# Electronics for dummies - McComb G.

ISBN: 0-7645-7660-7

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2 volts, you calculate the current in this way:

Current = ^ = 2 !vgts = 2 amps

You probably should check the power that you plan to run through the resistor to ensure that the resistor doesn’t burn up like Atlanta in Gone with the Wind. Calculate the power the resistor will draw by using another form of Ohm’s Law:

Power = R x I2= 1 Q x (2 amps)2 = 4 watts

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Chapter 7: Understanding the Basics of Electronics Circuits 149

Using this equation you know roughly how much power the resistor will draw based on your estimate of the amount of current in the circuit. Try to stay 25% below the power rating of the resistor or it could get REALLY hot.

In most cases, a 10-watt resistor can withstand the demands of a simple electronics project. If you’re burning out 10-watt resistors right and left, you’ve moved beyond electronics hobbyist to master electrician, and you need to buy a much more advanced electronics book.

What a Team: Capacitors and Resistors

Batman and Robin. Butch Cassidy and the Sundance Kid. Capacitors and resistors . . . Huh? It’s true: Capacitors and resistors often team up in an electronic circuit. In fact, a capacitor and resistor arranged in a circuit make up one of the basic building blocks of electronic circuits, such as the one shown in Figure 7-8.

+V

R1

Figure 7-8:

A circuit that joins together a capacitor and a resistor.

C1

-----V OUT

TO REST OF CIRCUIT

So why do these two make such a great team? That’s what this section is all about.

How the dynamic duo of resistors and capacitors works

A capacitor stores electrons, and a resistor controls the flow of electrons. Put these two together, and you can control how fast electrons fill (or charge) a capacitor and how fast those electrons empty out (or discharge) from a capacitor.

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150 Part III: Putting It On Paper

The larger the value of a resistor, the less current flows through it for a given voltage, which means it takes more time to fill a capacitor. Likewise, larger capacitors require more electrons to fill them up, which means they take a longer time to charge. By picking the combination of capacitors and resistors, you can determine your project’s charge or discharge time.

Turning things on and off

It turns out that the voltage out (Vout) depends on how full the capacitor in your circuit is. The closer to full, the higher Vout. The closer to empty, the lower Vout. Because components use different levels of Vout, you can pick values of resistance and capacitance to turn circuits on and off at a certain frequency or after a certain amount of time.

What if you want a capacitor to charge in 30 seconds? You have a 15-microfarad capacitor handy in your parts bin (well, who doesn’t?); using a 2-megohm resistor sets the time that it takes the capacitor to get to two-thirds of its capacity.

Filling the capacitor to two-thirds of its capacity often gives a high enough Vout to turn on the next component in the circuit. If it doesn’t, try a smaller resistor so that the capacitor fills up faster. You can generally do things that simply; take a capacitor that you have handy and calculate how many ohms you need to get close to the desired seconds of delay.

You can calculate the time to fill a capacitor to two-thirds of its capacity using something called an RC time constant. Simply multiply the values of the resistor, in ohms, by the capacitor, in farads, and you get the time it takes to fill the capacitor up to two-thirds of its capacity. (In Chapter 1, we discuss how to change 15 microfarads to 0.000015 farads, a procedure we follow in the equation below).

RC time constant = R x C = 2,000,000 ohms x 0.000015 farads = 30 seconds

Giving voltage fluctuations the boot with capacitors

You can use the ability of capacitors to gather and release electrons to smooth out voltage fluctuations. A given voltage level across a capacitor produces a certain number of stored electrons. When the voltage starts to rise, the capacitor stores more electrons, which dampens

any rise in voltage. When the voltage drops, the capacitor releases some of its trapped electrons, which dampens the drop in voltage. Power supplies that convert AC to DC often use capacitors to smooth out fluctuations in voltage.

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Chapter 7: Understanding the Basics of Electronics Circuits

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If you want to fine-tune the delay, use a resistor with a slightly smaller value than you need and add a potentiometer (a variable resistor that allows for continual adjustment of resistance from virtually no ohms to some maximum value) in series with the resistor. Because the total resistance is the sum of the value of the resistor and the potentiometer, you can increase or decrease the resistance by adjusting the potentiometer. Just tweak the potentiometer until you get the delay you want. Note that we cover potentiometers in more detail in Chapter 4.

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