Posts tagged electricity

Solar Power Class: Semiconductors and the P/N Junction

Holiday’s over, class, and it’s time to get back to our solar power lessons.  Today, let’s talk about semiconductors, and a slew of other things along the way!

What is a semi-conductor?  Pretty much what it sounds like.  If a substance which conducts electricity (think copper wires) is a conductor, and one that does not (rubber) is called an insulator, then a semi-conductor is right in between. Basically, atoms with only 1 or 2 electrons in their valence (outermost) orbit conduct electricity more easily than those which have a full or almost full outer valence (7 or 8).  The first are conductors, the latter are insulators.  So semiconductors are those with 3, 4, or 5 valence electrons, which means that they are neither particularly inclined nor disinclined toward conductance.

By taking silicon (which has 4 valence electrons) and doping it, as it is called, with an element which has either 3 or 5 electrons, materials with a net positive or negative charge can be created. This has to do with silicon’s natural crystal forming tendencies.  If the material used had 3 valence electrons, then the overall material has a positive charge and it is called a P-type material.  If it had 5, then the material holds a negative charge, and is called an N-type material.

All interesting enough, but things really get going when you place the two together.  The P/N Junction is the innovation that pretty much ushered in the electronic age.  The particular application of this material to solar power was actually one of the first experiments done with it, by Bell Labs back in the 50s.  When a P-type material and an N-type material are placed back to back, and a circuit is completed wiring the two pieces together, the sun excites electrons in the negatively charged layer (remember, this layer has extra electrons) and as the voltage (or pressure) builds to a point at which they overcome barrier resistance, they begin to jump to the positively charged layer where holes in the crystal structure await them.  From there, the circuit exits the P-type material and travels back to the N-type material via the wire circuit to restore the electron balance in each material.  If we hook up a battery amongst the wire circuitry, it is those bounced electrons which we store and call energy.

Okay, time for a break.  Join me back here tomorrow for the next installment of our solar power class.  In the meantime, for your reading pleasure, check out The Light Revolution, a great book about the necessary influence of the sun in our health and buildings.  Full review soon!

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Photovoltaic Class

To all my fellow travelers on the road to solar living! If you’re following along with the solar power class, be sure to note that the class now has its own page on the site (it’s at the top of the page on the right-hand side), where you can find all the lessons in one easy to remember place. If you are following along, stop by and say hello to your other classmates by commenting on that page, so we can facilitate discussion and learning for all involved. Also, if you find great resources that everyone will want to know about, post them there!

For my fellow Los Angeles classmates who are checking in, though all the information now posted is publicly available to all who visit the site, there will eventually some information and resources posted which are specific to our class that will be available only via password. By commenting and saying hello, I will know your email address to send you a password when such information is posted.

See you there!

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Solar Power Class: Kirchoff’s Laws

Whew! It’s been a busy couple of weeks in my photo-voltaic class. I was starting to fear that I’d have to take a math class to keep up with all the formulas! When I posted the first week’s lesson, I realized later that I’d given out misinformation, which is the danger of posting about something you don’t yet understand! So, from this point out, I’ll just post the lessons after I’ve been tested in the contents, that way you’re always getting the information someone has TOLD me I understand. Since we had a test this past weekend, here’s a new dose of mathematical fun!

This week: Kirchoff’s Laws. Last time, I discussed Ohm’s Law of DC power, which interrelated voltage, current, amperage, and power, and provided several formulas you can use to figure out any of the above for a circuit. If you’d like to review, check out the original post here. Now, let me repeat a few pertinent facts: a series circuit is when you basically hook everything up in a big loop, positive end to negative end in a chain. See the diagram below:

A parallel circuit is one in which the positive and negative ends are “shunted” together (parallel circuits are sometimes called shunts) creating a ladder effect. Again, see the diagram below:

Series circuits are called voltage divider circuits, because though a common current flows across the wire, at each stop along the way, voltage is dropped. These are two important concepts: 1. current is common. 2. voltage is divided along the circuit. Parallel circuits are the opposite. Though a common voltage flows through all the wires, the current is divided between the different potential paths. Therefore, in a parallel circuit, 1. voltage is common, and 2. current is divided along the circuit. Series circuits are called “voltage dividers” and parallel circuits are called “current dividers”. VERY IMPORTANT is you want to know how to manipulate these circuits later on.

There are even two formulae which will help you to calculate a voltage or current at any particular point along a circuit. Say you have three resistors along your circuit. In a series circuit, a voltage divider, if you want to know the voltage of resistor “b”, you would use the voltage divider formula: Erb = Et (Rb/Rt), where t represents total and Er is the voltage drop. Here’s an example:

In a series circuit with a total resistance of 100 Ohms, and a voltage of 120V, resistor b has a resistance of 25 Ohms. The total voltage drop across resistor b would be:

Erb = 120 v ( 25 Ohms / 100 Ohms ) = 30 V

Now, if you aren’t sure what the resistance of a particular resistor on the circuit is, then Kirchoff’s Law of Voltage (for series circuits only!) comes into play. His law states that the total voltage minus the voltage of each resistor, etc on the circuit will always equal zero. In other words, the Total Voltage equals the sums of all the voltage drops along the path. Here’s the official equation: Et – E1 – E2 – … – En = 0, where the circuit has n resistors. So if you know that one resistor has a voltage drop of 25 v and the third has a voltage drop of 50 v, and the total voltage is 130 v, then 130 – 25 – E2 – 50 = 0, and E2 = 55 v. Got it?

Now, on to parallel circuits, ones you’ll see a lot of in battery configurations. Because parallel circuits are current dividers, they need a separate formula for figuring out current drops around the circuit. This is called the Current Divider Formula (using Current at Resistor b): Ib = It ( Rt / Rb ). As with the voltages of a series circuit, if you need to know the current drop at a particular point, Kirchoff had a law for that, too. It’s called Kirchoff’s Current Law (for parallel circuits), and it states that the total current minus the current drops along the way, equals zero. So It – I1 – I2 – … – In = 0, where the circuit has n resistors.

Now, this is a LOT of information to absorb, especially in practice, so let me stop here for now, and we’ll pick up here tomorrow with the rest of the lesson. It seems like way too formulas to ever be useful, but once you get to solving practical equations with them, it’s not too bad. But let’s save that for the next lesson, sleep on it, and I’ll see you in class tomorrow~

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