Connect multiple DC sources to common ground

[jfouts]

I am working a project to remote monitor a PLC panel for lamp status (On/Off).
Each lamp has its own 12VDC source voltage.
I have worked up a board with voltage regulators connected to the positive side of the lamps, in order to control the output voltage which then feeds a Raspberry Pi.

Typically I would connect all grounds together for a single common ground, but since these lamps are being fed from multiple different pieces of equipment on their own isolated circuits, should I be concerned about connecting all of my grounds together?

I don't know why I am second guessing myself on this one, I guess I am just looking for encouragement that I am not losing my mind.

 

[jaggedben]

I find your description a bit lacking. But my first blush reaction is that if the 12V sources are all truly isolated from each other and not likely to become connected in any way at the source then you are probably okay. If there's no polarity reversal, intended or accidental, then all the more so.

The applications I'm familiar with that have similar issues may not be really similar at all, so take my comments with a grain of salt. It might also help to clarify whether by 'ground' you mean the typical electronic usage of a common negative return, or an actually earthed connection.

 

[Smart $]

... should I be concerned about connecting all of my grounds together?....

They are not grounds until they are all connected together and to ground.

 

[mgookin]

I am working a project to remote monitor a PLC panel for lamp status (On/Off).
Each lamp has its own 12VDC source voltage.
I have worked up a board with voltage regulators connected to the positive side of the lamps, in order to control the output voltage which then feeds a Raspberry Pi.

Typically I would connect all grounds together for a single common ground, but since these lamps are being fed from multiple different pieces of equipment on their own isolated circuits, should I be concerned about connecting all of my grounds together?

I don't know why I am second guessing myself on this one, I guess I am just looking for encouragement that I am not losing my mind.

Hello jfouts and welcome to the forum.

You are using nomenclature related to a subject which is very touchy on this forum as related to building & premises wiring (grounding) and that's what generated the former responses. It's "very touchy" because most of the world has a substantial misunderstanding of what earth grounding is and does. You had no way to know that; let's put that aside because you're talking about DC circuits.

For the purpose of your post, and your project, you're talking about a common bus for the -DCV path as we see on most any DC circuit schematic. Your question is whether you can use this common bus when voltages in the circuit derive from different DC sources.

This would be no different than having your car battery supplying voltage, then your engine starting and the alternator providing another voltage source at a slightly higher voltage. Another example is the PSU in your PC and having a battery on the mobo.

You'll be fine. Good luck with your project.

 

[jaggedben]

... For the purpose of your post, and your project, you're talking about a common bus for the -DCV path as we see on most any DC circuit schematic. Your question is whether you can use this common bus when voltages in the circuit derive from different DC sources. ...

Sometimes the common bus is connected to the chassis of the device, which in turn is connected to the grounded conductor of a power source. So sometimes 'gnd' in electronics design is actually grounded.

If more then one of the DC sources is grounded, then any intentional or accidental switching of polarity can result in a short when you have a common bus for multiple sources. That's why I put caveats in my response above regarding isolation and grounding. It certainly sounds from context like there's unlikely to be any polarity factor in the OP's project, but he didn't actually say. In these situation you either need to keep polarity straight or have isolated, ungrounded sources, AFAIK.

Also, it's unclear to me how the Raspberry Pi connects to the circuits and whether it, too may have a common connection somewhere else. When line power finds its way onto control and communication circuits, that's a recipe for an engineering failure, if not a fire. I'm familiar with one application where communication has to be optically isolated when using a common power bus. (To be clear, that application has nothing to do with the solar power systems I normally post about in this forum.)

 

[kwired]

If more then one of the DC sources is grounded, then any intentional or accidental switching of polarity can result in a short when you have a common bus for multiple sources. That's why I put caveats in my response above regarding isolation and grounding. It certainly sounds from context like there's unlikely to be any polarity factor in the OP's project, but he didn't actually say. In these situation you either need to keep polarity straight or have isolated, ungrounded sources, AFAIK.

I am not seeing how you can get a short circuit if each separate source is only bonded to the others at one point, doesn't matter if AC or DC volts either.

Think about how pretty much all the electrical systems with a connection to public grid are all bonded to one another, yes they are typically AC systems, but there are many DC systems also bonded to the same grounded conductor both positive and negative grounded systems, yet each system is only looking for current to return that originated from same source. You can maybe develop undesired paths at times but have no short circuits if the ungrounded side of all sources remains isolated from everything other then paths that lead through a load to get to the grounded side.

 

[SceneryDriver]

I am working a project to remote monitor a PLC panel for lamp status (On/Off).
Each lamp has its own 12VDC source voltage.
I have worked up a board with voltage regulators connected to the positive side of the lamps, in order to control the output voltage which then feeds a Raspberry Pi.

Typically I would connect all grounds together for a single common ground, but since these lamps are being fed from multiple different pieces of equipment on their own isolated circuits, should I be concerned about connecting all of my grounds together?

I don't know why I am second guessing myself on this one, I guess I am just looking for encouragement that I am not losing my mind.

While connecting the negative sides of your power supplies may work just fine, there exists the possibility of introducing noise issues on the "ground" rail, as well as difference-of-potential issues that may cause your power supplies to have problems; probably not, but how much will your machine's downtime cost? This is what interposing relays are made for. Place the coil of a 12VDC relay in parallel with each of your lamps to be monitored. Use the relays to switch the inputs to your Pi on and off. Use flyback diodes across the relays' coils to eliminate the negative going transient generated by turning off the relay.

Be aware that the Pi's inputs are not really designed for an industrial environment, and don't do well with voltage transients and induced noise. Keep your wires from your relays to the inputs reasonably short, and consider using pull-down resistors on your inputs to swamp any induced noise. If you have to run the input wiring for the Pi more than a few feet, strongly consider twisted pair cable. It's a real help when it comes to keeping noise out of low voltage digital inputs. Shielded twisted pair is even better; earth the shield at only one end to keep the noise out.



SceneryDriver

 

[gar]

151206-2116 EST

jfouts:

I can not understand your post.

I am working a project to remote monitor a PLC panel for lamp status (On/Off).

This seems to imply you have a single PLC, and that there are indicator lamps on the PLC. I can assume some lamps are connected to PLC inputs and others to PLC outputs. If these are the only lamps to be monitored, then it is logical to expect these are powered by some common power source.

But you then say

Each lamp has its own 12VDC source voltage.

What does its own 12 VDC source mean? How are these sources related? How are they connected together, and/or how are they isolated? We need a circuit diagram.

I have worked up a board with voltage regulators connected to the positive side of the lamps, in order to control the output voltage which then feeds a Raspberry Pi.

Why voltage regulators from the lamps?

A quick search seems to imply Rasberry is a 3.3 V device and its I/O has to relate to this. Maximum and minimum transition levels are about of 0.54 V and 2.31 V. Between 0 and 0.54 for a 0 state, and between 2.31 and 3.3 for a 1 state.

Apparrently there is an internal pull up or down resistor of 50 k. I believe you can use a 4N35 as an input. At 25 C the 4N35 off leakage should be less than 50 nA. On the 4N35 input side you size a current limiting resistor to adequately saturate the 4N35 output transistor. This provides you complete isolation of every input up to something less than 5000 V. If you do this, then you don't care how your PLC indicators are related to each other so long as you stay within the breakdown limit of the 4N35.

For a data sheet see http://www.vishay.com/docs/81181/4n35.pdf .

.

 

[jaggedben]

I am not seeing how you can get a short circuit if each separate source is only bonded to the others at one point, doesn't matter if AC or DC volts either.

"...bonded at one point...". That's the rub. If there's actually more than one point of connection then you have problem.

You can take multiple sources which are truly isolated, serving separate loads, and connect one wire from each source to a common bus. I merely say 'a wire' because it doesn't matter if that wire is positive or negative, or if each source is using the common bus for the same pole. You can switch the polarity back and forth at random on each of the sources individually and it still doesn't matter.

Now say that the sources aren't truly isolated. Maybe it's just a bunch of DC rectifiers all derived from the same AC source with no isolating transformers. Now when you switch polarity on one of them you have a short through the common bus back to the AC source.

Or say two of the negatives of the sources are grounded. Now if you switch the polarity on one of them (on the load side of the grounding point) you short through the common bus and ground.

That's all I was trying to say. When you are dealing with multiple sources on a common bus you'd better really know the nature of those sources or you could be blind to potential problems.

 

[kwired]

"...bonded at one point...". That's the rub. If there's actually more than one point of connection then you have problem.

You can take multiple sources which are truly isolated, serving separate loads, and connect one wire from each source to a common bus. I merely say 'a wire' because it doesn't matter if that wire is positive or negative, or if each source is using the common bus for the same pole. You can switch the polarity back and forth at random on each of the sources individually and it still doesn't matter.

Now say that the sources aren't truly isolated. Maybe it's just a bunch of DC rectifiers all derived from the same AC source with no isolating transformers. Now when you switch polarity on one of them you have a short through the common bus back to the AC source.

Or say two of the negatives of the sources are grounded. Now if you switch the polarity on one of them (on the load side of the grounding point) you short through the common bus and ground.

That's all I was trying to say. When you are dealing with multiple sources on a common bus you'd better really know the nature of those sources or you could be blind to potential problems.

I agree there is a problem with rectified outputs from a common source, those are not separate systems they are a sub component of the common source. If you take an AC distribution system and intermix parts of different feeders or branch circuits you have the same thing.

 

[Besoeker]

They are not grounds until they are all connected together and to ground.

Well said, that man!

 

[jfouts]

I apologize for not getting back to this sooner.
I kept waiting for an email notification that someone had posted to this thread


I see that I should have been more descriptive in my original question.


Yes, when I used the term ground I was not speaking of earth ground, but rather DC Negative.


While I have not yet actually visited the site that I will be installing at, if memory serves correctly the DC circuits in PLC's are bonded to the chassis, and presumably the chassis is bonded to earth ground.


This project is for a radio repeater station located at the top of our highest surrounding mountain.
Since the site is usually covered in snow, I was hoping to prep the project as much as possible in hopes of making it a one trip visit.


The station is run by a generator.


I mainly want to grab the generators 'low fuel' indicator lamp signal.


Attached is a quick draw of the circuit using a limited component library.


mgookin,
Thanks for your examples of car & PC. That made perfect sense.


jaggedben,
Polarities changing should not be an issue.


SceneryDriver,
Thank you for mentioning noise. Since this is a radio station, and all of the equipment is in relatively close quarters noise is likely to be an issue, and I had intended on using UTP. I will switch it to shielded.
Working up a board using relays may be a safer option and I will do a test run with them.
As far as Pi not being designed for industrial, can you recommend a microcontroller type other than custom building one?


gar,
I dont have a circuit diagram for the equipment yet, but as I understand it:
The generator powers the building.
Some of the equipment such as the radios run directly off of the generator.
Some of the equipment runs off its own dedicated 12v battery.
All of the 12v batteries pull a trickle charge from the generator.
There is a panel with switches, buttons & lamps.
This panel lights a lamp when there is a fault, while the buttons/switches allow you to acknowlege alarms or turn on/off/restart equipment.
Perhaps PLC was the wrong term since ive never actually seen the unit in person.
When I saw it poorly taken picture of it, it looked like an old school PLC, not that new fancy touchscreen business.
I chose a 3.3v regulator(LD1117) for the Pi inputs because I am not a fan of burnt Pi, and I had some on hand from a previous project.
I like the idea of using the optocoupler & will look into that further.


Thanks for all of the replies & Thank you for having me here!

 

[jaggedben]

I'm still not sure I understand how the circuit works, but at this point I'm mostly just hoping to pick up some knowledge myself, not criticize.

If there are multiple batteries and charge controllers then without knowing everything about the charge controllers there could be another parallel connection, possibly an unintended return path and overcurrent. But those concerns admittedly seem somewhat remote and theoretical in the diagram.

The concern about polarity isn't just for intentional switching, it's also an engineering thought; (i.e. do you fry the whole thing if you accidentally connect a wire wrong because you haven't had your coffee yet?)

Optical isolation could certainly help put such concerns to rest.

 

[gar]

151208-2211 EST

jfouts:

Your diagram implies that all your pilot lights are tied in common with a negative rail, and this negative rail has to be common to the negative common of the microcomputer. If this is true, then you need some moderately low load resistance on the output of your voltage regulators. Possibly 100 ohms. Ion = 2.2/100 = 33 mA. This is to reduce noise problems and provide a load on the regulator.

If the bulb voltage is 24 V, then regulator dissipation would run about 0.7 W. In this case 1000 ohms would be a better load.

If the pilot lights do not derive from the same power source, then your circuit likely has problems.

But I don't recommend your approach vs a 4N35 approach.

Using a 4N35 there has to be more discussion on how to avoid noise problems at the microcomputer inputs.

.

 

[jfouts]

Thanks for all of the comments.
Your concerns are much appreciated.

I am currently reworking the circuit some to add in some safety measures & noise reduction.
I will post back when complete.

 

[Smart $]

Thanks for all of the comments.
Your concerns are much appreciated.

I am currently reworking the circuit some to add in some safety measures & noise reduction.
I will post back when complete.

What is your end purpose? That is, what are you going to task the Rasberry Pi with?

 

[mgookin]

What is your end purpose? That is, what are you going to task the Rasberry Pi with?

From what I gather from this thread he has a remote radio transmitter atop a mountain. There is no utility power; everything runs on a genset. There are multiple pieces of equipment which have status indicators. All status indicators have different DC power sources. Each indicator indicates a trouble event. He wants to aggregate the trouble codes to the Raspberry Pi. From there I don't know and I'm not familiar with that flavor of Pi. Assumption is he's going to try to get it to send a text to his cell phone or similar to provide real time status information from a remote location.

Examples of trouble events are low fuel, low power, etc.

 

[jfouts]

The Pi will simply send out an email notification when an alarm is triggered.

Also just for kicks I googled 'earth ground' and came across several entertaining articles describing the confusion with the term 'ground'
Wow! It is a touchy subject!
I will attempt to correct my use of the term.

 

[GoldDigger]

Also just for kicks I googled 'earth ground' and came across several entertaining articles describing the confusion with the term 'ground'
Wow! It is a touchy subject!
I will attempt to correct my use of the term.

"Correcting" your usage may be impossible since someone will take issue with any usage you come up with.
But you can try to confirm to the usage in the NEC.

 

[mgookin]

The Pi will simply send out an email notification when an alarm is triggered.

Also just for kicks I googled 'earth ground' and came across several entertaining articles describing the confusion with the term 'ground'
Wow! It is a touchy subject!
I will attempt to correct my use of the term.

Here's a great start: https://www.google.com/?gws_rd=ssl#q=mike+holt+video+on+grounding+versus+bonding