Newbie Q re physical current flow - Page 2 - diyAudio
Go Back   Home > Forums > Design & Build > Parts

Parts Where to get, and how to make the best bits. PCB's, caps, transformers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 12th September 2002, 09:52 AM   #11
Warp Engineer
On Holiday
 
AudioFreak's Avatar
 
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.
  Reply With Quote
Old 12th September 2002, 03:29 PM   #12
haldor is offline haldor  United States
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
  Reply With Quote
Old 12th September 2002, 09:24 PM   #13
dc is offline dc
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
  Reply With Quote
Old 13th September 2002, 12:17 AM   #14
Warp Engineer
On Holiday
 
AudioFreak's Avatar
 
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.
  Reply With Quote
Old 13th September 2002, 12:29 PM   #15
haldor is offline haldor  United States
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
  Reply With Quote
Old 13th September 2002, 01:48 PM   #16
dc is offline dc
diyAudio Member
 
Join Date: Dec 2001
Location: NYC
Thanks much, guys. Things are much clearer, now!
  Reply With Quote
Old 14th September 2002, 04:40 PM   #17
Electrons are yellow and more is better!
diyAudio Member
 
peranders's Avatar
 
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
Tube Buffered Gainclone in work |Thread || Diamond buffer |Thread for the group buy | Wiki
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
newbie can multiple EE cores be paralled for better current handling abel2b Power Supplies 4 2nd December 2008 12:10 AM
Please help a newbie with PSU current requirement for Aikido linestage ncc Tubes / Valves 1 24th September 2007 03:34 PM
Do you think this cabinet restrict air flow andy2 Multi-Way 3 6th August 2005 11:36 AM
current flow, which direction? garbage Tubes / Valves 16 31st October 2004 07:16 PM
Scanspeak Flow Resistance DirkD Multi-Way 1 20th August 2004 01:58 PM


New To Site? Need Help?

All times are GMT. The time now is 06:53 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2