Solved! Multimeter test for power - where do I attach the black probe?

merudo

Estimable
Feb 20, 2018
10
0
4,570
2
The backlight of my LCD screen isn't powered anymore. Maybe my motherboard isn't supplying power to the LCD screen?

Basing myself on the laptop schematics, I'm trying to test if various components of the laptop provide power. I know I'm supposed to put the red probe on the location where power should be provided.

However, where do I put the black probe?

I think it needs to be connected to a ground, but which one?

I know that for testing DC power supply, the black probe can be put on the outside of the DC connector. But what if I test other parts, such as the LCD backlight power pin?

 

merudo

Estimable
Feb 20, 2018
10
0
4,570
2


Not sure what you are trying to achieve by being so negative :)

My investigation suggested a mosfet (AO3409) was defective. I bought a new part & replaced the defective mosfet - the job involved some tricky soldering, but I made it work.

Now the backlight works again!
 

Dugimodo

Distinguished
Sep 22, 2011
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The outside of the power connector should still work, or you can trace the motherboard track from it to the nearest convenient location.
Honestly though if you don't know enough to find a ground I don't know what you are going to achieve here. Not trying to be rude about it, but perhaps you are out of your depth.

I think it's more likely the backlight has failed than the power supply to it, but either could be the case. I guess what you are attempting might confirm which if you identify the correct spot.

Actually it should be as simple as putting the meter directly across the backlight connections, if there's power it's dead and if there isn't it's a supply issue.
 

merudo

Estimable
Feb 20, 2018
10
0
4,570
2


Not sure what you are trying to achieve by being so negative :)

My investigation suggested a mosfet (AO3409) was defective. I bought a new part & replaced the defective mosfet - the job involved some tricky soldering, but I made it work.

Now the backlight works again!
 

Dugimodo

Distinguished
Sep 22, 2011
67
0
18,610
18
Well that's a great result, it's just that you made it sound like you were more likely to break something than fix it by not knowing how to find a ground connection. Generally someone who knows how to Identify and replace a mosfet would also know how to find the common ground rail in most circuits.

Looks like my fears were unfounded.
 
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