Earth grounding, multi PCB chassis, and mounting standoffs.

Hello,

I am building my equipment chassis for an off-grid DC only astronomical observatory.

My current chassis is a steel alarm panel box that is 8" deep, and about 16"x18". I intend to install a mini-ITX motherboard w/12V wide Pico-PSU, 2 Velleman K-8055 usb boards, an Ethernet relay board, a rb433 routerboard, a RS-232 wattmeter, and several relays inside. My skill comes from PC building primarily, and so far I have mounted the mini-ITX board via metal standoffs that ground to the chassis like in a standard PC enclosure. This is where I sort of loose my way. The rest of the boards have mounting holes, but that aren't ringed with ground from what I can tell. Im trying to decide if I need to tie a ground for each PCB to the chassis (ground loop problem?), or to a common ground bus such as a terminal block, then to the chassis. Additionally should I remove the mini-ITX board and use nylon standoffs, then ground one of the mounts to the a common ground bus like mentioned in the previous sentence?

I am concerned about adding noise to the system as my USB CCD camera is sensitive to noise. The whole system will be powered by a solar array to a Xantrex C40 charge controller and a 230Ah 12V flooded battery (and possible a Low voltage disconnect). Additionally several 12V devices will be powered external to this box, and I am thinking of using a terminal strip on the exterior of the chassis for these. The Xantrex I was planning on attaching to the exterior of the box as well on one side or the other. Tying its earth/ground lug to the chassis and on out to the real earth ground outside the observatory.

Really I am just looking for some advice, I understand plenty about getting all the equipment working together, I just don't want to endanger the equipment, or myself, or introduce unnecessary noise.

Thanks!

You are making a nicely complex system! Fortunately, most of it looks digital, the relays you mention I think are part of a motor-control circuit.

My advice would be to use the same rules like when you assemble a computer on a motherboard. All grounds should be connected to a common point with heavy copper strips, and the DC power bus blocked to the ground with good 0.1 uF and 10 uF capacitors, as close to the processors or data-processing devices.

As your camera may be sensitive to interference, use a suitable connector for its cable installed in the metal wall of the enclosure so the shortest cable section is inside, exposed to pulse interference. Run the power lines (relays) to motors away from data and camera cables; use preferably a screened cable for them.

The USB camera cable and PC control possibly also USB are screened, so the main potential interference would come from your boards. if enclosed in a metal case and well grounded, no problem. The DC lines from your solar source and to the motors should be blocked to GND with the capacitors like above.
The only lines not to be blocked are those carrying digital data.

If you have an oscilloscope, connect a searching coil to it (100 turns of #30 wire on a 1"2" diameter plastic support) and look if there are interfering pulsed signals travelling over any cable. You can also use the ferrite barrels on the cables, to cut such interference. Good luck!

Awesome, thanks for the advice. When you say "All grounds should be connected to a common point with heavy copper strips" do you mean the just the grounding lugs/plane of each board, or the power ground going to barrel connectors for each PCB inside the chassis? (remember these are all discreet PCB boards each with it's own power connector) The PCBs that have grounded holes are currently grounded to the chassis via metal standoffs, the ones that don't (just holes drilled in empty PCB) I imagine should get the stranded copper cable tied to a ground. Or would just the 12V- be sufficient for the non-standoff grounded PCB.

I plan on using ferrite clamps on each usb cable, and on the power leads coming in and out of the chassis.

Thanks again!

Bryan

Dear Brian:

I would not rely upon grounds by only the metal standoffs. The best way to make grounds secure is to solder a copper strip, 0.5-1.0 cm wide, to each board ground (I would choose the widest ground I can see on the board, close to side connector(s)), and join them all into a good size screw in the metal case wall, again close to DC power and/or data connectors.
Or, you can start without it, and add one/by/one if you detect any problem. The search coil is also helpful. The digital boards often have several clock oscillators with quartz crystals that run independently on different frequencies. Those clocks radiate harmonics; I remember a nice rake from 10 kHz to >50 MHz on a spectrum analyzer when I tested a small digital PBX with one or two boards linked with a flat multi-conductor.
I would mainly care to separate DC power lines and data lines; the camera cable may be well screened between its connectors, so the only vulnerable section might be just inside the main case.

Regards, Jiri