| Author |
Message |
willi
Joined: Oct 20, 2008
Posts: 13
Location: Portland, Oregon
|
 Posted: Tue Nov 02, 2010 2:20 pm Post subject:
 Understanding Basics
Subject description: AC, DC, Ground, etc. |
  |
|
Hi,
I've done a fair bit of reading and researching online, and a fair bit of experimenting with circuits in the real world, but from time to time I realise how little I actually understand about electricity and circuit design. Some of these concepts I wish to understand may be very basic, and my misunderstanding might prevent me from knowledgeably relating my real question. Hopefully there are others who can also benefit from seeking answers to some of these questions!
Some of my most basic questions revolve around electrical ground, and the differences between AC and DC.
As best I understand, current (DC) is the unidirectional flow of electric charge. There can be positive or negative DC voltages.
Is ground relative? Does ground always = 0v? Measuring a 9v battery with a volt meter in proper orientation shows a reading of roughly +9v (presuming an ideal and fully charged battery, of course). Reversing the test leads shows -9v on the meter. But if a circuit uses only a single 9v battery, the negative lead is considered ground and of course reading ground to ground with the meter will show 0v. I'm under the impression this is called a single power supply. Let's say we do something very simple with this supply, like power an LED and current limiting resistor. What is the difference between -9v and 0v/ground in this circuit?
Is it possible to build a different, more complicated circuit that requires +9v and -9v and 0v ground, and power it from this single 9v battery power supply? Is that as simple as connecting the positive and negative leads together to form the 'ground', and if so, is it the circuit to be powered that prevents this from being anything other than a direct short? Isn't the direct short still the path of least resistance to ground?
What would happen if the supply leads (perhaps coming from another prior circuit, instead of a simple battery) were at +10v and -9v? Is the result +1v, and how does that relate to ground? Is it possible to modify a circuit designed for +9v/-9v so that, provided component operating specifications are not exceeded, the circuit can run on a +9v/0v supply only, and 'float' the ground to 4.5v? Sorry if these are silly questions. Perhaps it depends on the circuit?
Connecting two 9v batteries (+)--(+/-)--(-)can provide +18v, 0v ground, -18v, and that this is called a dual power supply. Let's say I connect an oscillator to this supply, and sweep the output from +18v to -18v and back. Is the result any different from AC, and if so, how? What is the difference between AC and modulated DC? Is rectification basically the same thing as 'demodulation'?
How can one plug or lead be 'hot' in AC, if both wires reverse polarity at the same frequency?
On some of this stuff, I think I might be getting a little confused by some differences between voltage and potential. I'm trying to understand Ohm's Law and other basics more and more every day! Thanks very much for taking the time to read my post. I sincerely look forward to your replies! |
|
|
Back to top
|
|
 |
blue hell
Former site admin
Joined: Apr 03, 2004
Posts: 24864
Location: The Netherlands, Enschede
Audio files: 353
G2 patch files: 320
|
| Posted: Tue Nov 02, 2010 3:10 pm Post subject:
|
|
|
 willi
| Quote: | | Is ground relative? Does ground always = 0v? |
Voltage is always relative to some other voltage, it is the voltage difference that makes current flow. So indeed ground is relative, it is an arbitrarily chosen reference that we label at a zero V olt potential.
Yes ground is relative, it a a chosen reference voltage that we assign the value 0 V to. It is a choosen reference for the circuit we are talking about, and two isolated systems can have grounds that have a different voltage wit
| Quote: | | I'm under the impression this is called a single power supply. |
Correct.
| Quote: | | Let's say we do something very simple with this supply, like power an LED and current limiting resistor. What is the difference between -9v and 0v/ground in this circuit? |
There is no difference.
| Quote: | | Is it possible to build a different, more complicated circuit that requires +9v and -9v and 0v ground, and power it from this single 9v battery power supply? |
Yes, that can be done, but it is not completely trivial.
| Quote: | | Is that as simple as connecting the positive and negative leads together to form the 'ground', and if so, is it the circuit to be powered that prevents this from being anything other than a direct short? |
So indeed that would just be a short - a zero ohm path (idialized wire) from one pole of the battery to the other ... that would be a black hole like thing where infinite current would flow (idialized battery).
| Quote: | | Isn't the direct short still the path of least resistance to ground? |
So, yes 
| Quote: | | What would happen if the supply leads (perhaps coming from another prior circuit, instead of a simple battery) were at +10v and -9v? Is the result +1v, and how does that relate to ground? Is it possible to modify a circuit designed for +9v/-9v so that, provided component operating specifications are not exceeded, the circuit can run on a +9v/0v supply only, and 'float' the ground to 4.5v? Sorry if these are silly questions. Perhaps it depends on the circuit? |
These are not silly questions! (that black hole I mentioned was for a reason, an example where we still fail to understand gravity, and run into a similar paradox - or so it seems to me) It is more like a "wrong model" ... maybe have a look at http://en.wikipedia.org/wiki/Kirchhoff%27s_circuit_laws for the Kirchhoff Voltage and Current laws - these are important in understanding electrical circuits.
| Quote: | | Connecting two 9v batteries (+)--(+/-)--(-)can provide +18v, 0v ground, -18v, and that this is called a dual power supply. |
It is a dual supply, yes, but it would provide -9V 0V + 9V - I can see you reasoning here from the earlier short circuit example.
| Quote: | | Let's say I connect an oscillator to this supply, and sweep the output from +18v to -18v and back. Is the result any different from AC, and if so, how? |
Except from the Voltage range being -9 .. 9 - yes it would be AC.
| Quote: | | What is the difference between AC and modulated DC? |
It is possible to have an AC voltage mounted onto a DC voltage ... that is a varying voltage can be decomposed into an AC part and a DC part. The DC part would be the average value, and the AC part would be the varying bit. A pure AC signal would be a varying signal mounted onto a DC value of 0.
| Quote: | | Is rectification basically the same thing as 'demodulation'? |
No, but rectification (followed by filtering) can be used to demodulate an Amplitude Modulated signal - that is, an AC signal that varies in amplitude dcan be demodulated to a DC value that follows the AC amplitude ... the resulting DC would be varying of course with the original AC signal's amplitude, so it would have an AC component in itself again.
| Quote: | | How can one plug or lead be 'hot' in AC, if both wires reverse polarity at the same frequency? |
By defining one of the wires to be ground ... the other one would have a voltage that moves above ground, then back to below ground etc. For AC mains voltage the situation is a bit more complicated than this, the one chosen to be at zero volts is not actually the same thing there as ground ... ground here is usually meant to be a safety ground, and the zero volt wire usually has about the same voltage as the safety ground, but this need not be the case. Nevertheless, the hot wire fluctuates more. In older systems (or three pahse systems) there can even be two (or three) hot wires with respect to the safety ground.
| Quote: | | I sincerely look forward to your replies! |
I hope this helped a bit ... |
|
|
Back to top
|
|
|
willi
Joined: Oct 20, 2008
Posts: 13
Location: Portland, Oregon
|
| Posted: Wed Nov 03, 2010 9:54 am Post subject:
|
|
|
| Blue Hell wrote: | | willi |
Greetings! Thanks for the reply! Your post was indeed quite helpful. I'm encouraged to see that I sort of had some of the basic concepts correct, but I really wanted to get some confirmation before I started taking false assumptions to heart. Anyway I had a few further comments and questions:
| Quote: | | Quote: | | Is it possible to build a different, more complicated circuit that requires +9v and -9v and 0v ground, and power it from this single 9v battery power supply? |
Yes, that can be done, but it is not completely trivial. |
Would you happen to have a link to such a circuit, or an explanation of one? I ran across a website the other day that described a circuit to create a DC power supply from an AC source, and it was helpful. It seems like understanding how to convert in the opposite direction would be useful... Once you have converted to AC voltage, the voltage can easily be converted to any other voltage with passive coils, if I understand correctly..?
| Quote: | | Quote: | | Connecting two 9v batteries (+)--(+/-)--(-)can provide +18v, 0v ground, -18v, and that this is called a dual power supply. |
It is a dual supply, yes, but it would provide -9V 0V + 9V - I can see you reasoning here from the earlier short circuit example. |
Ooops, my mistake! It's been a while since I built a circuit that used this type of power supply.
| Quote: | | Quote: | | What is the difference between AC and modulated DC? |
It is possible to have an AC voltage mounted onto a DC voltage ... that is a varying voltage can be decomposed into an AC part and a DC part. The DC part would be the average value, and the AC part would be the varying bit. A pure AC signal would be a varying signal mounted onto a DC value of 0. |
Very interesting!
| Quote: | | Quote: | | Is rectification basically the same thing as 'demodulation'? |
No, but rectification (followed by filtering) can be used to demodulate an Amplitude Modulated signal - that is, an AC signal that varies in amplitude can be demodulated to a DC value that follows the AC amplitude ... the resulting DC would be varying of course with the original AC signal's amplitude, so it would have an AC component in itself again. |
Incredible. Presumably these signals are decoded / computed based upon their frequency. As such, signals of different frequency can be overlaid on each other in a sort of 'complex waveform', right? Is there some theoretical maximum limit to this? I guess at the low end the limit is 'patience' or 'life', and the high limit in the real world comes down to signal to noise levels, waveform bounce, and other things that internet providers and tech engineers deal with regularly.
| Quote: | | Quote: | | How can one plug or lead be 'hot' in AC, if both wires reverse polarity at the same frequency? |
By defining one of the wires to be ground ... the other one would have a voltage that moves above ground, then back to below ground etc. For AC mains voltage the situation is a bit more complicated than this, the one chosen to be at zero volts is not actually the same thing there as ground ... ground here is usually meant to be a safety ground, and the zero volt wire usually has about the same voltage as the safety ground, but this need not be the case. Nevertheless, the hot wire fluctuates more. In older systems (or three pahse systems) there can even be two (or three) hot wires with respect to the safety ground. |
Yeah, I was asking in reference to AC mains, such as the 100v 60Hz supplies we have here in the US. If one wire fluctuates more I think I understand. Can you feel the difference between the two leads?  
| Quote: | | I hope this helped a bit ... |
Definitely! But I am then lead to wonder a little further about things like AC coupling and DC coupling, and blocking capacitors. For example, MOTU makes some cool software for interfacing analog CV to a DAW, allowing the DAW to do nifty things like autotune a VCO. But they require that the interface does not have blocking capacitors. I think I understand that a capacitor will remove some degree of -voltage- difference as it absorbs and discharges current. But wouldn't this remove the audio signal that the interface would be trying to capture?
Thanks again! |
|
|
Back to top
|
|
|
Clack
Joined: Aug 08, 2005
Posts: 438
Location: Walthamstow - london
Audio files: 5
G2 patch files: 1
|
| Posted: Wed Nov 03, 2010 4:24 pm Post subject:
|
|
|
| willi wrote: |
| Quote: | | Quote: | | Is it possible to build a different, more complicated circuit that requires +9v and -9v and 0v ground, and power it from this single 9v battery power supply? |
Yes, that can be done, but it is not completely trivial. |
Would you happen to have a link to such a circuit, or an explanation of one?
|
How I understand (someone correct me if im wrong) is the reason you have dual polarity power supplies is that some components (Transistors) conduct mostly in 1 direction and sound sources are AC (oscillating around 0v) so you need to add a DC offset to the signal
this can be done with resistors across the power supply set up as a voltage divider, the input signal AC is added to this 'bias' voltage, so the AC signal is seen at the halfway point. This is what's done in many guitar circuits as fancy bipolar supplies are a bit overkill and 2 batteries are bulky/expesnive to replace.
http://www.muzique.com/lab/buffers.htm
A dual voltage power supply - removes the need for this complex biasing (lowers part count!) and provides a rigid 0v point as different loads on the resistor divider can cause this reference voltage to 'bend'
the problem is that your limited to 9v rather than 18v with 2 batteries so a lot of circuits are hard to convert and balance correctly. It is hard to get some opamps to work across such a low voltage.
there are other options but they tend to rely on black box IC circuits which I dont like! ( if it becomes obsolete its hard to repair!!!)
The charge pump IC from Maxim that cleverly converts a voltage to a dual one.
http://www.geofex.com/circuits/+9_to_-9.htm
| willi wrote: |
Definitely! But I am then lead to wonder a little further about things like AC coupling and DC coupling, and blocking capacitors. For example, MOTU makes some cool software for interfacing analog CV to a DAW, allowing the DAW to do nifty things like autotune a VCO. But they require that the interface does not have blocking capacitors. I think I understand that a capacitor will remove some degree of -voltage- difference as it absorbs and discharges current. But wouldn't this remove the audio signal that the interface would be trying to capture?
Thanks again! |
A simple way of looking at it is that a capacitor in series is a high pass filter - it does not pass frequencies below a certain frequency, a DC voltage can be seen as a frequency that is 0Hz. So it will get filtered out, this is put on the input of sound-cards as they need to eliminate any DC offset to prevent distortion.
AC coupling is when you use a capacitor to connect 2 circuits so only an AC signal passes
DC coupling is literally a hardwired link and it passes both AC and DC
_________________
Clacktronics.co.uk |
|
|
Back to top
|
|
|
|
Forum index » DIY Hardware and Software » Developers' Corner
FAQ
Calendar
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
Chat Room
