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Probably a very basic DC question

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Dave_J_G

New Member
I have a reasonably good knowledge of the fundamentals of electronics / electricity, but there is one think I’ve never got my head around. I’m sorry if this is a very basic question, but I have never found a satisfactory answer to it on the web / in a book.

Firstly, I think I get the concept of alternating current. Am I correct in thinking, that the positive phase of the sine wave represents current flow in one direction, whilst the negative phase represents current flow in the opposite direction? If this is the case, then, if the sine wave is perfect, (i.e. both the negative and positive phases are of equal magnitude and duration) how can there be a net current flow in either direction?

My second question relates to direct current. I understand that DC does not fluctuate. On figures that explain it, it is generally represented on a chart with a y axis from –X to +X, with the DC line generally residing in the positive half, and is therefore +X volts DC.

However, as there is a negative portion to this chart, am I correct in assuming you can have negative direct current, and this is why you sometimes see on circuit diagrams +Xv and –Xv. If this is the case then:

1. What exactly does negative DC represent?

2. Is this only relevant in dual power supply circuits. I.e., sometimes you just see circuits that have +XV and then 0V, which I always took to be positive and negative sides (of say a battery). In some (dual supply?) circuits, you see +xV, -xV and ground. If you were connecting a battery / power supply to this, how would you supply + and – DC, and where would ground go?

I just can’t get my head around how you can have negative DC, what it represents, and how it is different to + DC. Are there circuits with just –DC?

Sorry again if this is basic.

Thanks.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
You're thinking DC is an absolute thing - it's not - it's purely relative.

As an example, if you measure the voltage of a battery relative to the negative connection, then you get a positive voltage. But if you measure relative to the positive connection you get a negative voltage. Nothing is any different, it's just relative to where you measure from.
 

Dave_J_G

New Member
Thanks for the reply.

I see.

So, if you see a circuit diagram with +9V and -9V and ground symbols on it, rather than the standard battey sign with a thick short line an thin long line, is this effectivly the same thing? I saw a circuit with this +9V and -9V and it said you would nead a dual power supply? would a 9V battery work? also, where on this circuit would ground go to if its no from a AC plug?
 

smanches

New Member
You can't get your head around it because it's the same as +V. You're thinking about it too much.

Voltage is just the difference of potential between two points. +Vdc between two points is -Vdc if you reverse your test probes. Usually the only need for having both +Vdc and -Vdc power sources is if you have to work with AC signals.

You are correct that AC net current flow is zero. It's not the flow of current that does the work, but your use of the current. Current is used when it gets dissipated into heat. This is when you start "losing" current.

(That last part was really hard for me to get into words. Someone please clarify it much better.)
 

BrownOut

Banned
You can't get your head around it because it's the same as +V. You're thinking about it too much.

Voltage is just the difference of potential between two points. +Vdc between two points is -Vdc if you reverse your test probes. Usually the only need for having both +Vdc and -Vdc power sources is if you have to work with AC signals.

You are correct that AC net current flow is zero. It's not the flow of current that does the work, but your use of the current. Current is used when it gets dissipated into heat. This is when you start "losing" current.

(That last part was really hard for me to get into words. Someone please clarify it much better.)


Or you can transfer the current into motion, as in an induction motor. Essentially, you are using both the positive and negative flow of current to do work. The fact that no net current flows does not mean it doesn't do work. Remember, power is voltage times current. In an AC circuit, power is positive in both the negative and positive current directions.
 

smanches

New Member
I was thinking about motors, but isn't all the power still dissipated into heat, since magnetic fields are regenerative? I lack knowledge here.

I should say it's dissipated into radiation, so to include RF transmitters and lights.
 

dknguyen

Well-Known Member
Most Helpful Member
I was thinking about motors, but isn't all the power still dissipated into heat, since magnetic fields are regenerative? I lack knowledge here.

I should say it's dissipated into radiation, so to include RF transmitters and lights.

No. That would mean the motor no longer a motor, but an heat/EM source like a radio, light bulb, or x-ray because there is no energy left to be outputted as mechanical motion. I don't know what you mean by "magnetic fields are regenerative". To me that sounds like a buzz word. The magnetic fields being generated do work. The creation of the BEMF to oppose the voltage source for the motor is not regeneration. It does not "regenerate" anything.

You can't get your head around it because it's the same as +V. You're thinking about it too much.

Voltage is just the difference of potential between two points. +Vdc between two points is -Vdc if you reverse your test probes. Usually the only need for having both +Vdc and -Vdc power sources is if you have to work with AC signals.

You are correct that AC net current flow is zero. It's not the flow of current that does the work, but your use of the current. Current is used when it gets dissipated into heat. This is when you start "losing" current.

(That last part was really hard for me to get into words. Someone please clarify it much better.)
As for the AC and "zero net current flow", you might think of it as a propogating wave like sound waves or water waves.. The air/water molecules have "zero net movement" (sometimes not even in the same direction as the wave!), but that doesn't mean the energy of the sound wave has "zero net movement". In fact, it is propogating in a single direction. You are transferring ENERGY, not necessarily electrons.
 
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smanches

New Member
No. That would mean the motor no longer a motor, but an heat/EM source like a radio, light bulb, or x-ray because there is no energy left to be outputted as mechanical motion. I don't know what you mean by "magnetic fields are regenerative". To me that sounds like a buzz word. The magnetic fields being generated do work. The creation of the BEMF to oppose the voltage source for the motor is not regeneration. It does not "regenerate" anything.

Regenerative in the fact that the current used to create the field is recreated when the field collapses. This is for static inductors though. Although in a motor wouldn't this be apparent power - real power?

Again, I have almost no knowledge about the inner workings of motors from the energy standpoint. How exactly the electricity eventually turns the motor shaft and the various energy conversions that happen.
 

dknguyen

Well-Known Member
Most Helpful Member
Regenerative in the fact that the current used to create the field is recreated when the field collapses. This is for static inductors though. Although in a motor wouldn't this be apparent power - real power?

Some energy is recovered when the field collapses, but nowhere near all. ***The more energy an inductor (aka the motor winding) stores, the more current passes through it approaching infinity. When the field collapses, all the stored energy is released in the form of inductive flyback voltage spike and current.

We both know that current in the motor windings does not increase to infinity the longer you run the motor because the BEMF opposes this. The motor's speeds (increasing BEMF) until an equilibrium is reached between the BEMF and the voltage source minus voltage drops across winding resistance, etc and other losses. In other words, the motor speeds up until the BEMF fully opposes the motor voltage.

In terms of our discussion this means the motor's speed is determined by the equilibrium between rate energy is being added to the winding inductance and the rate energy is being drawn from the winding inductance (via the magnetic fields which use that energy to produce motion). *** implies that an inductor storing a fixed amount of energy will have a fixed current flowing through it.

If the BEMF was not present, in other words if the magnetic fields were not drawing stored energy from the inductor to do work, (ie. just a static inductor storing energy) the current in the winding would increase towards infinity (or saturation in real life) because you are just accumulating more and more energy in the inductor.
 
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crutschow

Well-Known Member
Most Helpful Member
So, if you see a circuit diagram with +9V and -9V and ground symbols on it, rather than the standard battey sign with a thick short line an thin long line, is this effectivly the same thing? I saw a circuit with this +9V and -9V and it said you would nead a dual power supply? would a 9V battery work? also, where on this circuit would ground go to if its no from a AC plug?
If it says +9V and -9V then you need two separate batteries or supplies (since the total voltage difference from the plus side to the minus side is 18V). You could use identical batteries and connect one with a negative terminal to ground and the other with a positive terminal to ground. A ground is just a common point for signal returns on a circuit. It may or may not go to earth ground.

All AC signals have energy and it's identical in plus and minus directions. The plus and minus refer to current and/or voltage direction but not power. If you multiply a negative voltage by a negative current you still get positive power.

In an AC motor, the current creates a rotating magnetic field which generates a force to couple the electrical energy to the motor rotor, causing it to turn and delivering the energy to a rotating load. An ideal motor would transfer all the electrical energy to the rotor. In practice there are losses due to the resistance of the windings, and magnetic eddy current and hysteresis loses.

A motor does have some inductance, but that's incidental to it's primary purpose and not necessary for its operation. Under normal operating conditions the back EMF which limits the motor current is caused by the magnetic field from the rotating rotor and not its inductance. That's why the motor current is very high under stalled rotor conditions, being limited only by the winding resistance and the incidental winding inductance.
 

Dave_J_G

New Member
Thanks for all your replies

Crutschow; So, if a circuit diagram has 2 points, one saying +9v another saying -9V, is this the same as a circuit diagram just having the battery symbol with +18v on the positive side and 0V on the negative side?

Hypothetically, if an 18V battery existed, would this work rather than using 2 9V batterys. Or is +9V and -9V used so different parts of the circuit can use 9V or 18V?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Thanks for all your replies

Crutschow; So, if a circuit diagram has 2 points, one saying +9v another saying -9V, is this the same as a circuit diagram just having the battery symbol with +18v on the positive side and 0V on the negative side?

Hypothetically, if an 18V battery existed, would this work rather than using 2 9V batterys. Or is +9V and -9V used so different parts of the circuit can use 9V or 18V?

If it says +9V and -9V then it needs a split supply 9-0-9, a single 18V supply wouldn't do, because you need the centre tap.

Opamp circuits usually use split supplies, although you can often generate a split supply from a single higher one.

Look at it this way:

You're in New York, stood at the bottom of the Empire State building, looking up you think "that's a look way UP (+ve)". If you now go to the top you would then say "that's a long way DOWN (-ve)". If you're half way up, hanging out of a window, you would say "that's a fairly long way UP, and a fairly long way DOWN (split supply, +ve and -ve)".
 

Dave_J_G

New Member
Thanks Nigel

That makes sense now.

One other thing then, in such circuits, there is often a ground point. I understand that in a high voltage appliance, the ground would connect to an earth pin etc.

However, on DC circuit diagrams, you often see +XV and - XV. Then certain connections throughout the circuit will lead to the ground symbol (The triangle made up of parallel lines).

What is this point, what physically would you wire those connections to? And if you don’t need to wire them to anything, why represent the connection on the circuit diagram?
 

smanches

New Member
The ground point in circuits is just a frame of reference for measuring all other voltages in the circuit. It has NOTHING to do with earth ground whatsoever.

The physical ground connection is usually the negative side of the power supply for a single supply system. In a dual supply it's usually a center tap of a transformer, or the center tap of two batteries put together.
 

Dave_J_G

New Member
Thanks everyone for your replies, this all makes far more sense now.

Based on all above then:

If you look a the circuit in this link:

Function Generator Circuit | www.circuitstoday.com

It has a +15V and a -15V. Therefore

1. Is the total voltage going into the circuit 30V?

2. Why is it +15 and -15 when there doesn't appear to be any part which goes to the centre of the two voltages. I.e, hypotetically if this were two 15 V batteries, there appears to be no connection that would connect to the centre of the 2 cells.
 

crutschow

Well-Known Member
Most Helpful Member
One other thing then, in such circuits, there is often a ground point. I understand that in a high voltage appliance, the ground would connect to an earth pin etc.

However, on DC circuit diagrams, you often see +XV and - XV. Then certain connections throughout the circuit will lead to the ground symbol (The triangle made up of parallel lines).

What is this point, what physically would you wire those connections to? And if you don’t need to wire them to anything, why represent the connection on the circuit diagram?
The ground point is typically the reference point (common) where all signal voltages are measured. Say you have a 1V signal, the reference point for that is circuit ground. The ground symbol is not needed but it saves showing a lot of wires on a schematic, making it less cluttered. All points with the ground symbol are wired together. On circuit boards this is often done with a ground plane consisting of a separate, complete layer of copper.

The function generator circuit you reference uses a plus and minus supply and it does have a total of 30V on the circuit. Two supplies are used so that the output voltages will be symmetrical around zero volts (i.e. a sinewave output peaks would be plus V and minus V. Thus the center of the two voltages is the common reference for the signal out.

If you don't need a symmetrical output voltage, then you could use a single supply voltage for the circuit.
 

audioguru

Well-Known Member
Most Helpful Member
The function generator circuit uses a dual polarity supply of +15V and -15V. 0V is where the two supplies are connected together. There is not 30V.

The datasheet for the obsolete ICL8038 function generator shows that the triangle and sine-wave outputs have their average output DC voltage at 0V when it is powered from dual polarity supplies and the signal swings positive and negative around 0V. Therefore 0V is the common output wire, not -15V.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
The web page yopu linked to is really complete rubbish, it only shows part of a function generator, the rest consists of opamps to condition the outputs, which is basiclaly why it has split supplies, in order to keep the output centred around 0V.

As AG has mentioned, that chip has been obselte for many years, and was also never very good back when it was made.
 

xpi0t0s

Member
Voltages are relative; there is no such thing as "absolute voltage". It's relative to circuit ground (just a term we use to designate 0V and from which all other things are measured). Circuit ground itself may be different from earth potential, and your local earth potential may differ from my local earth potential - depends on many things possibly including whether or not there's a thunderstorm around.

In the circuit, regardless of the merits of the chip, yes, there is a 30V difference from +15 to -15. The square, triangle and sine outputs are relative to 0V and presumably swing from +15 to -15 according to the function shape. You can look at this however you like; you can rename the +15 and -15 to +30 and 0 if you want, and the outputs are then relative to +15V, swinging from +30 to 0. Or you can redesignate +15 as 0V and -15 as -30, and have the output relative to -15V. Or you can add some completely spurious number, let's say 250, and mark the voltage supply +265 and +235 with the centre point being at +250V. But it's actually more helpful to designate the centre point as 0V, hence the +15V and -15V.

You can do this with ANY circuit, take Nigel's circuit at for example. There's no reason we can't specify a +259V input and redesignate his "ground" as +250V, and you could even wire the whole lot up to mains electricity and actually run the whole thing at +250V, but the circuit then becomes hazardous for no benefit because the whole thing still has a potential difference of only 9V, or 5V after the regulator.

In fact if you use the term "potential difference" instead of "voltage" this may help make things clearer. PD and V are exactly the same thing and by its very name PD implies we're looking at differential values rather than absolute, so this could be more helpful than trying to find some mythical absolute zero voltage to compare everything else with.
 
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