3
\$\begingroup\$

I'm using a 9V battery with a 5v regulator as a power supply. What I can't grasp is why I place the regulator in the (+) lead of the battery. If electricity is electrons flowing, and the excess electrons come from the (-) terminal of the battery, why the heck is the regulator placed in the (+) lead instead of the (-)? Wouldn't putting it in the (-) regulate the electrons coming through?

To further add to my confusion, when watching a video about using a 9V as a small LED power supply, the guy connects the 470 ohm resistor on the (-) lead to protect the LED. Why is it that one person regulates the (+) lead, and another the (-)? I understand that there is probably some difference in function between the regulator and the resister (I'll look it up later, I swear), but the principle seems the same - manipulating the electricity sourced from the battery.

This leads into my other question about grounds and the negative lead. I'm going to be working with TTL chips on a breadboard:

14 pin => +5v

7 pin (labeled as ground) => (-)

So in this scenario, the (-) lead is being used as a ground? This seems to conform to the ground black wire used for car batteries and electric guitar pick ups - stuff I am a little more familiar with. But with my recently acquired knowledge, this leaves me scratching my head as to why the source of the electrons (the negative lead) isn't being connected to the 14 pin instead, which is essentially what I was rambling about in my first questions.

Please! Enlighten my poor wretched soul!

\$\endgroup\$
  • 1
    \$\begingroup\$ It's just a common convention, not all systems assume that negative is ground - some cars have a positive ground electrical system. I believe it was Edison that determined the convention of labeling positive and negative charges - before electron flow was discovered. \$\endgroup\$ – Johnny Apr 23 '13 at 19:35
  • \$\begingroup\$ possible duplicate of Help me understand the relationship between positive, negative, neutral and ground in AC and DC \$\endgroup\$ – Kaz Apr 23 '13 at 19:53
  • \$\begingroup\$ Think of a Newton's cradle - when a swung ball hits the stationary balls, the force makes the outward ball move almost instantly. Electricity is more like the force that moved through the stationary balls (i.e. an electromagnetic wave), not the balls actually moving (i.e. electron drift). It's quantum mechanics at its finest. That 'force' is more-or-less everywhere in the circuit at once, so it doesn't matter if you put stuff in the positive or in the negative line, it still influences the circuit in the same way. \$\endgroup\$ – Adam Lawrence Apr 23 '13 at 21:35
  • \$\begingroup\$ "Why is it that one person regulates the (+) lead, and another the (-)?" - Because it doesn't matter to the device being regulated. It only matters in the bigger picture where various regulated circuits may be sending signals to each other, etc. That's why we choose a common signal level (ground). And that common signal level happens by convention to be at the negative potential. \$\endgroup\$ – marcelm Nov 14 '17 at 13:34
2
\$\begingroup\$

You mentioned two possible issues: type of ground in use and direction of of current flow (source of the electrons).

There are four common types of ground:

  • Earth - Literally, a copper rod shoved deep into the ground.
  • Chassis - The common connection using the frame of something.
  • Digital - The common connection for digital circuits.
  • Analog - The common connection for analog circuits.

Realistically, all of these grounds are the same, but they are usually kept separate for the most part, only being connected together in a single place to avoid loops, and often using an inductor. It is called "ground" because of the original reference point being the actual ground, as in the a rod shoved into the earth. However, anything not connected to the earth (such as a battery powered device, or a vehicle) still employs a common node commonly called a ground.

When you use the negative terminal of the battery as a "ground" it is really just the common node (return) of the circuit. A circuit has to have a closed loop for current to flow, and using the negative terminal ensures that all of the components have an identical reference point.

Occasionally, separate portions of the circuit will be completely isolated, each with its own "ground," but again, this really just means the common node of that portion of the circuit.

As far as the "source of the electrons," that is a tricky matter to discuss. Typically, in the US at least, current is shown flowing from positive to negative. While this is not entirely accurate, that is the convention used for modern circuit design. Take a look at the answers to this question: Does electricity go from negative to positive or vice versa?

Your voltage regulator is placed at the positive lead because that is the convention used to design the components - nothing too mystical about it. It is the positive voltage potential that is being regulated. The negative terminal is acting as the reference point (common node) in the circuit.

In the example of powering an LED from the 9V battery, it doesn't matter if the resistor was placed between the positive terminal and the LED, or the LED and the negative terminal. It is not "regulating the voltage" so much as restricting the current. What does matter is the orientation of the LED. The anode has to have a higher potential (voltage) than the cathode for current to flow, so the LED anode is connected towards the positive terminal while the cathode is connected towards the negative terminal.

The position of the resistor is insignificant, it just needs to be there because the current flowing through the LED at 9V would immediately burn it up. The LED more likely drops 2 or 3V. The remaining voltage is dropped across the resistor, and it is this value that determines the current through the LED according to Ohm's law: V = I*R.

I_LED = ( V_battery - V_LED ) / R, where V_battery - V_LED = V_resistor.

\$\endgroup\$
  • \$\begingroup\$ Thank you for your succinct answer; you addressed all of my questions. The provided thread was helpful, but your explanation of convention was good enough. My take away is that it is simply what you said it was: A convention. \$\endgroup\$ – user22979 Apr 24 '13 at 5:13
2
\$\begingroup\$

In many cases, the selection of a particular supply terminal as the common voltage reference is a function of design inertia. Chips that use the negative rail as the common voltage reference are more common than those which use the positive rail, which prompts board designers to use the negative rail as a common voltage reference, which leads to the design of more chips that follow that convention, etc. Interestingly, early PMOS chips including the 4004 and some shift registers used in the Apple I used the +5 rail as the common reference, but regarded as "high" any signal that wasn't more than about 3 volts below it. Such chips could thus interoperate with TTL chips of the day, which used ground as a reference.

One notable place where the positive terminal is ground is in POTS (Plain Old Telephone Service) lines. If there is any leakage in the insulation, a buried cable which is at a positive potential relative to the surrounding dirt will be much more subject to corrosion than one which is at a negative potential. Having the wires be at -48VDC and ground minimizes corrosion.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.