Re: newbie DC question

From: Bob (nimby1_notspamm__at_earthlink.net)
Date: 12/28/04 

Date: Tue, 28 Dec 2004 03:22:13 GMT

"Rob Snyder" <rsnyder@reversedthebrokenglass.com> wrote in message
news:qJ3Ad.38117$ld2.15952316@twister.nyc.rr.com...
> Greetings.
>
> I've been studying electronics on my own for a little while now, and
> there's one basic concept I can't seem to get my head around. I
> apologize in advance for the really basic question, but I just can't
> find the answer and it's keeping me up at night.
>
> I thought I understood DC, at least at a primitive level. In circuits
> with a battery, things make sense - there is a positive and a negative,
> current flows from the negative through the circuit to the positive...
> makes sense.
>
> Where I get lost is when I'm looking at a circuit with an AC to DC
> transformer. Typically, the output of this, after the rectifier, is a
> positive DC source, a negative DC source, and ground.
>
> What *is* this ground thing? Does current flow from the negative output
> to ground? From ground to the positive? Both? Neither? What they heck am
> I missing?
>
> Most explanations I've read just say something to the effect of "ground
> being a reference, with the current being more negative or more positive
> than ground". Needless to say, I can't turn that into something I
> understand.
>
> I appreciate any guidance anyone has to offer.
>
> Thanks!
>
> Rob Snyder

Rob,

Great question.

The term "ground", in electronics, is often misused. Technically, if you
have a ground connection it means that some part of a circuit is connected
to earth (or the ground).

People often use the word "ground" when they really mean "common". A
circuit's common is merely any node in the circuit that has been given the
name "common" (or "ground") by some human. Typically a circuit's common is
another name for one its power supply connections, and it's often also
connected to a piece of equipment's chassis (metal). Sometimes this "common"
is also connected to earth (ground) -- usually via the third wire (green) of
a three wire electrical outlet.

As you probably know, if you have a battery and you connect a passive device
(e.g., a resistor) across the two terminals of that battery, the current
(conventional) will flow from the more positive lead of the battery to its
more negative lead. The current can be made to flow from negative to
positive, but this would require the inclusion of some type of
energy-storing device like a battery, capacitor, or inductor in the circuit.

It's very typical to have the output of a transformer hooked up (to diodes
and capacitors) such that you end up with three nodes -- a "common", a
positive supply (+), and a negative supply (-). This will act the same as if
you've connected to batteries in series (plus of one to the minus of the
other), with the middle node being what you're calling "common". No current
will flow into the earth (ground) with this hookup because it's not
connected to ground. If you connect a resistor between your (+) and common
then some current will flow. The same amount of current will flow if you
connect that resistor between common and (-). More current will flow if you
connect that resistor between (+) and (-).

I haven't discussed the (very real) capacitance that is always present
between any conductor and earth. This will allow currents of that circuit to
flow to earth, for time-varying signals of that circuit, even though there's
no apparent connection between your circuit and earth. For now, however,
don't worry about this.

Hope this helps.

Bob

Relevant Pages

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