Common Grounds

[Renfield]

Is the BCS fussy if it shares ground with everything else in the system (i.e. the relays, heaters, thermistors)?

The plan is to have all of the grounds bussed together in the control box with one of the GNDs on the BCS tied to the bus.

Not having to switch both signal and ground on the ON/OFF/AUTO switches will make it a lot easier to find switches.

Thanks,

Ren

[JonW]

Yes, you want to share grounds just as you have stated. If you introduce something like a 12V DC power supply to switch relays or valves, you also want to attach that ground to the common ground with the BCS.

[brewdude]

Is there a benefit other than simplified wiring by having a common ground between the 5v, 12v, and 120v systems?

I didn't even know that it was possible to do this. Need to study up on wiring somr more.

[ChuckO2999]

I believe that you do NOT want to share low voltage DC grounds with the AC ground. If you have any leakage of the AC to the ground you will get an AC component in the DC wiring.

In general, connect your AC ground (not neutral) to the input power ground, possibly through a GFCI. Connect your DC returns (Negative side) together but keep them isolated from the actual ground.

[Renfield]

I'll design the system to have separate gound busses for "high" voltage and low voltage.

If the low voltage ground is floating slightly above or below the high side ground I can't see where there should be any problems.

Ren

[ECC]

Ren, I agree. If the DC ground is floating a little higher or lower than the earth ground, it shouldn't matter at all. The power transformer is only 2 prongs, so unless you tie one of the GND pins of the BCS, its going to float. In theory there should be no problems with this setup. 99% of all electronic devices operate in this manner.

However, I've had a handful of customers reporting temperature fluctuations, which can be solved by grounding the BCS (connecting the GND to ac earth ground). There are a few threads in this forum that show this. I haven't experience this myself, but it seems that the only explanation for this behavior is if somehow the different references are 'bleeding' through, perhaps through the probes. I've talked to the probe manufacturers, and they assure me that the housing is electrically isolated from the sensor leads. People have also seen this behavior when using long runs of unshielded wire.

I previously added this statement to the wiki to cover it.
http://www.embeddedcontrolconcepts.com/ ... ng_the_BCS

Grounding the BCS
The BCS-460 and BCS-462 are powered by a two-prong transformer, which creates a DC operating environment that is not referenced to earth ground. If spurious sensor readings are observed due to ground loops, the GND pin of the BCS should be connected to the system ground.

So if in doubt, ground it. We haven't had any reports of problems with common grounds.

[Renfield]

Excellent. Thanks for the definitive reply.

Ren

[TrickyDick]

Probably a silly question, but I'm going to ask anyway.

On the BCS-460, I see there are three terminals/connections labelled GND. Are these internally connected within the BCS ?
I'm using a separate 12VDC supply in addition to 120VAC in my enclosure. I'm thinking I will likely require making a common ground.
If so, does it matter to which terminal on the BCS I make this GND connection to?

Also, it seems advised by several to isolate the 120VAC ground (not neutral, though from High School electrical class, it seems that the ground and the neutral are connected in the circuit breaker box anyway, circa 80's - maybe my memory is rusty) from the DC ground.

Thanks

TD

[JonW]

The internal grounds of the BCS are all the same. There are multiple terminals of GND because there are so many items that attach to the ground.

You want to tie the AC ground to the DC ground.

[TrickyDick]

Thank You.

Some of my 12VDC components have only a +12V input and a ground (PID Display Modules, 8ch OSCSYS Driver Board) I hope this does not pose any significant problems. I see that some folks have wired their BCS using a Common 12VDC PSU instead of the included PSU brick supply, and then have stepped down voltage for each component that requires DC supply. I'm not necessarily sure it would be worth my effort to fabricate some resistor step down circuits to try and accomplish the same. I have heard the Driver Board I'm using is sensitive so I guess I'll wait and see. My basic electrical and electronics knowledge is rather rusty, so thanks for the help.

I think I will ground everything to common ground using the enclosure chassis and see what happens.

Thanks again for the support.

TD