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Keep polarity in mind when using diodes. The stripe on a diode marks its cathode.
housing. The metal housing includes a heat sink or a mounting stud so that you can affix the diode securely on a heat sink. A few diodes use the same packaging as transistors (which we describe in the next section).
Which way is up?
All diodes have what amounts to positive and negative terminals. The terminals go by special names: The positive terminal is called the anode, and the negative terminal is called the cathode. You can readily identify the cathode end of a diode by looking for a red or black stripe near one of the leads. Figure 4-8 shows a diode with a stripe at the cathode end. This stripe corresponds with the line in the schematic symbol for the diode. Itís important that when you follow a schematic to build a circuit you orient the diode with the line facing the specified way.
As we talk about in the section ďDiode Mania,Ē earlier in this chapter, diodes pass current going in one direction and block current going in the other. So, if you insert a diode backward in a circuit, either the circuit doesnít work at all or you damage some components. Always note the orientation of the diode when you use it in a circuit. Double-check to make sure that you have it right!
Fun, fun, fun With light-emitting diodes
If bright lights turn you on, you can appreciate the curious behavior of semiconductors: They emit light when you apply an electric current to them. This light is generally very dim and only in the infrared region of the electromagnetic spectrum. The light-emitting diode (LED), such as the light that glows yellow or green when your computer is on, is a special type of semiconductor expressly designed to emit copious amounts of light. Most LEDs are engineered to produce red, yellow, or green visible light, but some special-purpose types emit infrared, blue, and even white light.
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82 Part II: Aisle 5, Component Shack: Stocking Up
A resistor inserted in series with an LED is used to limit current to the LED.
LEDs carry the same specifications as any other diode, but they usually have a pretty low current rating. An LED has a PIV rating of about 100 to 150 volts, with a maximum current rating of under 50 milliamps. If more current passes through an LED than its maximum rating allows, the LED burns up like a marshmallow in a campfire.
LED specifications indicate both the maximum current rating, usually referred to as forward current, and the peak current. The peak current is the absolute maximum current that you can pass through the LED for a very short period of time. Here, short means short ó on the order of milliseconds. Donít confuse forward current with peak current, or you may wreck your LED.
Resistors, meet LEDs
You use a resistor, such as the one in Figure 4-9, to limit the current to the LED. You select the value of the resistor to maintain the current below the maximum current rating of the LED. The calculation is simple, and for most LEDs and 5 or 12 volt circuits, you can use common resistor values that get you in the right ballpark.
We list common resistor values in Table 4-6; the values are selected based on the ratings of most LEDs.
Table 4-6 Resistor Values Used with LEDs
Circuit Voltage Current Limiting Resistor Value
3.3 to 5 volts 330 ohms
6 to 9 volts 560 ohms
12 to 15 volts 1Kohms
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Chapter 4: Getting to Know You: The Most Common Electronic Components
You can always select a higher value resistor, which simply makes the LED glow less brightly. If you select a lower value resistor, you run the risk of burning out the LED. Because most LEDs cost just a few pennies a piece, you can experiment with resistors of different values and not break the bank. Make it a game to see how bright you can make your LED before you make it go up in smoke ó just kidding!
If you want a more accurate calculation, you need to know the forward voltage drop through the LED, in addition to the LEDís maximum current rating. Most standard brightness LEDs have a forward voltage drop of about 1.5 volts. The latest crop of ultra-bright LEDs may have forward voltage drops exceeding 3.5 volts.
The calculation for desired forward current, in egghead terms, is this:
R = (Vs- Vf) / I,
R stands for the value of the resistor, in ohms, that you want to use.
Vs represents the supply voltage. Itís measured in volts.
Vf is the forward voltage drop through the LED. This is also measured in volts
If stands for the forward current (in amps) that you want to pass through the LED. You can use the maximum current rating of the LED or something less for the forward current, but never use more!
Suppose a circuit is powered at 6 VDC and the forward voltage drop through the LED is 1.2 volts. You want a forward current of 40 mA (thatís 0.040 of an amp). Substituting these values in the calculation, you get: