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Part number: Used if the component is standard, such as a transistor or integrated circuit, or you have a manufacturer’s custom product part.
For example, a part number may be something like 2N2222 (that’s a commonly used transistor) or 555 (a type of integrated circuit used in timing applications).
A value: Used if the component doesn’t go by a conventional part number. Component values are typically used for non-solid-state parts, such as resistors and capacitors. For example, when indicating a resistor, the value — in ohms, K ohms (thousands of ohms), or megohms (millions of ohms) — could be marked beside the resistor symbol and/or the reference ID.
To indicate the circuit’s proper function, the schematic may also include additional specifics about a component. For instance, you commonly can assume that, unless otherwise noted, all resistors are rated at X or J8 watt. When the circuit requires a different wattage — say a 1-watt or 10-watt power resistor — you may write that wattage beside the symbol in the schematic. In other cases, you may note any special considerations in the parts list, or as an addendum to the schematic.
Reference ID primer
Components such as capacitors are often identified in a schematic using a letter, such as C for capacitor, followed by the Reference ID number (such as C2). The number identifies a specific capacitor, and can be used in a parts list where you note the precise value of the capacitor to use, if that value isn't also printed beside the capacitor symbol. The following letters are among those most commonly used. Note that some components use abbreviations instead of single letters, but the intent is the same:
C - Capacitor
D - Diode
IC (or U) - Integrated circuit
L - Inductor
LED - Light-emitting diode Q - Transistor R - Resistor RLY - Relay T - Transformer XTAL - Crystal
In various headings in this chapter when you see a letter in parenthesis (as in the heading that follows) it's reminding you of the letter abbreviation for that component.
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Chapter 6: Reading a Schematic 131
Ă The schematic diagram of a capacitor reflects its internal construction: two Ă conductive plates separated by a small gap. The small gap and its contents are called the dielectric. As we discuss in Chapter 4, the dielectric can be air, liquid, or some form of insulator (such as plastic or mica).
Capacitors can be either polarized or non-polarized. Schematics show the polarity with a + (plus) symbol, though on the capacitor itself, the polarity can be marked with either a + (plus) or - (minus) symbol next to one of the leads.
Crystals and resonators (XTAL)
You use crystals and resonators to provide an accurate time base for electronics circuits. When you use these components with the appropriate oscillator circuit, the crystal or resonator generates a series of pulses, sort of like a metronome. The symbol for a crystal looks a lot like a capacitor, except that a crystal symbol has a rectangle between the end plates.
You can find many kinds of diodes out there, including rectifiers, Zeners, and light-emitting diodes (LED). Figure 6-4 shows an assortment of the most common diode types: The standard rectifying diode, a Zener, the LED, and the photodiode. You can use LEDs as indicator lights and photodiodes to detect light. The sensor for your VCR’s remote control is an example of a photodiode. And you commonly find bridge diodes in power supply circuits that convert AC voltage to DC.
Symbols for different types of diodes.
C~ Inductors are coils of wire that you often see used in radio frequency (RF) circ cuits, such as AM radios and transmitters. The symbols for various types of inductors are quite similar to each other and easy to spot; the main difference among inductors relates to what makes up the core. The most common core materials are air and iron.
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132 Part III: Putting It On Paper
Operational amplifiers (U or IC)
Operational amplifiers are a type of integrated circuit because they are actually circuits within a circuit. They combine, in a single component, all the necessary circuitry to amplify a signal. Schematics commonly use the symbol that you see here for almost any amplifier, not just an operational amplifier. The basic operational amplifier (op-amp) has two inputs (one shown with a + sign, the other with a - sign) and a single output.
You use relays to open and close a circuit while using another voltage (typically a smaller one) as a control. Relays differ from one another in the number of contacts that they contain. The symbol you see here is a double-pole, singlethrow (DPST) relay. When working with relays, be sure to keep the control voltage (shown connected to the coil) separate from the contact voltage (shown connected to the contacts of the relay) because the two voltages may differ and are not intended to be switched.