Fixing A Ground Loop

[iamdarkyoshi]

So I have a project that involves powering a monitor and desktop motherboard from six lithium batteries. The problem I have is that the monitor needed 16vDC so I could not use one of the rails from the main PSU, an M2-ATX-HV. The monitor is therefore driven off of a cheap buck regulator from China. Here is an image of it:

The heatsink on the inductor and the bottom of the board are my mods, the board has been electrically insulated from the heatsink.

I noticed a possible problem though:
The ground rail appears to have a diode on it. 

Anyway, the issue is that the monitor (VGA only) is displaying a lot of scrolling lines on it, and it only happens when the PC and monitor are driven off of their respective supplies connected to the same battery pack. If I connect the monitor through the buck regulator to my bench supply, the scrolling lines go away. I hate VGA.

What would be the solution here? The ATX supply and buck regulator share a power source, and I think we are getting power flowing through the VGA cable's shield, causing interference. I tried putting a common mode choke on the input of the monitor with a 470uf capacitor, and it did not make much of a difference.

Could it be that diode on the DC/DC letting power flow through the shield on the video cable instead of the diode? I haven't the slightest idea on where to begin.

[Enderman]

did you try running the monitor with 12v?

a lot of the time electronics will work with voltage below what is specified

[spwath]

Could you use a ground loop isolator that is usualy used for audio?

[iamdarkyoshi]

3 minutes ago, Enderman said:

did you try running the monitor with 12v?

a lot of the time electronics will work with voltage below what is specified

Cuts off at 13. Already tried it. Dad got the monitor for 5 bucks off Craigslist because the PSU was missing. It normally takes 14v. I run it off of 16v for a while, takes less current because the voltage is higher. With my scope I am seeing about 1v of ripple on the input to the main ATX supply...

[iamdarkyoshi]

5 minutes ago, spwath said:

Could you use a ground loop isolator that is usualy used for audio?

Unfortunately no as this is a DC power signal not an AC line level signal. If they made one for the video however, it would probably work but would look terrible

[spwath]

Just now, iamdarkyoshi said:

Unfortunately no as this is a DC power signal not an AC line level signal. If they made one for the video however, it would probably work but would look terrible

Ah, just thinking, as I had one spare that i could have sent to you with keyboards.

[iamdarkyoshi]

2 minutes ago, spwath said:

Ah, just thinking, as I had one spare that i could have sent to you with keyboards.

Ya. Considering the amount of ripple coming from the ATX DC/DC, I feel like putting common mode chokes everywhere to see what happens

[Scheer]

Have you tried bridging from the -5 and 12v, which would effectively give you 17v?

 

It will likely pull more power than the -5v can give though...

[iamdarkyoshi]

2 minutes ago, Scheer said:

Have you tried bridging from the -5 and 12v, which would effectively give you 17v?

 

It will likely pull more power than the -5v can give though...

Likely wouldnt carry enough power, my DC/DC likely doesnt have it, and that would set the monitor's ground 5v below VGA ground, which basically means shorting out the -5v rail

[bob345]

might need a dc filter choke depending on the kind of interference that is being caused. If you have a oscilloscope it would be pretty easy to figure out a solution.

[iamdarkyoshi]

Just now, bob345 said:

might need a dc filter choke depending on the kind of interference that is being caused. If you have a oscilloscope it would be pretty easy to figure out a solution.

I do, a brand new DS1054Z