# Electronic Circuits and Projects Forum

## Understanding Electronics Basics #1

1. Originally Posted by KeepItSimpleStupid
Just jumping on the train again.
Good man, glad we didn't lose you :-)

2. A circuit consisting of a 12 V voltage source and two 6 ohm resistors can be analyzed as follows.
12= 6I+6I ---> I=1 or 0=-12 + 6I +6I which is the generic form of the equation

This also means the the voltage across each resistor is 6V

Now Power = VI
If we did the same thing we would have (12)(1)W = 6(1) W+6(1) W
In the generic form of the equation
0=-12 W + 6W + 6W

Power is a negative quantity.

Power generated, by convention has a negative sign. Power dissipated, a positive sign.

Yea, it might make sense to have it the other way, but just like conventional current that's the way it is.

The analysis is generic and is generally known as a "loop equation". The algebra gets messy, but it is appropriate for multiple adjacent loops.

We don't talk about -10 MW generators, but when we analyze, they are negative. Your 100 W light bulb dissipates say 100 W of energy as heat.

3. Alright!!. Now where were we...

Originally Posted by KeepItSimpleStupid
I have to interject, the concept of negative power.

Positive power is power dissapated.
Negative power is power generated otherwise...

If you had a 10 MW generator and 1 million customers consuming 10,000 W
10 MW = 1000 * 1,000,000
0 = -10MW + 1000 * 1,000,000

or
Power consumed = power generated
All very true. However:

Any motor, for instance, by its very nature, while turning, is also a generator. Electric Motive Force (EMF) is said to be the "power" that causes the motor to turn. All well and good. But as it turns, it's also generating what's called a Counter-EMF (CEMF) that flows in opposition to the EMF (this can set up so called "Standing Waves", [which are really bad for causing energy losses] but that really is down the road).

Anyway, as in KISS's example, yes, there is power generated that does no useful work due to losses that are abundant. But keep in mind that a 10MW plant does not always produce at full capacity. The main power producing apparatus is throttled to the load. This does not eliminate losses, but it does reduce unnecessary loses. And, in essence, CEMF can be a power saving phenomenon.

So, for DC, since by way of a bridge rectifier system (and other ways) we can move the negative element of the sine wave to the positive side of ground we are, thereby, now making it part of the "power" side of the equation. This is as you saw in the excepts you introduced in a previous post.

It's good to see that that your curiosity is leading you to search out the answers to some of the questions you have.

Sometimes it's necessary to explain a fundamental concept in such a way is isn't exactly complete or in all cases true. Certainly in a number of fields thee are ambiguities and nuances that are difficult to grasp until a certain underlying concept is understood.

That's when you know better what questions to ask.

And you are rapidly getting there.

4. Another example with your trying to say that a negative wave does nothing. Take a three phase motor which as 2 windings energized with 3 sine waves 360/3 degrees apart. When the sine wave is negative it could be pulling rather than pushing or vice versa, hence it is doing work.

5. A DC motor can act as a generator as well as a motor. In fact the voltage produced by an unloaded generator is proportional to speed so V =k(rpm) or some constant k times the rpm. The motor also has a DC resistance, so there is an IR drop across the windings.

So the Voltage V across the motor is V = Vm - IR; where Va is the Voltage across the motor. If you can compensate that IR drop, you can gain speed control of the motor.

A recent application note from Linear Technology illustrates the earlier fact. The motor is pulsed for nearly the entire time. When it's coasting the voltage is measured and fed back to the speed control.

At the same time torque is proportional to current.

6. Sorry guys, baled & had one of those rare visits to pub
Back on it tomorrow
CBB, tried to self help when site was down but I know you know the way forward, never could there be amore eager student :-)
Night all

7. Hello! Anybody home??...

8. Originally Posted by cowboybob
Hello! Anybody home??...
You noticed I'd gone then sorry seem to have been unindated with jobs to make up for my playtime on bike
it was maybe a tad too early in the year but had a great day out although the cold took it's own toll on me, then Lou decided if I had enough time to go out riding, I needed more jobs to do most out the way & the rest can wait until weds morning, so finally, I feel whacked before I start but can sqeeze in a start to kick this along again

Originally Posted by cowboybob
Alright!!. Now where were we...

Sometimes it's necessary to explain a fundamental concept in such a way is isn't exactly complete or in all cases true. Certainly in a number of fields thee are ambiguities and nuances that are difficult to grasp until a certain underlying concept is understood.
That's when you know better what questions to ask.

And you are rapidly getting there.
ok, totally agree with you on that, please proceed

Originally Posted by KeepItSimpleStupid
Another example with your trying to say that a negative wave does nothing. Take a three phase motor
erm, yeah, maybe we'll leave that a while

It's nice to be back

9. CBB:

Now you have time to work on your triac stuff. You shouldn't be complaining. <G>.

10. See, Graham? Another addiction to deal with...

Your right, KISS. Prob has been that our DSL link to the world has gotten very bad, lately. Keep checking the stupid thing and wasting time doing it.

On to Triac play, yeah!!

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