What is the convention for -Vs? 1

[autonub]

I'm used to working with closed loop diagrams from college and during my research. However, in practice it is very common for the main lines from the output of an AC/DC power supply to continue from one page or sheet to the next. At the bottom and top of each line it is necessary to state the voltage so other people can follow the schematic. I know for -Vs there are technically several "truths." However, I'm curious what the standard practice or convention is when tagging Vs- (the negative voltage source potential or node; the wire connected directly to the negative terminal of the DC output).

From my perspective, simply saying DC GND makes the most sense for -Vs. However, you could say, for a 24Vdc power supply as an example, -24Vdc, as -24Vdc would be the potential with respect to +24Vdc (essentially flipping the leads of your multimeter in this case). Or you could even say 0Vdc, since -Vs in this case is your reference or ground potential (even though it may not actually be 0V with respect to real ground). But what is the most widely recognized practice?

I checked IEEE 315A among other standards but didn't find a clear answer. Does a standardized method for describing -Vs exist? Maybe "-Vs" is the answer? Please provide source(s) with your answer. Although I'd be interested to read your personal preferences, what I'd really prefer is an established method by an organization such as IEEE.

Thanks!

 

[IRstuff]

"However, you could say, for a 24Vdc power supply as an example, -24Vdc, as -24Vdc would be the potential with respect to +24Vdc (essentially flipping the leads of your multimeter in this case). "

THAT IS A DEFINITE NO-NO. The voltages listed must be measurable relative to something that's designated as "ground."

Most systems would have the most negative node arbitarily designated as earth ground. The only time a -Vs would have a voltage level would be if there were both positive AND negative supplies, with ground in the middle somewhere, as in the case of most op amps. In fact, I would suggest that you crack open a copy of the "The Art of Electronics," which has gobs of circuit diagrams, or pop open a few op amp datasheets.

TTFN
faq731-376

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[VEBill]

On complex systems where there are a zillion wires crossing many sheets, each wire has an assigned signal name (e.g. "X27B417") and they use sheet and zone references "27G11" (Sheet 27 Zone G/11) to jump between sheets.

Power supplies are treated no differently.

 

[autonub]

Thanks for the replies. I'm plenty familiar with operational amplifiers from college. My senior project actually used several 741 op amps. I have not read the book "The Art of Electronics" though. I'm not confused about understanding the way electronics work here. I'm only curious about the standard method for showing -Vs on an electrical/electronic diagram.

The company I work for is a military contractor but when they hired me they didn't have an EE working for them, so there are some bad habits I've noticed and have attempted to correct over the years (these "bad habits" were obviously never a danger to personnell or equipment, but rather mere inconsistencies and nonstandard approaches to commonly known standardized topics).

We use wire numbers on wire tags on our drawings to match the physical labels, as is the standard practice for any engineering or assembly company, which makes good sense especially for assembly and troubleshooting. However, for DC lines coming off the output the company has always simply used "0VDC TO LINE #####" or "0VDC FROM LINE #####" for -Vs. The purpose of my original post here was to determine if "0VDC" is correct based on an established standard and, if not, find some evidence to show a standardized alternative. I prefer to standardize everything I do, even when a specific standard is not required. This is better than choosing something merely because it's true and makes sense.

 

[davidbeach]

Grounded DC? Real DC systems are fed out of 60 large glass jars. ;-) And they're all provided with ground detectors to issue a warning if any of it becomes grounded. And then there's that weird stuff that Bell Labs came up with years and years ago where they work with 48VDC and deliberately ground the positive terminal of the battery.

I agree with VE1BLL that every wire should have a unique designation. The wires that are connected to DC negative in my world all have a designation ending in N while their counterparts on the other side of the battery all end in P.

 

[VEBill]

Complex systems will have multiple power supplies, often distinct and separate even if the very same voltage. In other words, one +28V may be kept very separate from another +28V. The same thing applies to 115vac, even if they could be traced back to the very same supply, they're still very distinct circuits after the Circuut breakers in the distribution center. Even grounds (all eventually common at some point) are often kept distinct.

For this reason, what exactly a certain wire carries is often too complex to safely summarize. Thus the system wiring diagrams (system level) just "give up" giving hints - so to speak - and resort to using the universal solution I described previously.

Of course, many schematics (typically for a circuit card assembly) will use the usual ground symbol(s) and the floating T junctions (with label for voltage) for the common power supply rails. There's less risk with a PCB because you can't really incorrectly rewire it (internally) - e.g. cross wiring analog and digital grounds.

Every organization should have a drafting manual that refers to whatever obsolete and outdated standards they've failed to keep up with. The systems themselves might last decades, watching drafting standards du jour come and go.

 

[itsmoked]

I've seen no general standard though several organizations have had internal ones.

I use the following when no other is required.
24VDC
24VRTN (as in 24V return) This avoids immediate comparisons to ground. and the confusion of using +24V and -24V I'm sometimes confronted with when the -24V is actually just 0V.

If both polarities are present:
+24VDC
24VRTN
-24VDC

Alternatively we use:
24VDC
0VDC

or

+24VDC
0VDC
-24VDC

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

It depends a lot on where in the World you are.

I like the Germans here. Siemens has been using the following system for decades. It is easy to use and understand and it doesn't need lots of special symbols - they even avoid their umlauts.

All positive voltages have a P.
The signal "ground" (grounded or not) is BS (Bezugsspannung i.e. Reference Potential).
The more common signal ground (and now, it is usually tied to GND) is M (for Masse or Mittelpunkt)
All negative voltages have an N.

Example:
+24 V P24
+15 V P15
+5 V P5
0 V BS
0 V M if grounded
-15 V N15
-24 V M24



Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[autonub]

Thanks for all of the interesting replies. I was leaning toward simply using "DC GND1 TO LINE #####," "DC GND2..." , etc, because this way I could make it easier to differentiate between the various -Vs lines from the various DC power supplies, but perhaps simply calling them all "0VDC" and relying solely on the line numbers is adequate. I'll revert back to the traditional 0VDC in this case then.

Thanks again!