Resistors for serious dummies.

[Robert Reed]

Inspite of all I have read here I still like the water analogy. Remember that we are dealing with someone who has no concept of electricity. Once this is understood then it is time for Ohms law and the finer points of electron flow.

[dacflyer]

Hmm water analogy,and open circuit...
if you go that route...you will have electricity pouring out all over the floor {:>O
lol...

[Michael J]

Sorry wrong post.<p>[ October 11, 2005: Message edited by: Michael J ]</p>

[jimandy]

In my bag of water analogy tricks, a capacitor is a rubber membrane suspended inside the pipe. Water, in this case, cannot "flow" through the membrane, but pulses of water on the source side will cause pulses on the drain side. Of course, I can't see any real value in pulses of water, except perhaps, in dental hygiene.

[Chris Smith]

Transistors as a water amplifier is easy. <p>Its called a pressure or flow regulator. [Fet VS Transistor]

Some water pressure is diverted to the gate or control valve, and used to control the main flow or quantity of pressure in the main pipe.<p> Transistors don’t amplify the pressure or quantity in the main pipe, they simple control the main pipe [pressure or quantity] available, by using a weaker signal at the valve head or gate/ base. <p>The amplification part comes from a weak signal controlling a strong availability of current/ pressure in the main pipe. <p>It either passes or restricts, based on a weaker signal, similar to your hand on a valve. Same goes with the water regulator. A small pressure, can control the main pipe on, off, or in-between and even oscillations. <p>Same goes for all water analogy and electronics. It works.

Caps are that membrane, stopping the flow, storing the pressure in elasticity, and pulsing signals from one side to the other. <p>Transformers being magnetic, don’t need any relationship. Apples and Oranges. <p>BUT,...The simplest correlation here for the transformer would be a “water motor” driving a “water pump”. Pump diameter, hose size, GPM and rate of spin [hz] all included.<p>You could call them or relate them in other ways, but magnetism and pressure/flow are not the same thing so it only has to have some kind of relationship to understand it. <p>Fets are merely a different gate or control valve configuration. <p>Chokes and coils are flywheels or brakes where they boost or drag the effort by smoothing out or disrupting flow through a pipe from the use of collimating or vortex effects. [Laminar VS Vortex]<p>The water pipe resistance is increased or decreased acording to these two effects, affecting the overa all flow.<p>[ October 11, 2005: Message edited by: Chris Smith ]</p>

[philba]

all in all, I see people really stretching to make the analogy work. If some one really wants to understand electronics, then they should understand the real thing, not an analogy. I've taught at the college level and found that analogies always diverted people's thinking from the end goal. <p>Just one man's opinion.

[Chris Smith]

You should always use what works. I know several students who spent weeks trying to figure out the AMP and found it in three seconds after I told them to think “Water”.

One student was over 60, and a plumber, and it took him even less time. <p>So what ever works for the individual, is right.

[rshayes]

It's probably best not to use the water analogy. Yes, it is used in some very elementary textbooks. But it becomes more complicated than the real thing about the time you consider a circuit with two components or more. And it doesn't work very well for nonlinear parts such as the LED circuit he mentioned.<p>Ohm's Law relates three different quantities: voltage, current, and resistance. Voltage is a loose tem for Electromotive Force (EMF). This is a force which tries to cause charge to move through a conductor. The motion of charge is considered the current and is measured in ampreres. The resistance is an expression of how much force is required to make a certain current flow, and is measured in ohms. A normal resistor follows Ohm's Law:
V = I x R
This simply means that the voltage across the resistor (V) is equal to the current (I) through the resistor times the value of the resistance (R).<p>This equation can be written two other ways:
I = V / R
R = V / I<p>There are two other basic rules that apply, called Kerkoff's Laws. One is that the sum of the voltages around a closed circuit is equal to zero. The second is that the total current flow into a node is also zero. A node is a connection between two or more components. On node is often considered to be at zero volts and is called ground. This is often the negative terminal of a positive power supply or battery.<p>An LED does not act like a resistor. It is very nonlinear. The voltage across an LED is roughly proportional to the logarithm of the current plus a constant voltage. This means that doubling the current through an LED will increase the voltage by only a small amount, and that doubling the current again will increase the voltage by the same increment. A red LED has a voltage drop on the order of 1.5 volts plus a small amount due to current flow. This is usually shown as a graph of current as a function of voltage. Once current is flowing through an LED, a small change in voltage can easily cause excessive current flow.<p>Consider the series LED circuit. Assume that the negative terminal of the 9 volt battery is ground. Start with the voltage acrosss the resistor:
Vr = I x R
Add the voltages of the LEDs:
V= Vr + Vrd + Vgd + Vgd
NOw add the battery voltage. Since the previous parts have had their negative terminals first as we went along the circuit and the positive battery terminal is the first one encountered, the battery voltage is reversed, or given a negative value. Adding in the battery:
V = Vr + Vrd + Vgd + Vgd - Vb
By Kerkoff's Law, this voltage is zero. The voltage across the red LED is about 1.7 volts and the voltage across the two green LEDs is about 2.5 volts. Putting these into the equation gives:
0 = Vr + 1.7 + 2.5 + 2.5 - 9
or:
0 = Vr - 2.3
Substituting Vr = I x R and moving the other term to the other side gives:
I x R = 2.3 volts
or:
I = 2.3 / R
This indicares that the resistor value determines the current through the circuit. Using the other form of Ohm's Law:
R = 2.3 / I
allows the resistor value to be selected to determine the current. For a 10 milliamp current, the resistor value would be about 230 ohms. The nearest standard values are 220 ohms and 240 ohms, so one of these would be chosen.

[Chris Smith]

When we over complicate an equation in the learning process, or split hairs while your still in the crib, confusion occurs. A bad learning process for any beginner.<p>To understand why we need a resistor with a Led is actually more simple than the exact science of what’s going on, describes. Too much water pressure blows the pipes, or the led, or anything else.<p>KISS ,....best describes the learning process for beginners. <p> A restriction in the lines, a choking effect reduces the pressure and flow and thus saves the Led from extinction. <p>Any thing after that is the next stage of the learning process of the physics involved. <p>Pie charts are the same, they are made simple to “understand” things in a simple way, not to complicate matters. <p>Beginners don’t need complications, they need ways to visualize in simple terms. Water pipes and pie charts are a summary device, not a complete analysis. <p>Even the word “analogy” is what it is, a comparison of one event, told in “other words” as a link of understanding for the mind. Its meaning is “ similarity in some respects between things otherwise unlike” <p>Once the mind grasps, it starts to learn, and everything in science is built on the back of something else. <p>Lock up the mind with too much complexity, and it stops learning. <p>Beginners start off at the bottom and climb. The bottom of the pyramid of learning is “Always” simple, where complex sits at the top. <p>So water analogy is the perfect learning tool for many. And that’s all it is, a learning tool, a way of visualizing things to come. It is never intended to replace ohms law, merely a way of visualizing the current in another environment, a imaginary one of the mind. <p>When some one says “why is a resistor needed with a Led”, that is the bottom of the learning pyramid and that mind isnt asking to be confused or dazzled with all of the facts just yet. Baby steps like learning, come one at a time.

[jimandy]

I hear you Chris.<p>My experience with teaching electronics is with students with zero level of knowledge. The starting point is always Ohm's Law and the first hurdle to overcome is their grasp of not just the relationships defined in the law, but the need to understand the three factors of the law individually. That's where the water analogy seems to work. Carrying it much beyond that is not necessarily useful, and may be time consuming.<p>However, using the analogy in reverse can sometimes be useful. For example, I worry about my old water heater blowing out and flooding the basement while I'm out of the house. In thinking of a possible solution (albeit not a practical one) I hit on the analogy of a GFI implemented in plumbing. If the water flow into the heater exceeds the outflow from the normal "output" side of the heater then something is wrong and the system should disconnect from the source.

[Robert Reed]

Way,way back when I had no concept at all of electricity, the water analogy was used to open the doors to this subject and it made quite a convincing picture in my mind. Since then a lot of learnig and experience has gone behind the belt. But even today I can look at most analog cicuits and immediately see the water flow going through the circuits ( only nowadays its electron flow with a lot more sophistication). But as crude as this is, it has enabled me to diagnose most analog circuits quickly and effortlessly. My point being--don't knock the water flow.

[Chris Smith]

Jimandy<p>A friend sent me an article on the heater / water pipe leaking situation. [basment flooding and water damage. [esp in wall damage]<p> Wireless and hard wired solutions have been commercialized to prevent this exact situation. <p>I'll see if I can find it for you.<p>http://www.watercop.com/<p>[ October 13, 2005: Message edited by: Chris Smith ]</p>