Shorted diode, help to identify it

[Ilyushin]

Hi, I have this laptop and I need help to find with what to replace it

Packard Bell mit-drag-gt2
Motherboard Hannstar J MV-4 94V-0 0648

It was in short-circuit and I found that the cause was a shorted diode. I removed it and it has no resistance no matter what direction (anode to cathode or reverse)

With that diode removed, the board has no short-circuit.

I haven't found any schematic for this board, and in the PCB under the diode is printed its symbol, so it's not a zener (attached pic)

The diode only has a yellow stripe on it. I found on other laptops what seems the same diode but with black stripe instead.

Any help to identify it? Thanks

[budm]

Re: Shorted diode, help to identify it

"It was in short-circuit and I found that the cause was a shorted diode. I removed it and it has no resistance no matter what direction (anode to cathode or reverse)" OK, it is alittle confused here, you found diode that tested shorted when is still soldered to the board, then you removed that diode and tested again and it shows NO RESISTANCE NO MATTER WHAT DIRECTION, do you mean it shows < 1 Ohm in both direction or it shows infinite resistance?

[selldoor]

Re: Shorted diode, help to identify it

Well looks like it may not be shorted - when you have worked that out this may help with ID
http://www.marsport.org.uk/smd/sod80.htm

[budm]

Re: Shorted diode, help to identify it

That is a good link, selldoor.

[Ilyushin]

Re: Shorted diode, help to identify it

Sorry if it wasn't well explained

The diode removed, shows 0 resistance no matter what direction. This is the same values measured when it was soldered, and what caused the short in the mainboard between the +19v input and gnd

Looking in the link provided by selldoor seems that it should be a zener? but the symbol in the pcb where it was soldered is the symbol of a simple diode, not a zener

Or could it be a switching one as that link says?

[budm]

Re: Shorted diode, help to identify it

So the anode is connected directly to ground and the Cathode is connected directly to the +19VDC input? Is that correct?

[Ilyushin]

Re: Shorted diode, help to identify it

Yes, Anode goes to GND and Cathode goes to the +19v input through 2 "PL" that are inductors if i'm right.

Attached photo, where the red wire was soldered (for finding the short) is the +19 input

[atsio]

Re: Shorted diode, help to identify it

Usually Mitac designs (like yours) are using this http://my.mouser.com/ProductDetail/T...3NuWCW%252bQ==

[Ilyushin]

Re: Shorted diode, help to identify it

Usually Mitac designs (like yours) are using this http://my.mouser.com/ProductDetail/T...3NuWCW%252bQ==

thanks for your reply, but that link shows a Zener diode, and mine is not a Zener, based on the drawing of the pcb

[selldoor]

Re: Shorted diode, help to identify it

You cant always trust the markings on the boards - may have originally been going to have an ordinary diode then decided to switch to a zener and didnt alter the silkscreening.

[cdicky]

Re: Shorted diode, help to identify it

You have to ask yourself for why would a diode be connected to ground ??

Any chance you can follow and trace a circuit around it. It might help identify what its purpose was and figure out what it should be, rather than guessing what it was.

[atsio]

Re: Shorted diode, help to identify it

thanks for your reply, but that link shows a Zener diode, and mine is not a Zener, based on the drawing of the pcb

Do not take note of the drawing on the board. All the markings usually on boards are for diodes although there is a zener on that position

[Ilyushin]

Re: Shorted diode, help to identify it

thanks everyone for helping. So, can we conclude it's the zener atsio posted in #8?

#11 I'm afraid that's far above from my understanding...

[selldoor]

Re: Shorted diode, help to identify it

looks to me like it goes to r103 not to ground? there is a trace under your arrow?

[ktmmotocross]

Re: Shorted diode, help to identify it

its just diode on ground, replace with any of diode to 30v and u r good to go

[budm]

Re: Shorted diode, help to identify it

It looks to me from the pictures, the Anode is connected to two resistors (connected in parallel) PR51, PR66, the other two ends of the resistors are connected to the B+ large pad on the left side of the PQ10 (MOSFET?).
I doubt that this diode is used as a crowbar protection device in case the polarity of the power supply is reverse, which in that case, the Diode will be bias on and will cause the fuse to blow, the diode will be much bigger size to handle the crowbar current.

[selldoor]

Re: Shorted diode, help to identify it

It looks to me from the pictures, the Anode is connected to two resistors (connected in parallel) PR51, PR66, the other two ends of the resistors are connected to the B+ large pad on the left side of the PQ10 (MOSFET?).

Me too - I put the value instead of the board designator

[Ilyushin]

Re: Shorted diode, help to identify it

Hi everyone

So... I found in some monitor scrap-power board a diode that looked exactly the same, and that tested good when I removed it. (flow in one direction, not in reverse)

I put in the laptop board and there wasn't any short-circuit. Then I ensemble everything, connect to DC adapter and there was a slight "click" sound.

No power-up at all, and now the laptop board is again in short-circuit. I have done again the "method" to locate the cause of the short (bench power supply and where it got hot), and again is the diode's fault. Diode removed and no short

So, any suggestions? what can be causing the diode to die "open"?

[budm]

Re: Shorted diode, help to identify it

Since the Anode is not directly connected to Ground pad, with diode removed, what resistance do you get between the chassis ground and the Anode, Chassis Ground and Cathode? It seems to me that too much current was flowing through the diode and caused the damage to the Diode.