Motherboard ground connector not picking on mulitimeter

[Jaegar]

I have an MSI B450m bazooka plus motherboard, had a blackout and didn't power on pc till next day, I use a surge protector of course. Next day when trying to power it up, the pc wasn't powering up at all, I tried shorting the power on pins on the motherboard and still nothing, I also drained the cmos and nothing. I removed the psu tried it and it works. No any signs of burn on the motherboard, I tried testing with multimeter and one of the ground pin connector(ground 17 on the picture) on the board wasn't connecting with the ground (vga plate), while the rest were connecting using the multimeter. Is this noemal or is something wrong with this?

 

[Bill_Bright]

I don't know what you mean by VGA Plate.

I removed the psu tried it and it works.

How did you try it if it was removed?

Note Pin 17 is the return for Pin 16. These are the two pins you can use to test if a PSU is able to turn on, by shorting the two pins. This is commonly called the "paperclip test", as seen here. This test is NOT conclusive by any means. It only tells you if the PSU is able to turn on or not. It does not test to ensure each of the voltages are present or within tolerances. Nor does it test for ripple and other anomalies. If the PSU does not turn on with this test, it does tell you the PSU is bad.

A better way to test a PSU is with a tester as shown in that same link. It still is not conclusive because it only presents a small 10Ω "dummy load" instead of a variety of realistic loads. But it does tell you if all the required voltages are present and (at least with that load) their actual voltages. I keep one of those testers in my tool bag for house calls.

The only way to conclusively test a PSU is with an oscilloscope or dedicated power supply analyzer, under a variety of loads. Since most users don't have access to these expensive pieces of test equipment, or have the necessary training to use them, the next best way to conclusively test a PSU is to swap in a known good one and see if your problems persist, or go away.

I recommend you beg, borrow or steal a known good PSU from another computer, swap it in and see if your computer boots okay.

 

[Redwoodz]

I believe you should not have continuity from + voltage to ground. That would tell you have a short in the motherboard.

 

[ty_ger]

I believe you should not have continuity from + voltage to ground. That would tell you have a short in the motherboard.

I think you will. There is a lot of parallel circuitry that will add up to low resistance.




I wonder how you will know that the PSU is working properly when disconnected. Please describe your testing process.

 

[Bill_Bright]

I think you will.

Redwoodz is right. With power removed, there should be no continuity from +V to ground as that would be a short. It should measure as an open.

There is a lot of parallel circuitry that will add up to low resistance.

Ummm, that is not how it works. In fact, parallel circuits effectively increase the size of the conductor, essentially creates a bigger wire (like 2 stands instead of 1). And a larger conductor results in reduced resistance.

I wonder how you will know that the PSU is working properly when disconnected. Please describe your testing process.

Yeah, this I wondered above too. We need the OP to return with more information.

 

[qxp]

It would be surprising if the ground pin failed. It's possible that the motherboard has some sort of isolation (like an optoisolator) and pin 17 is not connected to ground. If pin 17 were connected to ground you should see a short between it and other ground pins on the same connector.

 

[ty_ger]

Redwoodz is right. With power removed, there should be no continuity from +V to ground as that would be a short. It should measure as an open.


Ummm, that is not how it works. In fact, parallel circuits effectively increase the size of the conductor, essentially creates a bigger wire (like 2 stands instead of 1). And a larger conductor results in reduced resistance.

I think you guys both misunderstood the OP's context. The OP is measuring the power and ground pins on the motherboard. Between power and ground, the resistance will measure very low. That is normal. You can't test for a motherboard short in this way.

Try it yourself.

It would be surprising if the ground pin failed. It's possible that the motherboard has some sort of isolation (like an optoisolator) and pin 17 is not connected to ground. If pin 17 were connected to ground you should see a short between it and other ground pins on the same connector.

Pin 17 won't measure continuity to other grounds. Or will measure high resistance. It is a switched input/output (depending on your point of view).



I think the OP's measurements so far are irrelevant or not conclusive.

 

[Bill_Bright]

I think you guys both misunderstood the OP's context.

IMO, the OP was not very clear. Or at least, I don't get a clear picture from his description. He said,

and one of the ground pin connector(ground 17 on the picture) on the board wasn't connecting with the ground (vga plate),

I don't know what a VGA plate is for one. That's why I asked for more information.

HOWEVER, after your comment and reading it again, he does say, "on the board". That said, if he meant Pin 16 (PS-On) then I believe you would be right. There should be other components in that circuit that would have resistance. But I note he said Pin 17 was not indicating a ground. That should. Unfortunately, I don't have a spare motherboard here to test.

In any case, IMO, that is not a proper test anyway. And I agree with qxp - unless there was physical damage to the motherboard, like broken pins or a screwdriver cutting through circuit traces, it is highly unlikely any connections on that 24-pin motherboard connector failed.

Bottom line: we need more information.

 

[qxp]

I think you guys both misunderstood the OP's context. The OP is measuring the power and ground pins on the motherboard. Between power and ground, the resistance will measure very low. That is normal. You can't test for a motherboard short in this way.


Pin 17 won't measure continuity to other grounds. Or will measure high resistance. It is a switched input/output (depending on your point of view).

My understanding of the OP post is that he/she tried to verify ground power connector connection by measuring continuity to a ground plane of the VGA card because it was easy to access. What OP found that all the ground pins of the motherboard connector except one - pin 17 - show continuity to VGA ground plane, as you would expect as PCI/PCIe connector has ground pins. The OP question is why is pin 17 not showing continuity even though it is marked ground on the spec. One possible reason is that the connection is poor, but it is more likely the motherboard was designed with pin 16 and pin 17 having a separate circuit that is activated with an optoisolator, or something similar that separates grounds.

The reason they did it is that specification calls for power supply pins 16/17 being safe to prevent high voltage reaching the case and/or power button in case of a fault. An optoisolator adds a bit to cost of the motherboard, but is a simple means to make the motherboard safe as well.

 

[Bill_Bright]

You are making a lot of assumptions I would not. The OP said VGA "plate". You apparently assume he means "ground plane". There is neither on a motherboard that I am aware of - except, maybe, some motherboards have a "backplate" for the CPU that may act as a ground plane. To me, the closest thing to a ground plane (or plate) for a graphics card would be the back of the computer case where the card's mounting bracket mates with the case's back panel. That whole back panel could easily be considered a ground plate.

I doubt they use optoisolators. They are relatively expensive, relatively big, and typically used in higher voltage, higher current circuits. As you suggested, it is more likely they use "something similar".

 

[qxp]

You are making a lot of assumptions I would not. The OP said VGA "plate". You apparently assume he means "ground plane". There is neither on a motherboard that I am aware of - except, maybe, some motherboards have a "backplate" for the CPU that may act as a ground plane. To me, the closest thing to a ground plane (or plate) for a graphics card would be the back of the computer case where the card's mounting bracket mates with the case's back panel. That whole back panel could easily be considered a ground plate.

A "ground plane" is a large copper filled area on PCB electrically connected (or considered) as ground.

Ground plane - Wikipedia

en.wikipedia.org

I doubt they use optoisolators. They are relatively expensive, relatively big, and typically used in higher voltage, higher current circuits. As you suggested, it is more likely they use "something similar".

They are commonly used in power supplies, and motherboards can be more expensive. So not impossible. The manufacturers get good pricing, they are not buying single units.

Looking at the image below next to the power connector you can see a largish chip with 8-pins. I don't think it is a switcher, because the coils above are connected to two smaller chips to their right. I suspect this is an optocoupler, or an integrated device that has an optocoupler.

https://image.made-in-china.com/2f0j00vMbhCHLEfwuK/Standard-ATX-Specification-Q470-Chipset-Industrial-Motherboard-Computer-Motherboard.webp

 

[Bill_Bright]

I know what a ground plane is. You can follow the link in my sig to see I might know a little about that - especially with my background with radio systems.

My concern is, that is NOT what the OP said. He said VGA plate. I am not assuming he meant the same plate. And if you note your own link, when it comes to PCBs, a ground plane is often located on the "bottom" of the board, not top where the card's socket it.

I also know what optoisolator is and where they are commonly used - and they are NOT commonly used in motherboards because as I noted, they are typically used with higher voltages and currents.

Even if the OP's board has one, the device used is off topic.

Looking at the image below next to the power connector you can see a largish chip with 8-pins.

I really don't understand you. That is not meant to criticize - just stating an observation. Why are you showing us a motherboard the OP does not even have? That makes no sense whatsoever.

As I noted above, we need more information so I see no reason to continue this discussion until the OP comes back with more information.