Electricity for Beginners c.1992 Chris Greacen The last Electricity for Beginners article in Home Power #31 used a plumbing analogy to explain how current flows in parallel and series circuits. We looked at Kirchhoff's Laws which tell us how to predict how much current will flow in different legs of a circuit. Now let's look at the fun part the pieces and parts that you can fit together to build circuits. Playing with these electronic parts is to play with one of the pleasant successes of capitalism. These little pieces are cheap, and you can freely build whatever your mind can dream up. The only rules you need to follow are the rules of electrical physics. So, my revolutionary comrades, lets get a closer look at two of these proletariat Legotm blocks: the resistor and diode, and how we might put them together to build some circuits. Some philosophy: an understanding of electronics is built up from lots of little understandings. Each of these components is explained starting with a plumbing model (complements of our fictitious inventor, Dr. Klge). Stop there if you want. This level of understanding will give you enough to often interpret what's going on in an already designed circuit. After the plumbing analogy I'll discuss some caveats usually limitations to the device. An understanding of these restrictions is necessary for designing circuits that work or work reliably. Circuits are holistic systems it is important to know who affects whom, how, when, and why. Resistors Resistors restrict the flow of electrical current. In the last issue we looked at a plumbing analogy for resistors: a section of narrow pipe or a pipe filled with gravel which restricts water flow, resulting in a loss of pressure or "head". A wide pipe, or a pipe with a small amount of coarse gravel is like a resistor with lower resistance. A narrow pipe, or a pipe with lots of fine gravel corresponds to a resistor with high resistance. Figure 1 INSERT GRAVELPIPE,RESISTOR The greater the flow in the pipe, the greater the pressure loss from one side to the other. Mathematically, V = IR, the voltage drop (V, measured in volts) is equal to the current (I measured in amperes) times the resistance (R, measured in ohms). This is Ohm's law, formulated by the German Georg Simon Ohm in 1827. It's not exactly a law. Ohm's law works very well for resistors and wires, but for other things (like diodes) it's not true at all. An ampere of current flowing through a one ohm resistor has a voltage drop of one volt. Voltage is named in honor of the Italian physicist Allessandro Giuseppe Antonio Anastasio Volta (1745-1827). The "I" for current comes from the French intensit. The Greek omega () is the symbol for ohms. Never thought basic electricity was so international, did you? Joule's Heating Law The voltage loss in a resistor causes heat. How much heat? The answer is given by Joule's law (the last law you'll have to look at in this article): P = IV. P is for power, measured in watts. For any component or appliance using dc electricity, Joule's law is always true. For ac electricity, Joule's law is always true if I and V are measured at the same instant. One ampere of current at one volt will make one watt of heat. A car head light draws about 10 Amperes at 12 Volts, so it uses 10 x 12 = 120 Watts. All electronic components make some heat. This Macintosh computer, from an energy perspective, is a glorified 40 Watt heater. Unfortunately, nearly all electronic components have degraded performance when they are hot, and if they get too hot, they die. Use Joule's law to figure out the heat a component will produce. Multiply the voltage drop across the component by the current through the component. Design your circuits so that they are comfortably below the maximum wattage ratings. Sometimes this is no problem, but often it determines what is practical and what is not. Joule's law is often called the "I squared R law" since for resistors (and wire) you can substitute V=IR (Ohm's law) into P = IV to get P = I(IV) = I2R. For more on wires and I2R losses, see "What's an Inverter" by Elliot Josephson in this issue. Diodes On to more interesting components! Diodes are one way valves for electricity. Check out Dr. Klge's plumbing diode, in Figure 2 below. If the voltage is higher on the anode (the side of the electrical symbol with arrow) than the cathode (the side with the vertical line), the diode lets the current flow through (Figure 2.a). In this case the diode is said to be "forward biased". If the voltage is higher on the cathode than the anode then the "valve shuts" current cannot flow (Figure 2.b). In this case we say the diode is "reverse biased". If you're curious what substances allow current to flow one way and not the other, see "How Photovoltaic Cells Work", Home Power #23. Figure 2a: Forward biased diode anode more positive than the cathode. In the plumbing analogy, current pushes the ball valve open INSERT KlugeDiodeFBPICT Figure 2b: Reversed biased diode. In the plumbing analogy, current pushes the ball valve shut. INSERT KlugeDiodeRBPICT Figure 2c: diode symbol. The arrow always points in the direction of forward biased current flow. INSERT Diode1N4001PICT Figure 2d. A typical diode, the 1N4001. Semiconductors all carry inane numbers which serve to mystify the art of electronics and provide job security for insecure electronic engineers. Where would you use a diode? The current in a typical American home is alternating current (ac) (figure 3a). It sloshes back and forth sixty times a second. Computers and radios and many other consumer electronics need current that flows in only one direction, called direct current (dc). Diodes are used to "rectify" the ac current to a rough dc current. Capacitors, which I'll cover in a future article, are used to smooth this waveform. Figure 3a: American household current. INSERT SinePICT Figure 3b: Half wave rectifier INSERT HalfWavePICT Figure 3c: Full wave rectifier INSERT FullWavePICT A single diode hooked up to an ac voltage lets only positive voltages through (figure 3b). Four diodes hooked together in a "full wave rectifier" let the positive voltages through, and bring the negative ac voltages positive (figure 3c). Pretty tricky, huh? Follow the circuit yourself to see how it works. Imagine a positive voltage on the top ac wire (A), and decide which diodes will conduct, then imagine the positive voltage on the bottom ac wire (B) and look at which diodes will conduct. One very important thing about diodes is they don't obey Ohm's law (V=IR). The amount of current a resistor will conduct is proportional to the voltage applied. In other words, if you double the voltage across a resistor, you'll double the current going through it. This is not the case with diodes. One never speaks of the "resistance" of a diode. Your average diode conducts very little current until the (forward biased) voltage reaches about 0.5 Volts. Then, roughly speaking, the diode "turns on", and will conduct all the current that is applied. Again, for a resistor, V = IR. For a forward biased diode, it's a good approximation to say that V 0.5 Volts, no matter how much current goes through it! Of course there's an upper limit on this current (one ampere for the 1N4001). If you exceed the limit you are penalized the cost of a new diode (about five cents for the 1N4001). How do you limit the current going through a diode? A common method is to put a resistor in series with it. Also, diodes can only hold back a certain voltage. Above this peak inverse voltage (PIV) a reverse biased diode "breaks down" and starts conducting. The PIV for the 1N4001 above is 50 volts, but they're available with PIVs up to 1000 volts. Usually diodes are operated at reverse bias voltages much less than their PIVs. LEDs One particularly neat diode is the Light Emitting Diode or LED. When current flows in the diode (typically only 5 milliAmps to 20 milliAmps is necessary) they light up. They come in red, yellow, green, and (for a high price) blue. You've probably seen them in digital clocks with lit up numbers. LEDs have a voltage drop form 1.8 to 2.5 volts, depending on color. They're one of the most efficient light producers around. But, because of their monochromatic color, they don't make great house lighting. You can buy or make a set of them as low-voltage Christmas lights! Figure 4 & 5 INSERSTLED+SymPICT NSERT LEDXmasLightsPICT Solar Cells Photovoltaic solar cells (PVs) are big, flat diodes. There's still a 0.5 Volt drop across each cell, but sunlight does the work of pushing electrons up this 0.5 Volt drop to generate electricity. For an explanation of how they work, see "How Photovoltaic Cells Work", Home Power #23 Zener Diodes I said above that diodes are usually operated far away from their peak inverse voltage. An exception is the zener diode. Zeners are designed to break down under a reverse bias voltage. You can get them with breakdown (or "zener") voltages anywhere from 3.3 to 75 volts. They work like pressure release valves, "letting off steam" when the voltage gets too high. Dr. Klge has assembled a plumbing zener. Let's see how it works. In figure 4a, the plumbing zener is forward biased. Pressure is lower on the bottom than the top, opening the right hand side ball valve. Current flows just like in a forward biased diode. This is no surprise a zener is a diode, and forward biased diodes conduct. Figure 6b: Forward biased zener diode current flows INSERTZenerFBPICT For the next two figures, though, the plumbing zener is reverse biased. This is the way we normally use it. The voltage (think of it as pressure) is higher on the cathode than on the anode. In Figure 6b the voltage is not high enough to push the left ball open against the force of the spring. No current flows. In Figure 6c the zener "breaks down". The pressure is high enough to open the spring, and current flows. Figure6b: Reversed biased zener diode with voltage below the zener voltage. No current flows. INSERT ZenerRBSmallVPICT Figure 6c: Reversed biased zener diode with the voltage above the zener voltage. The zener breaks down current flows INSERT ZenerRBHighVPICT When a zener breaks down, it makes heat. Remember Joule's law: P = IV. Here V is the zener voltage. I is the current conducted. You have to make sure that the zener won't try to make more heat than it can dissipate. For example, a 1 Watt, 12 Volt zener diode can't take more than 1/12 Ampere of current since 1/12 Amps x 12 Volts = 1 Watt. For a very small PV (1 Watt) module, you might consider an appropriate 15 Volt zener as a single component voltage regulator for charging a small battery. If a battery's voltage under charge reaches 15 Volts the zener shunts the current from the positive wire to the negative (or ground) wire. Warning: try this on a larger than a 1 watt panel and you'll blow up the zener. You can get larger zeners, but they're hard to find. Besides for bigger panels, there's better ways using transistors or integrated circuits; see the Home Brew in this issue. INSERT Figure7 ZenerRegPICT Advertisement: Transistors Home Power #33 Resistors and diodes are interesting in their own right, but if you need to, consider this a prelude to next issue's look at transistors. Transistors are the basic building block of any of today's circuits which either amplify something, or have to make some decision.