Should supply rails be negative?

Hello.

As we know, it is a historical accident that the charge of the electron is considered negative, and conventional current flows counter to the electrons. For a long time I thought that this wasn't of any consequence, since all the math was symmetrical: everything would work the same if we inverted the signs.

But with semiconductors this isn't true. Electrons are more mobile than holes, and that is why NPNs are in general a bit better than PNPs, and N-channel mosfets are better than P-channels. Ideally, we'd always choose the polarity of a circuit so that we'd primarily get to use N-type transistors.

However, there is another asymmetry that can prevent this, and that is the distinction between the ground and supply rails. "Ground" is the potential level that is connected to the chassis and possibly to earth. Sometimes, because of safety or EMI reasons, we want a circuit to be tied to ground, and that's why switches have to be high-side. But due to convention, high-side (i.e. non-ground-side) is at a positive voltage relative to ground, meaning that we either have to use P-type transistors or then resort to complex bootstrapping gimmicks (boost capacitors).

So let's summarize:

  • Physics says we should switch on the negative rail
  • Safety says we should switch on the non-ground rail
  • Convention says these two rails are distinct

To me this seems to imply that if compatibility with conventional circuits is not an issue, we'd be better off tying our positive rail to ground and calling the negative rail -V, and then doing easy, safe switching on the negative rail with N-type transistors. As an additional bonus, the flow of electrons in the circuit would finally seem natural.

Do I have this right?

Lauri

Reply to
Lauri Alanko
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No.

w.

Reply to
Helmut Wabnig

No. Far from it.

"Ground" (I prefer to say "zero volts", when discussing circuits) has different meanings, depending on whether we are discussing utility supplies, or theoretical circuits. Zero volts in a circuit may or may not be connected to true earth ground, depending on circumstances.

Supply rails may be either positive, or negative of circuit reference zero, and there can be several rails, some negative, some positive with respect to circuit zero. The whole circuit, positive rails, negative rails, and "zero", may be floated hundreds, or thousands of volts above true ground.

Some operational amplifier circuits are an example, having both a positive, and a negative supply rail, and a "virtual ground", which is at (nearly) zero volts, relative to the supply rails. by virtue of gain and feedback.

It's misleading to think of current as electron "flow". Electron displacement is a better description. Electrons travel quite slowly in conductors.

My advice to anyone getting interested in electronics is to forget about electrons. Think charges, voltages, fields, and currents. Become familiar with circuit theory. Learn, and practice, Kirchoff's Laws, Thevenin's, and Norton's theorems, mesh, and nodal analysis to the point where they become second nature.

Conventional current is a well-established norm. There's no point in re-inventing it to suit some philosophical principle. It works, get used to it.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)
Reply to
Fred Abse

That is most interesting, but not particularly informative. Would you care to elaborate where my reasoning went wrong?

Lauri

Reply to
Lauri Alanko

You don't have a clue. If you're truly interested in this, go to school and learn. And your amateurish pontificating turns everyone off. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

That may be true, but you didn't address my argument at all, so I'm not yet better off.

Did you even read my message? I didn't have any philosophical principle, but two practical points:

  • N-channel mosfets are preferable over P-channel mosfets for physical reasons that have nothing to do with conventions
  • A non-conducting (i.e. switched-off) part of a circuit should preferably be at the same potential level as the chassis or earth or at least some nearby large conductive surface, if such things are available

And from these I concluded:

  • The chassis/earth/surface should preferably be at a higher potential level than other rails so that we can easily use N-channel mosfets to control the connection between parts of the circuit and the other rails

My reasoning may well be wrong (I'm a newbie posting to .basics, after all), but if so, I'd like to be explained why. Your post wasn't very helpful in this sense.

So how do you choose which potential level gets called "zero volts"? If there is no reason to prefer one level over another and the choice is completely arbitrary (e.g. in an isolated, floating circuit where the rails have similar areas and shapes), then there is no reason not to do low-side switching, so my argument isn't relevant.

But if there is a reason to call a particular rail "zero" (perhaps due to the circuit topology, or due to the conductive area of the rail), then I'd guess the same reason also motivates having switched-off parts of the circuit connected to that rail.

(Of course the motivation may be very slight, if we are only dealing with low voltage and low frequencies.)

Yes, obviously. That's what I was suggesting in my post, after all: if there is a rail that is for some reason the preferred potential level for inactive circuits (whether we call it zero or ground or whatever), then it seems to me that it would make practical sense to have other rails have _negative_ instead of positive potential relative to that.

But convention makes this difficult: most systems are designed so that other rails have a higher level than the ground, and we need to interoperate with them. And available components show this bias as well: for instance, there are far fewer switching regulators available that use the _higher_ input voltage level as the reference.

So this is my concern: the choice of sign is by itself immaterial, but since we of course prefer to deal with positive instead of negative numbers, I fear it has induced practices that make us use semiconductors in a less than optimal fashion. It's not a very big problem, of course, but I still found it to be an interesting tidbit.

Still, I'd of course be delighted to learn that I have misunderstood something and my concern is unfounded.

Lauri

Reply to
Lauri Alanko

I think my amateurish pontifications are quite at place here. And I'm beginning to understand the motivations that led to the creation of this group in the first place.

Cheers,

Lauri

Reply to
Lauri Alanko

I'm not going to argue, just to say that most of the engineers on the planet are happy with conventions the way they are.

Go study electrical engineering in depth, study other people's designs, and try to understand why they did it the way they did.

Whatever polarity convention you use, it doesn't stop you from using any component any way you want. The only limit is your own imagination, and what the laws of physics dictate.

--
"Design is the reverse of analysis" 
                   (R.D. Middlebrook)
Reply to
Fred Abse

Amateurish questions are appropriate, amateurish pontifications are not. You are not yet competent to present tutorials. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

To ask if my idea is correct, I have to first present it, competent or not. There's no way around that.

Far too often I have encountered questions of the form "why does X do Y?" where X doing Y is something that follows quite naturally from all the rules of the system in question. Obviously the querier is misunderstanding something about the system, but their question doesn't give any hints as to what it might be. So the only way to respond is "why would you expect otherwise?" and hope for some clarification.

To avoid this to-and-fro I'm straight away presenting the reasoning behind my idea so that it is easier for a responder to point out exactly where I went wrong. So far, though, no one has bothered to make use of the opportunity.

Incidentally, this is also how science works: you put a paper up, with your argument and conclusions, and invite critique. The paper has to be detailed enough in order to be meaningfully criticized.

You, however, seem to be offended by the very act of someone daring to present a detailed argument without sufficient credentials. In a newbie group. That's like dissing an undergrad for having the gall to present a paper at a seminar.

Unlike, say, boasts about a relative's body count?

Sorry. I'm here to learn, not to fight, so I'll leave the quips at that.

However, conscience dictates that I'll at least try to make this group a nicer place, slim though the chances are. So let's make a feeble attempt. Here is a public service announcement:

You are being a dick. Seriously.

It is okay not to like beginners' stupid questions. The safest way to avoid them, of course, is to unsubscribe from .basics.

And it's okay not to like my questions in particular. In that case, you'd better plonk me away. I don't mind, that's what killfiles are precisely for.

But you are going out of your way to participate in a discussion with a) nothing of substance to contribute and with b) the express purpose of deterring someone from discussing a matter that they are trying to understand, and in particular from telling what their current conception is.

Again, personally I don't mind too much. I have seen enough misbehavior towards newbies that I'm used to it. Sometimes it's due to frustration with previous newbies, sometimes due to imagined slights, sometimes due to just general condescension. It's not unexpected. But it is utterly antithetical to the purpose of this group, and likely to scare less thick-skinned newbies out from this group, and _that_ I do mind.

I happen to love Usenet, and I still hope that new users would find this wonderful distributed discussion system that no one can censure or control. That's why I care about how a group looks to newbies. In fact I'm writing a magazine article about Usenet, and if a reader stumbles onto this group inspired by my article, I feel partly responsible for their experience. Seeing this sort of altercation in the history won't make it better.

Now consider what you are doing. You are responding to an earnest question about electronics design - that you for some reason find offensive - with "you are wrong, and I won't tell you why, go to school". This sort of behavior will:

  • _not_ convince anyone that they are wrong
  • _possibly_ convince them that this group is useless for learning (if even a regular recommends asking in school instead of here)
  • _conceivably_ convince them that people in electronics are horrible

Is this really the sort of contribution you wish to make to this group?

I realize outburst has most likely been in vain, but I'm counting on the off-chance that you simply don't realize how destructive and unreasonable you are being, and just need it to be pointed out.

So once more: you are being a dick. Stop it. Please.

That's all.

Lauri

Reply to
Lauri Alanko

I was wondering, are you by any chance related to a Mr Chang ??

Reply to
Rheilly Phoull
[snip]

You're dealing with Jim Thompson here, and lately he has shown increasing evidence of being a "dick", especially when he goes on his disturbing tirades about his desire to kill liberals, and his childishly sexist remarks about women, and as you have seen, his gloating about the death count of

someone who shares some of his DNA and most of his unpleasant attitude.

But to respond on-topic to your inquiry, even if it had some validity (and I'm not saying it does not), the conventions for circuits using a positive rail have become so deep-rooted that only a very compelling reason would

suffice to promote the alternative. Looking back at some of my very old circuits, it seems that I used a lot of PNP transistors such as the

2N1540 (which was also Germanium), and the circuit had a negative rail and a positive ground, which is harder for me to conceptualize.

Another reason for the positive rail and NPN or N-channel devices being more popular may be their closer correlation to vacuum tubes, which (AFAIK) do not have a PNP or P-channel counterpart.

I do appreciate the time and thought you put into your post, but I think it is more of a philosophical proposition or "thought experiment" such as Albert Einstein enjoyed. I think it is always valuable to think "outside the box" to some extent, and "play the devil's advocate", and toss around some ideas which may prove to be zany, but perhaps may stimulate some discussion.

Not everyone is a wacky as Jim Thompson, whose response may not be too surprising, but I do think you have been treated rudely, especially in the basics forum. Usenet may be on the road to extinction, and it will only survive by the influx and proper use of this resource by younger or newer people with electronics. I'm glad you recognize its unique value, and I hope you will stick around and help stabilize this resource.

Paul

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Reply to
P E Schoen

An alias >:-} ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

I wasn't responding to your "ideas", only to your "presentation".

Don't go away mad, just go away. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

"P E Schoen"

Another reason for the positive rail and NPN or N-channel devices being more popular may be their closer correlation to vacuum tubes, which (AFAIK) do not have a PNP or P-channel counterpart.

** That would be the origin of the B+ rail convention.

Early transistor radios use germanium PNP devices, so had positive ground supplies.

Most old motor vehicles ( pre 1950s) had positive ground systems, usually 6V too.

.... Phil

Reply to
Phil Allison

Can't see why. Physicists use conventional current flow, too.

Can't see that either.

Most serious electronics has multiple rails, both polarities. Often lots of them, eight or ten being not all that unusual.

Most electronic loads, like relays and LEDs, are low-side switched. That generally takes less parts than high-side switching. NPNs and n-fets are good low-side switches.

I wonder if things like uPs and FPGAs would use negative supplies if the convention had turned out otherwise. With CMOS, it's sort of arbitrary. Old p-channel digital ICs did use negative supplies. As does ECL.

Most ee's don't think about electron flow; we learned Ben Franklin's convention and stuck with it.

It is interesting that most people nowadays prefer to have *something* on a schematic that they can call "ground", even if it's not grounded. I was looking at some old tube circuits, and they tended to be a maze of connections, often with no ground symbols and nothing clearly being the circuit common. And no node names/off-pages... just lines connecting everything. I find such rats-nest schematics hard to read,

Ignore Jim. He's a misogynist who spends most of his time dreaming of killing people.

--
John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    
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Reply to
John Larkin

Ignore Larkin, but pray for him... he is mentally ill, but won't seek help. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

You aren't the first person to make the observation that negative should be positive. After all, we all learn that much in basic electricity.

And in spite of the "electronics.basics" moniker they don't suffer fools in this group.

Besides that, electrolysis electro chemical reactions aside, we've been doing it too long to change now.

If you are in electronics just think of electrons going with the arrow in diodes and bipolar transistor symbols and you won't go far wrong. If you are an aspiring physicist, no one cares what you have to say.

Reply to
_defaullt

Yup, I wonder why? (My old tractor originally had a positive ground 6V battery, now all switched to 12V and neg. to Gnd)

Though not at all an expert, ECL also uses a positive ground 'scheme'.

To give the OP a bit of a break there does seem to be some positive bias in electronics at present. I can buy lots of positive voltage references, but negative ones are not so common.

George H.

Reply to
George Herold

IIRC the switch to negative ground was to reduce galvanic corrosion.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Reply to
Phil Hobbs

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