Should supply rails be negative?

Discussion in 'Electronic Basics' started by Lauri Alanko, Sep 1, 2013.

1. Lauri AlankoGuest

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

No.

w.

3. Fred AbseGuest

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.

4. Lauri AlankoGuest

[Positive ground would make switching more practical]
That is most interesting, but not particularly informative. Would you
care to elaborate where my reasoning went wrong?

Lauri

5. Lauri AlankoGuest

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
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

6. Lauri AlankoGuest

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

7. Fred AbseGuest

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.

8. Lauri AlankoGuest

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

* _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

9. P E SchoenGuest

"Lauri Alanko" wrote in message

[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
www.pstech-inc.com
www.muttleydog.com

10. Phil AllisonGuest

"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.

supplies.

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

..... Phil

11. George HeroldGuest

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.

Huh??

13. Lauri AlankoGuest

Well, everyone deserves the benefit of doubt, even those who don't
give it to others. It was worth a shot.
Yes, obviously. The problems with running high-side n-channel switches
are relatively minor, and there are well-established solutions. But it
is annoying to think that there might be an easier way to do things
and only convention prevents it.
The popularity of N-type transistors is understood. What I don't get
is how that associates with preferring a positive rail, when to me it
seems like N-types would be more practical with a negative rail.
Yes, of course. What else?

A big part of learning about a system is studying different
configurations and trying to understand how they behave. It is
essential that the configurations are highly variable and illustrate
all aspects of the system, and not merely constrained to those
encountered in practice.

For instance, when studying physics, it is quite on par to consider
what happens when you send someone to space and back at light speed.
It doesn't matter that this sort of thing will never ever happen in
the real world: it illustrates how special relativity works, and you
_have_ to be able to explain what happens if you want to claim to
understand physics.

However, in many online discussions of electronics I have seen
something that I hesitatingly call an "engineering mindset": if a
question is not related to a practical real-world problem that someone
is trying to solve, it is not worth discussing. Those sort of people
will no doubt think that discussion of unconventional rail
configurations is a waste of time. (Those people also must think I'm
mad for getting joy from getting a circuit work purely in simulation
even if I have no intention of ever building it in practice.)

For comparison, suppose a beginning math student just learned about
numeral systems and exclaimed: "Hey, wouldn't it be better if we used
a duodecimal system? After all, we divide things into threes and fours
much more often than into fives!" A reasonable response would be
something like: "Probably, yeah. It's not going to happen, of course,
but you are right that it would have advantages." Even though the
proposal is utterly unrealistic, making it demonstrates that the
student has grasped something essential about the purpose of numeral
systems and the tradeoffs in their design, and that is to be
commended.

People with an "engineering mindset" would respond: "You're not
experienced enough to challenge such basic practices. Keep on using
the decimal system like everyone else."
Indeed. Thanks for your reply, the first decent one I got in this

Lauri

14. Fred AbseGuest

That's not what you said. You said:

"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."

In diodes and bipolar transistors, *conventional current* "goes with the
arrow", NOT electrons.

Voltage does not "go" anywhere, it just sits.

15. Fred AbseGuest

That's just to keep "other ranks" "in their place" ;-)