Newbie Q re physical current flow - Page 2 - diyAudio
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 12th September 2002, 09:52 AM #11 Warp Engineer On Holiday     Join Date: Oct 2001 Location: Queensland, Australia DC, When speaking about mains devices, where ground is earth all of your last statement is correct ... see the later replies by other users for more explanations regarding battery circuits where ground is usually not at earth potential.
diyAudio Member

Join Date: Aug 2001
Location: Columbus, Ohio
Quote:
 Originally posted by dc Keld, I meant to say that if a + voltage source, such as a +12V source, is connected to ground, then electrons are flowing from ground to the +12V voltage source(from a position which is negative, relative to the +12V voltage source to the part of the circuit with the greatest potential), right? So, that must mean that the electrons are being taken from the ground? Once those electrons are taken from the ground, wouldn't that leave the ground with a positive charge? Am I even close???
I think I understand what is tripping you up. The plus terminal is only positive compared to ground when the negative terminal of the voltage source is connected to ground.

If you connect the plus terminal to ground (directly or through your resistors) with the negative terminal disconnected (floating), the postive terminal will be at ground potential and the negative terminal will be at -12V. No current will flow through the resistors (except for a tiny burst of current to charge up the internal capacitance of the battery when you first make the connection).

When you connect the negative terminal to ground, current will flow through the resistors and the postive terminal will measure +12V. The ground connection is irrelevant to the current flow, the point is that current flows from one terminal of the generator to the other. Either terminal (or somewhere in-between can be connected to ground) without changing this.

If you have a voltmeter (or a 12 V lightbulb) you can try this for yourself and see how it works.

Phil

 12th September 2002, 09:24 PM #13 diyAudio Member   Join Date: Dec 2001 Location: NYC Haldor, Your last reply was new to me, so it took me a while to figure out what you meant. I'd like to restate your message to see if I've got it right. 1. A circuit is attached to a 12V battery. The potential difference between the + terminal and the - terminal is 12V, regardless of the direction assigned to current flow. 2. The battery is removed and replaced with an AC source. The AC comes through a wall outlet and is connected to a step-down transformer with a center tap. The center tap is connected to chassis ground and the secondaries are fed through a bridge rectifier, and then connected to filter caps to reduce ripple remaining on the line. After the caps, there is a total of 24V potential difference between the secondaries. Because the center tap is equidistant from either secondary and the center tap is connected to ground, one secondary becomes a +12V rail and the other one is a -12V rail. 3. If only the +12V is attached to, say, a common-emitter circuit, and the -12V is left unattached, then there is no closed circuit, and current will not flow? Based on schematics of partial circuits I've seen in textbooks, I was under the impression that ground could serve as a return, thus closing the circuit and allowing current to flow. After all, ground, at 0V has less potential than the +12V rail, no? Is it accurate to read Haldor's last post to mean that current will only begin to flow once both the + and - leads are somehow connected, whether or not via the ground? When both leads are connected, there is a 24V potential difference, regardless of whether the circuit is connected to ground, correct? If the center tap is not placed in the center of the transformer, but placed so that spacing between one secondary and the center tap is 3 times larger than between the center tap and the other secondary (75/25), does this mean that the resulting rail voltages will be +18V and -6V? How does "floating" the circuit change things? I was under the impression that connecting the +12V to chassis ground would create a closed circuit, allowing current to flow.... And that the same would be true with the -12V. I guess that would mean that there would be two loops, one for the +12V/ground and one for the -12V/ground. Now that I type it out, it doesn't seem right. I think a lot of my confusion stems from the voltage divider circuit shown in the opening pages of _The Art of Electronics_ and another schematic for a common-emitter transistor from the _Tab Guide to Understanding Electricity and Electronics_. Both of these show a + voltage source connected to ground through a series of resistors. I realize that these are partial circuits, but was confused trying to imagine from where the current was coming and to where it was going. I didn't realize that a - voltage was also required to be connected to create a potential difference and allow current to flow. Many thanks, brad
Warp Engineer
On Holiday

Join Date: Oct 2001
Location: Queensland, Australia
Quote:
 Originally posted by dc 2. The battery is removed and replaced with an AC source. The AC comes through a wall outlet and is connected to a step-down transformer with a center tap. The center tap is connected to chassis ground and the secondaries are fed through a bridge rectifier, and then connected to filter caps to reduce ripple remaining on the line. After the caps, there is a total of 24V potential difference between the secondaries. Because the center tap is equidistant from either secondary and the center tap is connected to ground, one secondary becomes a +12V rail and the other one is a -12V rail. 3. If only the +12V is attached to, say, a common-emitter circuit, and the -12V is left unattached, then there is no closed circuit, and current will not flow? Based on schematics of partial circuits I've seen in textbooks, I was under the impression that ground could serve as a return, thus closing the circuit and allowing current to flow. After all, ground, at 0V has less potential than the +12V rail, no? Is it accurate to read Haldor's last post to mean that current will only begin to flow once both the + and - leads are somehow connected, whether or not via the ground? When both leads are connected, there is a 24V potential difference, regardless of whether the circuit is connected to ground, correct? If the center tap is not placed in the center of the transformer, but placed so that spacing between one secondary and the center tap is 3 times larger than between the center tap and the other secondary (75/25), does this mean that the resulting rail voltages will be +18V and -6V? How does "floating" the circuit change things? I was under the impression that connecting the +12V to chassis ground would create a closed circuit, allowing current to flow.... And that the same would be true with the -12V. I guess that would mean that there would be two loops, one for the +12V/ground and one for the -12V/ground. Now that I type it out, it doesn't seem right. I think a lot of my confusion stems from the voltage divider circuit shown in the opening pages of _The Art of Electronics_ and another schematic for a common-emitter transistor from the _Tab Guide to Understanding Electricity and Electronics_. Both of these show a + voltage source connected to ground through a series of resistors. I realize that these are partial circuits, but was confused trying to imagine from where the current was coming and to where it was going. I didn't realize that a - voltage was also required to be connected to create a potential difference and allow current to flow.
2. Correct.

3. +12 to Ground thru a load (a resistor will do) is a loop and current will flow.
-12V to Ground thru a load is a loop and current will flow

As long as there is a difference in the voltage (potential difference) between the terminals and a load connecting them, current will flow.

Finally, let me restate that you dont want to use chassis ground as the return path of the power. Chassis ground is connected to the Earth terminal of the power point. All power should return thru the Neutral terminal which is connected via the transformer.

 13th September 2002, 12:29 PM #15 diyAudio Member   Join Date: Aug 2001 Location: Columbus, Ohio DC, AudioFreak is totally correct. The point of grounding the center tapped of the transformer secondary is two create two power supplies (one positive, one negative) and either one is able to supply current independently of the other (within limits). Phil
 13th September 2002, 01:48 PM #16 diyAudio Member   Join Date: Dec 2001 Location: NYC Thanks much, guys. Things are much clearer, now!
 14th September 2002, 04:40 PM #17 Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4 Aren't you nice guys to this DC guy? It takes guts to ask about Ohm's law in this forum. Still, we all want to help if we can. It can be challanging to explain fundamentals that we take for granted. __________________ /Per-Anders (my first name) or P-A as my friends call me PA03 LM4780 amplifier group buy, SIGN UP HERE for the group buy 0 boards left. 118 paid.

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