beginner's 'ground' question

[Robbs]

A very basic thing about grounds I'm embarassed I don't know:

Would


*********** (***********: some circuit
| | | with a positive terminal &
| | | multiple ground connections)
| | |
= = =

indicate that you'd need to build the following in reality?

*******************
| | |
| | |
| | |
| | |
+--->|---+--->|---+-------- V 0 (ground)

to prevent current from one ground connection travelling back up to
anotherpart of the circuit? Or is just

***********
| | |
| | |
| | |
+----+----+------- V 0 (ground

acceptable?

I have several books on electronics/electricity, but a basic, practical
explanation of the concept of 'ground' can't be found in any of them.
'Reference voltage' turned up a few things, but nothing i've been able to
apply in my head to building real-life circuits.

Also, I've seen the word 'rail' used in many posts in this group,
referring (I think) to the negative and positive terminals(?). Is that
correct? I suspect there's a subtle difference in how they're used.

Thank you for any help.

Robbs

 

[John Popelish]

Robbs wrote:

A very basic thing about grounds I'm embarassed I don't know:

Would

*********** (***********: some circuit
| | | with a positive terminal &
| | | multiple ground connections)
| | |
= = =

indicate that you'd need to build the following in reality?

*******************
| | |
| | |
| | |
| | |
+--->|---+--->|---+-------- V 0 (ground)

to prevent current from one ground connection travelling back up to
anotherpart of the circuit? Or is just

***********
| | |
| | |
| | |
+----+----+------- V 0 (ground

acceptable?

Just connecting them all together is the normal way. When you are
dealing with high gain or other noise sensitive circuits, the details
of where all the currents flow in the ground connections gets
important, because no connection is zero ohms, so every current
through every trace and wire drops a little voltage. But in general,
just connect them all together.

I have several books on electronics/electricity, but a basic, practical
explanation of the concept of 'ground' can't be found in any of them.
'Reference voltage' turned up a few things, but nothing i've been able to
apply in my head to building real-life circuits.

Also, I've seen the word 'rail' used in many posts in this group,
referring (I think) to the negative and positive terminals(?). Is that
correct? I suspect there's a subtle difference in how they're used.

Yes. Rails are just connections to the supply. They are called
rails, I think, because if they are drawn on the schematic, one is
often drawn across the top and one across the bottom, with the circuit
laying between the tracks.. or rails. The drawing convention most
common is positive rail across the top, negative rail across the
bottom, and signal flow generally left to right.

Thank you for any help.

Robbs
--

John Popelish

 

[Jonathan Kirwan]

On Fri, 18 Jul 2003 01:27:47 GMT, "Robbs" <burnzy@nyc.rr.com>
wrote:

snip

No diodes. They are all tied together. In some applications,
the circuits diagrams don't tell you everything you need to
know, for example "starring" a connection may not be shown,
under the assumption you'll know what needs it. But I don't
expect you to know about that or other things.

Normally, all grounds are just tied together somehow.

Also, I've seen the word 'rail' used in many posts in this group,
referring (I think) to the negative and positive terminals(?). Is that
correct? I suspect there's a subtle difference in how they're used.

Hmm. You will often hear it as "voltage rail" as "rail" has
several meanings (including a "rail of ICs.") A voltage rail is
just a very commonly used voltage level. For example, there may
be 5 or 6 connections to the positive terminal of a battery to
gain access to that voltage, directly. It might then be called
the "positive rail" then. You might also hear "ground rail" as
ground is just another voltage.

It's like a "hitching post" or "hitching rail" for horse leads,
sort of. Haven't you ever rode up to a rail and tied your horse
to one?

Jon

 

[Peter Bennett]

On Fri, 18 Jul 2003 01:27:47 GMT, "Robbs" <burnzy@nyc.rr.com> wrote:

I have several books on electronics/electricity, but a basic, practical
explanation of the concept of 'ground' can't be found in any of them.
'Reference voltage' turned up a few things, but nothing i've been able to
apply in my head to building real-life circuits.

The term "ground" is widely misused and abused when dealing with
electricity and electronics.

Sometimes, particularly in AC power wiring and some radio antenna
systems, "ground" really does mean "a connection to the earth".

However, most times it would be better to use the term "common" or
"reference point" instead of "ground". In these cases, "ground" is
just the point in the circuit that the designer decided to call "zero
volts" - it is where he puts the black lead of his voltmeter when
measuring voltages elsewhere in the circuit. There is nothing
mysterious or magic about this "ground" - it is just one connection in
the circuit.

Most often, "ground" or common is the most negative terminal of the
power supply (and everything connected to it), but it may sometimes be
the most positive terminal. In many analog circuits (audio
amplifiers, etc.), "ground" is the midpoint of the power supplies, so
you have both positive and negative power supplies for the circuit.


--
Peter Bennett VE7CEI
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