testing mobo outside of a case?

[Abram730]

Originally posted by: mindless1

Originally posted by: MrDudeManThere may be multiple paths in a case, but I think he meant simply connecting a single, alternate ground to the motherboard in some way. If that were the case, then it could be a significant issue. I'm not familiar with the grouding inside of a computer case, but I was under the impression that most of the grounding was done through the ATX connector, making a "single" ground connection.

Alternate paths to the motherboard in a case include the standoffs the board is mounted on, card brackets that are ground and touching or screwed to the case (ground), drive/other peripherals with grounded metal cases screwed to the system case.

Alternate paths regardless of case, may include video card with power connector, drives, sound cards with cables connected, video or capture/tuner cards with cables connected, other cards.

It would be a very unusual system to not have several ground loops. Sometimes the loops DO cause issues, like when it's not a digital signal but rather analog on integrated sound you may get noise on the output depending on system load. Sensitive amps->speakers are more likely to reveal that noise.

Yes the only "required" ground is through the board (or other parts' connector) pins/leads, but if there were a single alternate ground to the board, it should still work fine.

What types of problems could a grounded loop cause in a PC? If possible non shorted results could happen say a cheep Board and say a combination of plastic and metal standoffs or some such thing? Could there be a noticeable affect on system stability?

 

[manowar821]

It's already grounded through the ATX power connector. Don't set it on a metal sheet, just get a rubber pad or something nice for it to sit on.

 

[myocardia]

Originally posted by: manowar821
It's already grounded through the ATX power connector. Don't set it on a metal sheet, just get a rubber pad or something nice for it to sit on.

A piece of cardboard works great, if you don't have a piece of plastic the right size.

 

[mindless1]

Originally posted by: myocardia

Originally posted by: manowar821
It's already grounded through the ATX power connector. Don't set it on a metal sheet, just get a rubber pad or something nice for it to sit on.

A piece of cardboard works great, if you don't have a piece of plastic the right size.

I like to stack two magazines, as this is just about the right height (depending on mag thickness of course) that it allows enough clearance for cards' brackets to hang down over the edge of the magazines while plugged into the board.

Just don't use the original ESD packing material since much of it is conductive and can cause all kinds of odd problems like failure to POST.

 

[bobsmith1492]

Regarding ground loops: you can't really talk about them without a good picture... I wouldn't really call this good but it illustrates the problem.

Pic

You have to consider all wires as resistors/inductors rather than the same potential everywhere, especially when higher currents are involved as in audio amplifiers. The current flow is shown through the amp on the right; if the signal is referenced from ground at the source and connected to earth ground in the amp, some current will flow through the negative signal line. This extra signal (where Vnoise is a 60Hz signal) will be amplified, giving a nasty buzz.

In this case, it is easy to avoid the noise by using a differential input on the amplifier; common mode noise from the preamp is then rejected.

 

[Navid]

Originally posted by: bobsmith1492

Pic

Do you need a ground "Earth" connection for an audio amplifier in addition to the negative supply connection, which is usually called the "ground plane"?
Do you think a battery-operated hand-held radio has an Earth connection?
Do you think a battery-operated radio (or boom box) does not have an audio amplifier?

Can you show a link to a schematic diagram of any electrical/electronic equipment that shows current flowing through the Earth connection of an equipment like in your picture?
I am not talking about the negative connection of the supply. I am talking about the third prong of the AC outlet (Earth).