Optical Indicators

There are thousands of types of indicators, which vary in purpose, size, shape, and tech-nology. Optical indicators can be loosely classified into two groups. One group is designed to serve primarily as a light source or single-point indicator. The second group provides alphanumeric information and/or graphic information. We shall limit our discussion to the light-source group, and we will further divide this group into incandescent, neon, and solid state.

Incandescent Light Sources

Figure 2-45 shows the construction and schematic symbol for an incandescent lamp. The overall construction consists of a tungsten filament supported within an evacuated glass bulb. This has not changed substantially since the time of Thomas Edison. When current flows through the tungsten filament, it becomes hot and glows bright white.

Incandescent lamps are used as point indicators (e.g., a power-on indicator) and as light sources. Most of the lightbulbs used in your home are incandescent lamps. The range of voltage requirements extends from less than 1 V to more than 120 V. Current require-ments range from a few milliamperes to many amperes.

Incandescent lamps are very susceptible to mechanical vibrations or shock. It should be noted that the rated lifetimes are specified for a shock-free environment. Actual lifetimes

Exercise Problems 2.9 Exercise Problems 2.9

Tungsten filament

Schematic symbol

Electrical contacts Evacuated glass bulb

Filament support posts

Figure 2-45. Construction and schematic symbol for an incandescent lamp.

The incandescent lamp is useful as an indicator (point source) or as a source of illumination (diffused source).

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The lifetime of an incan-descent lamp is often shortened by high-voltage transients, inrush currents, and mechani-cal shock.

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may be much shorter. Incandescent lamps can also be damaged by transient voltages or currents. Transient voltages frequently occur on commercial power lines. Additionally, every time the lamp is turned on it is subjected to a current transient in the form of an inrush current. This occurs because the resistance of a cold tungsten filament is much less than its hot resistance. This allows the initial current to be much higher than the normal operating current. You may have noticed that the lightbulbs in your home burn out most frequently when they are first turned on. Figure 2-46 shows several representative incandescent lamps.

Neon Indicators

Figure 2-47 shows the construction of a simple neon indicator and a circuit diagram showing its connection. The indicator consists of a neon-filled glass bulb with two sealed electrodes passing through the glass. If sufficient voltage (75 V and up) is connected between the two electrodes, then the neon gas is ionized. You will recall that ions can participate in current flow. The voltage required for ionization is called the ionization voltage, or firing voltage, and varies between different types of bulbs.

Figure 2-46. Some typical incandescent lamps.

Neon lamps are primarily used as indicators.

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Neon bulbs have ioniza-tion voltages in excess of 75 V. Their deionization, or turnoff, voltage is less than the ionization volt-age.

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Neon-filled

glass bulb Electrodes

Wire leads

Current-limiting resistor (ballast)

Glass support and seal

V_A

Figure 2-47. The construction and application of a neon indicator.

Prior to firing, the resistance of the neon gas is quite high. Once the gas has ionized, the resistance between the two electrodes drops drastically. This can allow damaging currents (that will actually melt the electrodes) to flow. To prevent electrode damage, all neon lamps must be operated with a resistance (sometimes called a ballast) connected in line with the lamp. This is shown in Figure 2-47. Normal operating currents vary from hundreds of microamperes to tens of milliamperes.

It takes a higher voltage to ionize the neon gas than it does to maintain the ionized state.

Therefore, once the lamp has fired, the applied voltage must be reduced to a lower voltage to extinguish the lamp. The ionization voltage varies with the amount of external energy passing through the glass; older neons could not be ionized if they were cold and dark.

Modern lamps have a small amount of radioactive material to provide constant radiation and reduce this effect. Figure 2-48 shows two types of neon lamps.

Solid-State Light Sources

The solid-state light sources to be considered here are called light-emitting diodes (LEDs).

Figure 2-49 shows a cutaway view of an LED and the method of connection. Two types of semiconductor materials are formed into a small chip that is about 0.01 inch on a side. The junction of the two materials emits light when subjected to the proper voltage conditions.

Tiny (0.001-inch diameter) gold wire and conductive epoxy are used to connect the semi-conductor to the leads. An epoxy housing is formed around the leads and semisemi-conductor to serve as the body of the LED. The housing is transparent or translucent so that light can pass through it. LEDs are available in red, green, yellow, and blue. Some manufacturers even make LED housings with more than one color LED in the same package.

LEDs typically require from 1 to 30 mA at a potential of slightly less than 2 V. The LED is a polarized component. That is, it must be connected in a particular direction. More

Neon bulbs require cur-rent-limiting resistors to protect them from dam-age by excess current.

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Figure 2-48.

Some typical neon indicators.

Gold wire

Anode (longer) lead

Anode

Cathode (shorter) lead

Cathode Silicon

chip Plastic housing and lens

Figure 2-49. LED construction and symbol.

LEDs generally operate with less than 2 V across the semiconductor material.

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specifically, the negative potential must be connected to a terminal called the cathode, and the positive potential must be connected to the anode terminal. The cathode lead is generally shorter than the anode lead and/or the body of the LED has one flat side near the cathode lead so a technician can identify the leads. LEDs are easily damaged by excessive current flow so they must be operated with an in-line resistor (or some equiva-lent method) to limit the maximum current flow. Figure 2-50 shows several representa-tive LEDs.

1. What class of light source do the standard screw-in lightbulbs used in a home represent?

2. Which class or classes of light sources require an in-line resistance to limit current flow?

3. Which type of indicator must be connected in a particular direction?

4. If the operating voltage on an incandescent lamp were higher than its rated value, what would happen to its life expectancy?