Help identifying a component - M091 SOT23 (?)

[Nexxen]

I need to identify a component:

3mm x 2mm
2 legs on a side, 1 on the other (SOT23?)

Has M091 on top.
Two leads have +5V, the side with one had 0.


Thanks in advance!!

[fzabkar]

http://www.smdmark.com/en-US/search/code?id=m0

Try m0, mo, m09, m09, m0xx, etc. There are numerous permutations.

It would help if you were to tell us its circuit reference, eg Qnn, ICnn, Tnn, etc.

[Nexxen]

No idea what you are talking about. Unfortunately those are unknown acronyms.

It's on a motherboard, close to ram.

I'm a beginner, I know I'll be both dasappointing and frsutrating

I'll post pictures tomorrow.

Thanks

[fzabkar]

A component on a PCB is usually identified with text such as IC123 or U123 or Q456. "IC" and "U" are used to identify an integrated circuit, whereas a "Q" identifies a transistor, "C" = capacitor, "R" = resistor, etc.

[Nexxen]

Q748 (Q is the important info I guess)

Thanks, I never thought it actually mattered. Ok, C and R I knew 

[fzabkar]

Q748 (Q is the important info I guess)

I'm guessing it's an enhancement mode P-channel MOSFET.

If the voltages are 0V, 5V, 5V from left to right (centre pin at top), then the MOSFET is being switched on.

[Nexxen]

Pics

Components [M091] goes to [WW1 P6]

In pic 3 the 0V pad of M091 goes to Q747 (60N3L) mosfet, sharing it with componets PQ1 (D90N0 2L). On the left of the RAM slots.
I can't make where Q747 and PQ1 get their Vcc. Probably needs to be enabled.


https://pdf1.alldatasheet.com/datasheet-pdf/view/171603/STMICROELECTRONICS/D90N02L.html

[Nexxen]

I couldn't find a match.
I'll write to Asrock and hope they'll give me the answer.

I'm wondering if such a small component can be responsible for the whole board to power on.

[Nexxen]

AP2309GN
https://www.alldatasheet.com/datasheet-pdf/pdf/146907/A-POWER/AP2309GN.html

Could this one do it? I found it on another board, and it takes both +5VSB and +5V on the other, like my board does.

[fzabkar]

AP2309GN
https://www.alldatasheet.com/datasheet-pdf/pdf/146907/A-POWER/AP2309GN.html

Could this one do it? I found it on another board, and it takes both +5VSB and +5V on the other, like my board does.

I'm confused. Your original hand drawn diagram shows that the MOSFET is connected to the PSU's Power OK signal (which is open collector), but now you are saying it is connected to the +5VSB standby supply.

[Nexxen]

That's a mistake on my part. 
It's indeed +5VSB.

My apologies.

[fzabkar]

Before replacing the MOSFET, I would check the other connected components to confirm that there are no shorts to ground, or any other reason for the failure.

[Nexxen]

There were two burnt components:

- the mosfet
- a trace

The trace was rebuilt and no shorts, no dead shorts on PWR connector nor other components.
No shorts whatsoever anywhere. Checked again now.

This doesn't mean it may not die.

[fzabkar]

It looks to me as if the MOSFET is configured as a power switch. Maybe you could install a fuse between drain and source, and then see what happens?

[Nexxen]

The side with one pin,

- goes to a mosfet that on one leg then goes to power on the PS/2 connectors and other two components further down.

Without this it won't output the necessary V to PS/2 and those components.
To me IT IS linked to the switch on circuitry.

What kind of fuse? Values to suggest.

Sorry if I'm boring but I'm really that low on electronics knowledge, albeit with much enthusiasm! 

[fzabkar]

Maybe 0.5A or 1A fast blo? I still have a feeling that you've missed something. For example, if a MOSFET dies with a short between gate and source, then whatever IC is driving the gate can often fail, too.

[Nexxen]

This might take some time, I have to order them on aliexpress.

Fast blowing smd 0.5A any voltage?

Thanks for the help so far!!

[fzabkar]

Voltage doesn't matter. I'm not suggesting that you make this a permanent fixture. I'm simply suggesting that you wire an ordinary glass fuse between drain and source for testing purposes. Then, after you are sure that there are no other faults, you could replace the MOSFET.

Alternatively, you could use a momentary pushbutton switch plus fuse attached via wires.

[Nexxen]

There was a big bulge on the S/D side of it. Could have it been an overload from PS-2 or the previous owner touched the trace and it created some loop?

Like when you touch a trace with a metallic object and it makes the trace burn. It happened to me a few times.

Is it possible for the S-D bridge that a current going to it instead from may have caused the M091 to burn? A flow in the opposite direction??

[fzabkar]

I believe the keyboard and mouse ports are normally protected by picofuses or polyswitches. I guess someone could accidentally short something with a screwdriver when working one the motherboard. It's a strange one.

[Nexxen]

I got that, otherwise what would be the purpose of a dedicated component?  Ok, engineers cut corners but not this much I hope...
It's just to test, if it works it works.
If not it wouldn't have.

I added a pic of the damaged zone. In read how the V goes around + damaged trace (not +5VSB)


Time to bed. Read you tomorrow but thanks again

[fzabkar]

I haven't done much motherboard repair, so I can't help a great deal. I know that USB ports are also protected by polyswitches. You might try the people at badcaps.net. They do a lot of motherboard repairs over there. Someone may even have circuit diagrams.

https://www.badcaps.net/forum/forumdisplay.php?f=14

https://www.badcaps.net/forum/forumdisplay.php?f=15

[fzabkar]

Perhaps a fuse wouldn't be a good idea.

That part of the circuit appears to select either the 5VSB or VCC5 supply to power the USB/PS2/LAN etc.

[Nexxen]

+5VSB/+5V is selcted on another trace and that actually works (on the VSB/5V pin header I have continiuty with +5VSB pin and ATX).


I checked a little more and:

the dead SOT23 Drain connects to another sot23 that supplies Vcc to RT8105 (https://pdf1.alldatasheet.com/datasheet-pdf/view/1131962/RICHTEK/RT8105.html).

That component is responsible for BOOT and SOFTSTART.

My guess is that the blown SOT23 gives Vcc necessary to turn on the m/b, without it's dead.


The other two components the big mosfet connects to (red line on pics) are:
- https://pdf1.alldatasheet.com/datasheet-pdf/view/21744/STMICROELECTRONICS/74ACT08.html (data I/O)

- https://html.alldatasheet.com/html-pdf/218263/stmicroelectronics/st75185c/1945/1/st75185c.html (UART to SR-232)

These two, if not powered, maybe just won't allow communication from serial and or other I/O on the back of the board.
With a dedicated graphics card it should display something nonetheless.


Thanks for all the data, it's difficult to repair something if you don't know all that is around it.



Bottom line:
the +5VSB actually powers the soft-on power-on of the motherboard via the SOT23.
the +5V is the Gate allowing +5VSB to go to RT8105 and allow power-on ----> like a "power from ATX plugged, ready to power-on on demand"

Don't know what this circuit is but I have no other explanation.

[Nexxen]

I discovered that the other +5 to blown comp. comes from +5VSB too, but it goes through a cap and a resistor and then to the other leg (of the side that has 2 legs).

Basically the blown component gets +5 to both from +5VSB.

The burnt trace goes to the +5VSB/+5V 3-pin header selector. Indeed powering PS2 and USB (pin continuity with pin +5VSB from ATX)


BTW: there are no dead shorts on ATX nor anywhere else. Burnt trace could have been because of +V from USB?

The square component that gets Vcc from +5VSB is L6713A (https://pdf1.alldatasheet.com/datash...CS/L6713A.html)