolder (and search) a 2,5x2mm ic...

[budm]

Re: MOSFET question

May be you can may be 1A fuse or less between the S and D pin and see if the other circuits will function, that way if there is shorted circuit down the line you will blow the fuse instead of blowing the new MOSFET.
You do see the G pin Voltage toggle HIGH and LOW, right?