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The circuit in Figure 7-1 has the switch in the closed position. When it’s closed, it completes the circuit and allows electrons to travel from the negative battery terminal through the light bulb to the positive battery terminal. The light bulb contains a filament that heats up and emits light when the electrons pass through it.
On the other hand, when you have the switch in the open position, such as shown in Figure 7-2, there’s a break in the circuit. Because of this break in the circuit, electric current can’t flow. No current, no light.
The circuit with an open switch puts you in the dark.
A flashlight works in the same way. When you turn on the flashlight, a switch completes the circuit between the light bulb and the battery and allows electric current to flow. When you turn off the flashlight, you open the circuit, which prevents electric current from flowing.
Controlling the current with a resistor
Say you’re building a model railroad and you want to dim the light over the station platform. Just add a resistor to the circuit. Figure 7-3 shows the circuit in Figure 7-1 with a resistor added.
Adding a resistor allows you to dim the light.
In Chapter 4, we explain that resistors “resist” electric current (makes sense, huh?). Adding a resistor reduces the amount of electrons flowing through the circuit. When fewer electrons flow through the filament in the light bulb, the filament emits less light.
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144 Part III: Putting It On Paper
You can use Ohm’s Law (for a quick review of this handy rule, see Chapter 1) to calculate the amount of current flowing through this circuit before and after you add the resistor. If the resistance of the light bulb is 5 ohms and the battery applies 3 volts, then you calculate the current like this:
V 3 volts I R 5 ohms a6 amp
Here I represents the current, V stands for the voltage, and R represents the resistance.
When you add a 5-ohm resistor to the circuit, the total resistance of the circuit becomes 10 ohms, and you calculate the current as:
R 3 volts 1 “ R “ 10 ohms “ a3 amp
The resistor cuts the current running through the light bulb’s filament in half. This current cutting reduces the amount of light over your train station platform, allowing the tiny stationmaster to catch a few winks.
Parallel (or Series) Parking Your Light Bulbs
You can arrange components in series so that the same current runs through each component, or you can arrange them in parallel so that one batch of current runs through one component and another batch of current goes through another component, and so on. In the following sections, you can see just how series and parallel circuits work.
Circuits: The series
In the circuit in Figure 7-3, electrons flow from the negative battery terminal, through the light bulb and then go on to run through the resistor before reaching the positive battery terminal. You call this set-up a series circuit, meaning that the current runs through each component sequentially. You can calculate the total resistance of a series circuit simply by adding together the resistances of each component.
Figure 7-4 shows another example of a series circuit with 4 resistors.
To calculate the total resistance of this circuit, or Rt, simply add the values of all 4 resistors:
Rt = 220 Q + 33 Q + 10 Q + 330 Q = 593 Q
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Chapter 7: Understanding the Basics of Electronics Circuits 145
In a series circuit the current zips through each component, one after the other.
You can use this value of Rt with Ohm’s Law to calculate the current in the circuit. So, if +V (supply voltage) equals 9 volts:
I = ÒÃ = '593? = 0.015 amps or 15 milliamps
Why should I care about the total current in a circuit, you ask? There are two really good reasons:
^ Even the hardiest components can only handle a certain amount of current; for example, an LED would probably burn up if you ran more than 50 milliamps through it.
^ On the other hand, your power supply or batteries can only supply a given amount of current. The level of current calculated here, 15 milliamps, is no big deal. However, the next example uses over 1 amp of current, which raises the bar for your power supply or battery. Bottom line: To make things run, make sure that you have an adequate power source to supply as much current as the circuit requires for as long as you need it to run.
There is a potential problem that you may run into with series circuits: If one component fails, it stops the flow of current to every component in the circuit. So, if your spiffy new restaurant sign sports 200 light bulbs wired together in series and one burns out, every one of the light bulbs goes dark.
There’s a way to fix the problem of all components in a series circuit blacking out when one item fails. You can wire components in a parallel circuit, such as the circuit in Figure 7-5. With a parallel circuit, if you burn out a few bulbs in your restaurant sign, the rest of it stays lit. (Of course, you may be left with a glowing sign reading, “WORLD’S BEST FOO.” There are pros and cons to everything.)