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Ground question

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Mark_R

Member
Hi everyone,
Been lurking for awhile, figured I'd jump in.

I am designing a PCB which has a 12VDC input from a power supply. (the power supply is an off-board pre-made universal input deal) I need the 12vdc for various industrial I/O uses. I have a voltage reg on the PBC providing 5vdc for a micro controller. The question is; does logic ground ("VSS" I guess) get tied to earth / chassis ground or float?

If it matters, The PCB also has up to 240 VAC present at some locations where field voltage interfaces to the micro via opto-isolators.

This is probably very basic, but I can't find a definitive answer her or via Google.

Thanks.
 

BrownOut

Banned
IMO, logic power/ground should generally be isolated from other "industrial" signals. Without knowing anything about your power distribution and other signals, it's going to be tough to answer your question. I'm sure someone else has more insight.
 
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Mark_R

Member
More info;

The mains voltage 120-240VAC enters the PCB via a terminal strip, goes through fuses and exits the PCB via molex connector/cable to an off board 12VDC power supply. 12VDC returns to PCB 12VDC bus via ribbon cable.

There are 8 channels of input on the PCB which each monitor for the presence of an external "wet" AC signal (120-240vac) for channels 1-8, each passing to Pins 1-8 of a micro controller via an opto isolator.

Each channel can optionally (by changing a configuration switch) send 12vdc out of the channel to monitor a "dry" contact. Thus, the the 12VDC can exist out in "the real world" to monitor the dry contacts. The 12VDC + and - buss that supplies the remote contact power is also fused, in case of shorts on the field wiring, or in case someone accidentally connects voltage to a terminal configured for dry contact.

RS232 data passes from the micro to a modem for remote transmission of channel status.
 

MikeMl

Well-Known Member
Most Helpful Member
I'f I were doing it, First, I would keep the -12V separated from anything on the AC side (N or Ground).

Second, I would route a VSS (5V regulator common, bypass caps, anything that touches the uP) trace up to the edge of the board where the connector from the 12V supply comes in, but would connect it to its own pin on the connector, an place it next-to, but not connected to -12V.

Third, I would route a -12V trace where ever it needs to go on the board. Any bypass cap from +12V returns on -12V (not on Vss). Any bypass cap on the output of the 5V regulator returns to Vss (not to -12V)

To make the system work, eventually Vss must tied to -12V. You have two choices. 1. Install a jumper at the connector because Vss is next to -12V. 2. Carry separate wires back to the 12V supply, and common Vss to -12V there. Which you do in production depends on which one works better...

Do not intermix Vss and -12V traces on the board...
 
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Mark_R

Member
Mike,

Thanks, that makes sense. One question; wouldn't the -5 & -12 be tied together at the 5V Vreg? Would this defeat the isolation?

Sorry for the noob questions, it's been about 20 years since electronics class.
 

MikeMl

Well-Known Member
Most Helpful Member
Mike,

Thanks, that makes sense. One question; wouldn't the -5 & -12 be tied together at the 5V Vreg? Would this defeat the isolation?
....
No, there will be only one connection to the common of the 5V reg; Vss. That is why the Vss trace must ultimately connect to the negative of the 12V supply. Keeping them separate gives you the opportunity to cross-tie them in the two places I mentioned above.

The goal is to keep any of the current that flows along the -12V trace/wire separate from the VSS trace to prevent ground-loops that appear as common-mode between (for example) the reg common and the Vss pin on the uP. This is especially important if the uP has an A/D converter built into it, or if it interfaces to any sensors, etc.
 

Mikebits

Well-Known Member
If it were my board, I would keep all supply grounds separate until the end of the board (Where signals go off board), and I would tie them altogether and possibly to chassis ground depending on application. I think they call this a star configuration and it has worked for me in the past. Sorta like;

Dgnd
\__ chassis gnd
/
Agnd
 
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