Correspondence Lesson 4: Electronics & Lighting Controls

Let's begin by covering the basics of electronic devices and control technologies, then proceed to specific applications of lighting controls. Assuming you're from an electrical power background, a brief overview of electronics is pertinent to many of the lessons in this course. To many of us "power" people, the device names are intimidating: Zener diodes, field-effect transistors, PNP junctions,

Let's begin by covering the basics of electronic devices and control technologies, then proceed to specific applications of lighting controls. Assuming you're from an electrical power background, a brief overview of electronics is pertinent to many of the lessons in this course. To many of us "power" people, the device names are intimidating: Zener diodes, field-effect transistors, PNP junctions, and so on. But when you realize these are just fancy names for such things as automatic switches, a lot of the mystery fades away.

Electronic components. The first thing to remember about electronics is: The basic laws that apply to the operation of electricity (Ohm's law, Kirchoff's voltage law, Watt's law, calculations of parallel resistance) also apply to electronics. In reality, electronic work is not that different from many types of electrical work: The main differences relate to the amount of power used as well as the exotic-sounding names of many electronic components. Secondly, there are five things electronic circuits do that are not possible with normal electrical circuits:

  • Respond to very small signals and from those signals, they produce a much larger signal (e.g. how transistors amplify signals);

  • Respond much faster than electrical devices (such as relays);

  • Produce magnetic signals, such as radio waves, X-rays, or microwaves when operating at high speeds;

  • Respond to light (certain devices such as photocells); and

  • Control the direction of current flow.

With that general information as background, let's get into component details.

What is a diode? A diode, in lay terms, is a semiconductor with two electrodes passing electric current in one direction only. An ideal diode offers no impedance to current flow in one direction and an infinite impedance in the other. A typical solid-state diode has a very low forward resistance (resulting in a 0.5V to 1.5V drop) and a reverse current of a few milliamperes when blocking several hundred volts.

What causes current flow? A typical semiconductor diode has N-type and P-type materials. These are created by altering their crystal structure; in other words, some atoms within their crystal are replaced with atoms from another element.

An N-type material is created by adding atoms from an element that has more electrons in its outer shell than the crystal. This provides more free electrons. Since they have a negative charge, we call the material N-type ("N" for negative).

The same procedure is used to create P-type material, except that atoms from an element having fewer electrons in its outer shell are added to the crystal. This results in empty holes in the crystalline structure, and these represent positive charges; hence, the name P-type ("P" for positive).

When voltage is applied to a diode, the holes in the P-type material are filled with the free electrons from the N-type material; thus, the current flow from negative potential to positive potential. (In electron theory, current flows from negative to positive, whereas conventional current flow is positive to negative.)

The above is a simplified explanation; however, it provides justification for unidirectional current flow.

Which end is which? When you apply the proper polarity to a diode, it's called forward bias and results in forward current. When you apply the opposite polarity, it's called reverse bias. This results in a reverse current very close to zero.

As shown in Fig. 1 of the original article, the symbol for a diode has two parts: a straight line representing the cathode and a triangle representing the anode. As we discussed earlier, electrons flow from the cathode to the anode.

Manufacturers denote anode and cathode locations in various ways. One method has the diode symbol on the surface of the diode; another uses a band around the diode indicating the cathode; a third has the cathode end larger than the anode end; and a fourth has the cathode end beveled.

If you're unsure about the polarity, use an ohmmeter for verification. An ohmmeter's polarity is usually denoted by the markings on its face (` or 1) or by color coding (red for positive; black for negative). However, the ohmmeter's battery actually determines the external polarity.

To find the polarity (forward and reverse bias) of a diode, you place the diode in one direction between the known polarities of the ohmmeter and take a resistance reading; then reverse the diode's direction and take another reading. A low resistance reading indicates forward bias, while a high resistance reading indicates reverse bias. Since the ohmmeter's polarity is known, the diode end connected to the negative lead during forward bias must be the cathode; and the end connected to the positive lead must be the anode.

Testing diodes. While diodes are reliable, they certainly aren't indestructible. What can damage them? Anything from high voltages to improper connections to overheating. As such, you may have to verify the condition of a diode during troubleshooting of inoperative solid-state equipment.

Open and shorted diodes are the two most common problems. Here's how to interpret ohmmeter readings to determine a diode's condition.

Open diode. When taking two sets of resistance readings, one with test leads connected positive-to-anode, negative-to-cathode, and one with negative-to-anode, positive-to-cathode, high resistance readings are obtained in both cases.

Shorted diode. When taking two sets of resistance readings, one with test leads connected positive-to-anode, negative-to-cathode, and one with negative-to-anode, positive-to-cathode, low resistance readings are obtained in both cases.

Good diode. When taking two sets of resistance readings, one with test leads connected positive-to-anode, negative-to-cathode, and one with negative-to-anode, positive-to-cathode, the first connection provides a low resistance reading. The second connection provides a high resistance reading.

Diode capacity and derating. The amount of current at which a diode can operate without damaging itself is limited by two factors: the size of its heat sink, which helps dissipate heat; and its P-type/N-type material (PN) junction temperature rise.

Most diodes are rated to operate at 25oC, or at room temperature. This is a reflection of the diode's PN junction temperature rise. If the ambient temperature increases, the diode can't dissipate as much heat and must then be derated. This means you must reduce the diode's maximum operating current. Manufacturers offer derating tables to help find these limits.

Solid-state equipment manufacturers usually install heat sinks to permit internal diodes to operate at or close to maximum operating current. In retrofit or troubleshooting activities, you may want to install a heat sink to overcome frequent diode damage. Again, this information is available from solid-state equipment manufacturers.

Typical diode application. You can usually find diodes in rectifiers, gates, modulators, and detectors. Let's talk about rectifiers here. While AC power is generally the most available, many electronic devices require DC power for operation. As such, you must convert AC power to DC. Circuits that provide this conversion are called rectifiers, which use semiconductor diodes.

The simplest form of a rectifier circuit is the half-wave rectifier. The circuit includes a load resistor, RL, along with a diode across an AC source. A rectifier circuit affects the AC voltage by cutting it in half, or as is commonly said, rectifying it. Because the signal now travels in only one direction, it's called pulsating DC. This type of power supply circuit is only one in which diodes are used.

What's a Zener diode? A Zener diode is a type of diode that acts as a voltage regulator when installed by itself or with other semiconductor devices. This device is unique because we primarily use it to conduct current under reverse bias conditions. Standard diodes most often conduct current in the forward bias condition; as such, they can be damaged or destroyed if the reverse voltage or bias is exceeded.

Another name for the Zener diode is avalanche diode. This name is applied because the Zener diode usually operates in reverse breakdown.

How does a Zener diode work? To better understand how a Zener diode works, let's refer to Fig. 4. Here we see an operating characteristic curve for a typical Zener diode. Relatively speaking, a Zener diode's forward and reverse characteristics are similar to those of a standard diode. However, there are some very important differences.

Forward direction source voltage. When a source voltage is applied to a Zener diode in the forward direction, the result is a breakover voltage and a forward current.

Reverse direction source voltage. When the source voltage is applied in the reverse direction, the current remains very low until the reverse voltage reaches reverse breakdown, or what is commonly called Zener breakdown. At this breakdown point, a Zener diode will conduct heavily, or avalanche. This is the important difference between a standard and Zener diode: When a Zener diode conducts, it may continue to conduct for some time, and at considerable current, without damage in the reverse direction.

What's important about this characteristic? The voltage drop across a Zener diode remains almost constant, despite the very large current fluctuations.

How is voltage regulation maintained? The Zener diode acts as a constant voltage source because of resistance changes taking place within the PN junction.

When a source voltage is applied in the reverse direction, the PN junction's resistance stays high; thus a reverse current only in the microampere range will be produced. However, as the reverse voltage is increased, the PN junction reaches a critical voltage, and the Zener diode begins avalanching.

What's happening at this particular point? Once the avalanche voltage is reached, the PN junction's normally high resistance drops to a low value. The current then increases rapidly, but a circuit resistor or resistance RL generally limits it. The breakdown current usually will not destroy the Zener diode; however, it may be destructive if it becomes extremely excessive or if the Zener diode's heat dissipating capabilities are exceeded.

What are pertinent ratings? As we mentioned earlier, Zener diodes are designed to have a specific breakdown voltage rating, usually a close approximation of the necessary circuit control voltages. Examples include 4.7V, 5.1V, 6.2V, and 9.1V.

Zener diodes are manufactured to certain breakdown voltage tolerances, such as 520%, 510%, or 5 5%. For high-precision applications, they are available in the 51% range. Zener diodes are available in wattage ratings of one millionth of a watt to 50W; popular ratings are 1W and lower. These dissipation ratings are usually given for a specific operating temperature, which is usually an ambient of 25 DegrC.

This temperature will affect a Zener diode's breakdown voltage. As such, manufacturers usually list a Zener voltage temperature coefficient, which identifies the percentage of change in Zener voltage per degree of temperature change. Typically this change is about 0.1% per degree Centigrade.

How are Zener diodes tested? Either a Zener diode provides voltage regulation, or it fails. If it does fail, you obviously must replace it so the circuit can operate properly.

Sometimes, however, a Zener diode may fail only in certain situations. This type of failure is called an intermittent. To check for an intermittent, the Zener diode must be tested while it's in operation. Here, an oscilloscope is the best troubleshooting tool, because it displays the diode's dynamic operating characteristics.

Another workhorse of electronic equipment is the transistor. There are two types of transistors commonly used: PNP and NPN. As shown in Fig. 5, the PNP transistor consists of a thin layer of N-type material between two layers of P-type material. The NPN transistor has the opposite: a thin layer of P-type material between two layers of N-type material.

Transistors have three terminals: an emitter (E), base (B), and collector (C). These terminals have identical locations for both types of transistors. Note that the PNP transistor symbol has an arrow called the transmitter arrow pointing toward the base while an NPN transistor has the transmitter arrow pointing away from the base. In both, however, the transmitter arrow points away from the P-type material and toward to the N-type material.

Terminal arrangement. Transistors are available with two or three leads extending from their case. If you need a specific shaped transistor, you specify a transistor outline (TO) number. Note that the TO-3 configuration has only two leads or terminals. Many times, the transistor's metal case is used as the C lead.

You can identify specific leads by their spacing. For example, the E and B leads are usually close together, while the C lead is farther away. Also, the B lead is always in the middle.

Sometimes, transistors come with index pins. To identify terminals on such transistors, start with the lead closest to the index pin and work clockwise. The first lead is always the E lead; the B lead (always closest to the E lead) is next; and the C lead is last.

Transistor junction biasing. The base-emitter junction in a transistor circuit must always be forward biased while the base-collector junction must always be reverse biased.

Voltage in a transistor. There are certain voltages in the base circuit of a transistor. There are others associated with transistors, namely those in the collector circuit.

We have a bias voltage (VCC) applied across the emitter-collector circuit. This voltage will be divided between the emitter-base and collector-base junctions (based on their individual resistances), because these junctions are in series with each another. The voltage across the collector and emitter is VCE, and the voltage across the collector and base is VCB. VCB is greater than VCE because the collector-base junction has a much greater resistance than the base-emitter junction. However, base voltage VCB is greater than base-emitter voltage VBE.

SCR. Frequently used in solid-state equipment such as lighting dimmers and variable speed controls is the silicon-controlled rectifier (SCR). It's a semiconductor device having three electrodes: an anode, cathode, and gate. An SCR's anode and cathode are similar to those of a semiconductor diode.

How does an SCR differ from a diode? For one thing, it has the aforementioned gate electrode, which is its control point. For another, it will not pass significant current, even when forward biased, unless the anode voltage equals or exceeds the forward breakover voltage. Once this breakover voltage is reached an SCR will switch ON and become highly conductive.

SCR characteristic curve. An SCR operates like a regular Zener or avalanche diode in reverse bias: There is a small amount of current flow until avalanche is reached, after which the current increases dramatically. In forward bias, a certain value of forward breakover voltage must be reached before an SCR will conduct. In this mode, an SCR operates like a regular Zener or avalanche diode. In other words, there is a small amount of current flow until avalanche is reached, after which the current increases dramatically. And, as is the case with a Zener diode, this current can cause damage if thermal runaway begins.

When an SCR is forward biased, there's a small current, the forward blocking current. This current will stay relatively constant, at least until the forward blocking voltage is reached. At this point, which is called the forward avalanche region, the current will increase rapidly. Here, an SCR's resistance is very small. In fact, an SCR acts the same as a closed switch here, with the current limited only by any external load resistance. As such, a short in an SCR's load circuit will destroy the SCR if inadequate overload protection is provided.

Gate control. As mentioned earlier, an SCR works just like a mechanical switch: It's either ON or OFF. When the applied voltage on an SCR is below its forward breakover voltage (VBRF), the SCR fires (is ON). It will stay ON as long as the current stays above the holding current value; it will turn OFF when the voltage across it drops to a value too low to maintain the holding current.

How does an SCR's gate electrode come into play? When the gate is forward biased and current begins to flow in the gate-cathode junction, VBRF is reduced. The higher the forward bias, the less VBRF needed to get the SCR to conduct.

Once an SCR is turned ON by its gate current, this current loses control of the SCR's forward current. Even with its gate current completely removed, an SCR will stay ON until its anode voltage is removed. It also will stay ON until the anode voltage is reduced enough so the current is not sufficient to maintain a proper holding current level.

SCR applications. Basically, an SCR is used as a DC switch because of its many advantages over mechanical DC switching. These include arcless switching, low forward voltage drop, rapid switching time, and no moving parts. You can use an SCR for AC switching, although you need two SCRs.

Varying power to a load is perhaps an SCR's most prominent application. This is because of its ability to turn ON at different points in its conducting cycle; thus, it's useful in varying the amount of power delivered to a load. This type of variable control is called phase control. Don't confuse the term "phase" as used here with that pertaining to power distribution systems. Here, "phase" refers to the time relationship between two events, in this case, between trigger pulse and the point in the conducting cycle at which the pulse occurs.

Testing an SCR. You can "rough" test SCRs using an ohmmeter and test circuit. If an SCR does not respond as indicated for each of these steps, it's defective, and you should replace it.

Step 1. Set the ohmmeter on the "R 3 100" scale. Connect the ohmmeter's negative lead to the SCR's cathode and its positive lead to the SCR's anode. The ohmmeter should read infinity. (Resistance will actually be more than 250,000 ohms.)

Step 2. Close the switch. This will short circuit the gate to the anode. The ohmmeter should read almost 0 ohms. (Resistance will actually be about 10 ohms to 50 ohms; this range of readings will not register on the "R 3 100" scale.) Open the switch, and the ohmmeter should still read zero ohms.

Step 3. Reconnect the ohmmeters leads, positive lead to the SCR's cathode and its negative lead to the SCR's anode. The ohmmeter should read infinity. (Resistance will actually be more than 250,000 ohms.)

Step 4. Close the switch. This will short circuit the gate to the anode. The resistance reading should remain high because the SCR is reverse-biased and, therefore, can't conduct.

Step 5. Open the switch. The resistance should remain high because the SCR is reversed-biased and has no gate current.

Low-voltage switching. Probably the most widespread use of low-voltage controls is low-voltage switching. This technology operates by replacing power wiring between lighting fixtures and switches (typically individual conductors in conduit) with inexpensive, low-voltage cables. Italso replaces the more expensive 120V switch with a low-voltage switch.

You do this by reducing the voltage at which the switch leg operates. If these conductors operate at less than 50V, NEC Art. 725 covers them. This makes cheaper and easier-to-install conductors possible.

The new generation of low-voltage switches operates at 24VAC, and uses a Class 2 power-limited circuit. You can derive this type of circuit directly from a transformer, without any additional overcurrent protection.

Yes, you'll need an extra transformer to make this work, but small transformers of this type are inexpensive. You can install them inside some lighting fixtures or on to a standard wiring box through a knockout. Some of the new low-voltage devices combine the transformer, relay, and connections between them in one modular unit.

Applications. While we think of low-voltage switching as a system of lighting control, there are many other applications: energy management control of 120V loads; fire alarm shutdown relays; security system 120V load control; and controls for fans, pumps, signs, livestock feeders, HVAC units, lighting contactors, motor control circuits, exit lighting, pool lights, and yard lights.

Actually, low-voltage switching makes good sense any time you have to run a medium to long switching or control circuit. This list of equipment simply shows the most common uses; there could be many others.

Some people use low-voltage switching for dual switching (inboard and outboard pairs) of fluorescent light fixtures. There could be hundreds of possible industrial applications.

Lighting control. There are two primary ways of controlling lighting expenses:

  • Use more efficient lights, and

  • Use your light sources more efficiently (for fewer hours).

In this lesson, we won't cover reduced-wattage lighting technologies, but we will spend some time on the reduction of wasted lighting. Actually, reducing watts represents only about half of the potential for maximizing energy savings. Reducing operating hours through automatic controls is the other half.

Occupancy sensors are devices that ensure lights energize only when occupants are present.

Occupancy sensors save energy by automatically turning off lights in unoccupied spaces. When the sensor detects motion, it activates a control device that turns on the luminaires (lighting fixtures). If it detects no motion within a specified period, it turns the lights off until it senses motion again.

Occupancy sensors are suitable for a very wide range of lighting control applications, and you should consider them for every upgrade. You can install occupancy sensors to provide on/off control in several situations. You can use them with incandescent or fluorescent loads, and with bi-level control, capacitive-switching HID luminaires.

Most occupancy sensors have adjustable settings for both sensitivity and time delay. The sensitivity setting enables you to finetune the sensor for specific activities. In this way, you can detect normal motion without triggering responses to extraneous signals.

The time delay setting refers to the amount of elapsed time with no motion detected before the luminaires turn off. The time delay prevents the luminaries from switching off when people are in the room, but are moving too little or slowly for the sensor to detect them.

Some occupancy sensors provide daylight switching with their occupancy switching control. Typically, these won't shut off the lighting system due to sufficient daylight while people occupy the space. Rather, they'll prohibit the lights from turning on when an occupant enters a space when daylighting is sufficient. Make use of a trial installation to assess user acceptance of this technology.

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