#### Aux

##### Well-Known Member

AUTHOR'S NOTE: This is focused on direct current (DC) applications. Maybe I'll do another about AC later but for portabilizing you'll mostly being working with DC. That's the kind that comes from batteries, not from the wall (except after it goes through an inverter, usually in a "wall wart" or power brick).

*Let's go!*You may have heard the term "electricity", if you're not a time-traveller visiting from the Middle Ages. If you are, you may want to check out a new invention we have called "bathing". In fact, many people who may come across this guide may want to look into this wonderful activity and practice it daily. Anyway. "Electricity" really means "electric current at non-zero voltage". Those are big words, don't worry, we'll explain them.

Electric current is the measure of the movement of electric charge by electrons through a material. When that "material" is wires and electrical components connected by other conductive material (like solder, or in a breadboard) we call this "material" a circuit. We measure current in amperes (represented by A), often shortened to amps (after Andre-Marie Ampere, the guy who discovered it).

**REVIEW POINT:**

*Current is electrons moving electrical charge through something, and it's measured in amps (A). Something that we set up and we want current to move through is called an electrical circuit.*

Current will move through the circuit when certain conditions are met, but generally, everything just kind of sits there and the current is 0. Think of it like pipes full of water. If the pipes are just there, and you have some water sitting in them, nothing's going to move anywhere and the current will be 0. However, if we add a power source to "push" the electrons as a pump would push water, current starts moving through the circuit. What makes this slightly different than our pipe analogy is that unless power and ground are connected, no current will flow. Our electrical "pump" can't just push electric charge off into nothingness- it needs to have a "ground" to collect it.

This is the biggest problem I see with electronics noobs, they don't understand that current only flows from power to ground, and disconnecting the ground will have the same effect as disconnecting the power- in that nothing will be powered because there will be no electrical current.

**REVIEW POINT:**

*Without a power source and a ground, current through a circuit is negligible.*

So you may be wondering, how is current generated? Why does it happen? What makes something a power source?

If you know anything about nature, it's that it likes equilibrium. When you put hot and cold things together, heat energy transfers from the hot thing to the cold thing to balance out. For instance, when you put an ice cube in hot tea. The ice cube warms up and melts into water, and the tea gets cooler. Nature likes things balanced out.

One property objects have is electrical charge. It comes from electrons doing their electrony things, I won't get into the chemistry and particle physics of it. Just know that things can acquire positive or negative electrical charge by having too few or too many electrons respectively, and Nature wants to balance that out to equilibrium just like hot and cold. When you rub your feet on the carpet when it's dry you acquire a negative electical charge, and when you touch a metal doorknob (or your little sister, or anything with a higher charge than you) a small electric transfer occurs, just for a moment, to balance that charge by transferring some. This kind of electricity we call "static electricity", and it's not really electric current because it's a discharge (a transfer, not a circular flow in a closed system like a circuit), but it's a good example to get the idea across.

**CONFUSION ALERT: READ THE FOLLOWING CAREFULLY, BECAUSE IT'S SUPER IMPORTANT BUT HAS LIKE SIX DOUBLE NEGATIVES.**

Electrons have a negative charge. So having a negative charge means you have too many electrons. Positive charge? Not enough. So electrons move from negative to positive (too many -> too few). HOWEVER, CURRENT MOVES IN THE OPPOSITE DIRECTION OF ELECTRON FLOW. This means current flows from the positive charge to the negative charge. For the purposes of understanding the rest of this, you really only need to remember current goes from positive to negative.

**REVIEW POINT:**

*Electrical charge is a property of an object, just like temperature. When charges differ enough, an electric current will form to balance those charges from the positively-charged area to the negatively-charged area in a circuit.*

Now I'm going to take a minute here to talk about resistance, because you're probably wondering why the tiniest electrical differences don't balance out all the time and lightning isn't common all around your face. Resistance is another property materials can have. Quite simply, it means it's harder for electrical current to move through the material. We measure this in Ohms, represented by Ω and named after the German physicist Georg Simon Ohm. The opposite is conductance- a material with low resistance has high conductance, and vice versa. You see, electricity is lazy, and it wants to balance out in the easiest way possible. In order for electric charge to move, the difference in the electrical changes must create more energy than the resistance of the materials between them. (If you've studied kinematics in Physics at all, resistance is the same thing for current as friction is for movement.) Air has a high resistance, which is why you need to be very close to something before you can shock it with your built-up static. Rubber has a very high resistance, which is why we call it an "insulator" and use it as a barrier between differing charges that might otherwise start up a current- like rubber coatings on wire, so you can grab the wire without affecting the charge transfer with your body's own charges.

**REVIEW POINT:**

*Resistance (Ohms, Ω) is a measure of how much a material resists the flow of current. The difference between two charged areas must exceed the resistance of the material between them for current to flow.*

While we're talking about the difference between two charged areas, did you know that has a name? It's called voltage, and it's measured in Volts, represented by V, and named after Alessandro Volta- the Italian physicist who invented the battery, no big deal or anything. Voltage is NOT current. Amps are NOT Volts. Amperage is a measure of the amount of current moving between two points. Voltage is a measure of the differences between the charges of two points. This is another common confusion in electronics.

**REVIEW POINT:**

*Voltage (V) is a measure of the difference in charges between two points.*

You will often hear people say something like "2 amps at 12 volts". This means the difference in charge from the positive to the negative areas in the circuit is 12 volts, and the amount of current flowing is 2 amps. You will also hear people say a circuit "draws" or "pulls" a certain number of amps, that is amount of current a circuit lets through at a given time. Another useful unit is Watts (W), which measure "power". A watt is equal to the power used by one amp at one volt. Power and energy are often confused- energy is power over a certain amount of time. For instance, were we to run a circuit that takes 10 watts for 10 whole hours, we would have used 100 watt-hours of energy (10*10). Joules (J) and calories (cal) are other units of energy you might have heard of, and they have direct conversions to watt-hours (1 Wh = 3.6 J). So a watt is a measure of power- not energy. Power*Time=Energy.

**REVIEW POINT:**

*Watts (W) is a measure of power, where W=A*V. Watt-hour (Wh) is a measure of energy, where W*h=Wh.*

With all this talk of power and energy, let's touch on batteries for a moment. Batteries are actually little capsules with a chemical reaction going on inside them. The part marked (+) is slightly positively charged, and the part marked (-) is slightly negatively charged. They are called POWER and GROUND. When you connect the power to the ground, current begins to flow through the thing you connected them with, and the chemical reaction happens faster, keeping the (+) positively charged and the (-) negatively charged to keep the current flowing. In disposable batteries (like Alkaline) when this chemical reaction runs out, it's all done and you have to throw them away and buy new ones with fresh chemicals in them. In the old ones there's no more good chemicals to react and make energy to keep the positive postitive and the negative negative. With rechargable batteries like Li-Ion, LiPos, and NiCads, this chemical reaction is somewhat reversible. By feeding current through the battery backwards, it undoes the chemical reaction that made current in the first place and you're left with a fresh, new, ready-to-plug-in battery full of like-new chemicals. However this doesn't always happen efficiently and over time, you'll see your battery holds (and therefore provides) less and less energy.

**REVIEW POINT:**

*Batteries use chemical reactions to make a positive and a negative area. By connecting these two areas to a circuit, we can make current flow through the circuit. In rechargable batteries this reaction is somewhat reversible, so we can "recharge" it with more current by undoing the chemical reaction.*

Batteries are rated funny ways. A battery's capacity is its energy, which we've said is measured in watt-hours (Wh). So a useful thing to know is, if I'm running this many watts, how many hours will I get? People usually rate batteries in amp-hours (Ah), however... and as we've said, we need a voltage (V) to determine watts. What gives?

Well, it turns out a battery of a certain type will always provide about the same voltage- that's because the different charge the type of material creates between its positive and negative ends will always be about the same. For instance, Lithium Ion (Li-Ion) will always have 3.7V between their positive and negative. However, by putting two of these in series, we add the voltage- so we get 7.4 V. We'll cover that later. For now, we'll just say our circuit uses 2 amps at 7.4 volts.

Say we're seeing a Li-Ion that's rated at 5000 mAh (5000 milliamp-hours, aka 5 amp-hours) and we want to know how long that'll last.

First we want to find the watt-hour rating of this battery. So we multiply the amp-hours by the voltage (because A * V = W) and we get 18.5 Wh.

Our circuit uses 2 A * 7.4 V which is 14.8 W. So 18.5 Wh / 14.8 W leaves us with an optimal value of 1.24 hours of battery life.

**REVIEW POINT:**

*Amp hours are not watt hours, though battery capacity can be rated in both. Use A*V=W to convert to similar units before making comparisons or calculations.*

When we're talking about batteries and such, another electrical component that comes up is voltage regulators. Those do exactly what they say- regulate voltage. They usually have three pins. You can put in whatever voltage you want in the Voltage In (Vin) pin, and the regulator will make sure that on the Voltage Out (Vout) pin, only whatever voltage it's set up for will come out (as long as the Vin is greater than the Vout). The last pin is ground, and should be electrically connected to the (-) on your power source.

**REVIEW POINT:**Voltage

*Regulators take in a higher voltage on Vin, and spit out the lower voltage they're set up for on Vout.*

Another thing that will come up is resistors. Now, as you might have guessed before, a resistor is an electrical component with a pre-defined resistance. There is a very important equation called Ohm's law that governs resistors:

V = I*R

I means "current", and is just the mathematical way of saying A (for amps). This means the voltage (recall that voltage is just a difference of charge) between one end of a resistor and the other is equal to the current flowing through that resistor times that resistor's resistance, in Ohms. We call this the "voltage drop across the resistor" and it is super useful because if we know the current we have and the voltage we start with on one side, we can subtract the voltage drop and we'll know what the voltage is on the other side. More importantly, if we know what voltage we have and what voltage we need, we can calculate what value resistor we need. For example, let's say we have 5V at 10 mA (.01 A) and we need to drop that to 1.4 V so we won't blow up a red LED.

5 - 1.4 = voltage drop required of 3.6 V

V = I*R

3.6 = .01*R

360 = R

So a 360Ω resistor would do the trick. Current would flow from the battery (or other 5V source) at 5V and hit the resisitor. The power side of the resistor will be 5V, and the other side would be 1.4V. Here we would connect our LED's longer (positive) lead, then we'd connect our shorter negative lead to ground. Remember, for current to flow, it must always go back to the ground of the power source.

**REVIEW POINT:**

*V=I*R*

The last thing I want to cover here is parallel vs. series. Think of a television series- it's just one episode after the other (even if it's a show like Survivor, and you wish they would just stop). Wiring two or more things in series is the same, you connect them so they are one right after the other, when you're tracing the current flow. When you put things in parallel, however, think of parallel lines. They are next to each other, not one before the other. When you connect electrical components in parallel, you connect both their positive ends to the same point then both their negative ends to the same point. Putting two or more things in parallel or series affects their behavior.

Resistors in series: Their resistance adds, so to get the total resistance across all, add them all together.

Resistors in parallel: The current adds, which does funny things to V=I*R. Use the equation 1/Rtotal = 1/R1 + 1/R2 + ... etc

Batteries in series: Their voltage adds, so where one may have been 3.7, two would be 7.4 and three 11.1.

Batteries in parallel: Their current adds, so where it might have specified maximum 2 amps draw, with two you can get 4 or with three, 6.

**REVIEW POINT:**

*Parallel adds the voltage/resistance, series adds current.*

And now that your brain has exploded...