Guide to Basic Electronics Theory--Other simple components

Home | Glossary | Books | Links/Resources
EMC Testing | Environmental Testing | Vibration Testing

To this point in the book, you have read about passive components. In other sections, you can look at active components, such as tubes and transistors. An active component is one that is capable of amplification. A passive component does not amplify and does not need a power source other than the signal passing through it.

The most important types of passive components have already been covered. These components are resistors ( section 3), capacitors ( section 6), and inductors ( sections 8 and 9). This section will discuss a few additional passive devices that are used in many electronic circuits. You will mostly be working with switching devices. The last portion of this section will turn to motors and lamps.

What is a switch?

In the circuits described so far, the only way to stop the current from flowing is to physically disconnect one of the components. Obviously, this would be impractical in most types of equipment. Yet, a continuous current flow is generally undesirable—power is wasted, continuous operation might be annoying or even dangerous, and it might interfere with the operation of other circuits. Also, it’s often necessary to selectively allow current to pass through various sub circuits or components at different times. The solution to these problems is a simple device called a switch.

T of switches

A switch is simply a device that makes and breaks an electrical connection, thereby completing or opening a current path. The simplest type of switch is the knife switch, which is shown in FIG. 1. When the handle of the switch is in the position shown in FIG. lA, no current can flow through the attached circuit, because there is not a complete path for the current to follow. If the handle is moved into the position shown in FIG. 1B, the metal handle completes the circuit and current can flow.

- --1 A knife switch. Switch opened (A); switch closed (B).

A switch like this is quite simple to make and use, but it’s rarely used in modern circuits because it’s quite bulky, and the electrical connections are exposed, which could mean a serious shock hazard.

A more common type of switch is shown in FIG. 2. This kind is called a slide switch. When the slider (movable portion) is in the position shown in FIG. 2A, the circuit is open and no current can flow. But, when the slider is moved into the position shown in FIG. 2B, the metal strip on the bottom of the slider touches the two connections to the external circuit, thus completing the current path.

- --2 Construction of an SPST (single-pole, single-throw) slide switch. Switch opened (A); switch closed (B). Slider

Another popular type of switch is the toggle switch, shown in FIG. 3. It operates in a manner similar to the slide switch, but the slider is in the shape of a ball that rolls into and out of position.

Still another commonly used switch type is the pushbutton. This device is discussed elsewhere in this section.

- --3 A toggle switch.

SPST switches

The switches you have been reading about so far have only two connections. Because the slider, or pole, makes contact in only one position, this form of switch is said to be a single-pole, single-throw switch. This name is generally abbreviated as SPST The schematic symbol for a SPST switch is shown in FIG. 4. This switch is usually shown in its open position for clarity. The same symbol is used for most kinds of switches (that is, slide, toggle, or whatever).

- --4 Schematic symbol for an SPST switch.

When the switch is open, there is a break in the circuit. No current flows. When the switch is closed it acts like a simple piece of wire, and current can flow through it. See FIG. 5.

- --5 Equivalent circuits for an SPST switch. Switch opened (A); switch closed (B). Circuit closed; Circuit open

--- 6 SPST switch in a parallel circuit.

Another way an SPST switch can be used is shown in FIG. 6. This circuit has two resistors in parallel, but if the switch is open, current can flow through R1 only. Electrically, R2 does not exist. However, when the switch is closed, current can flow through both resistors. R1 and R2 act like any parallel resistance combination. Refer ring again to FIG. 6, if the battery generates 6 V, and each resistor has a value of 100 ohm?, the milliammeter will indicate 60 mA when the switch is open. However, when the switch is closed, connection R2 is in parallel with R1, the total effective resistance in the circuit drops to 50 ohm, and the current increases to 120 mA.

SPDT switches:

A single switch can be used to control two separate circuits if a third connection is added, as in FIG. 7. When this switch is in the position shown in FIG. 7A, the current can flow between connector 1 and connector 2. Connector 3 will be open (disconnected). Moving the slider into the position shown in FIG. 7B will allow current to flow between connector 2 and connector 3, but connector 1 will now be left open.

- --7 An SPDT (single-pole, double-throw) slide switch. Switch opened (A); switch closed (B).

Because this type of switch has a single pole and two possible positions, it’s called a single-pole, /double-throw or SPDT switch. The schematic symbol for this kind of switch is shown in FIG. 8. The slider can be shown in either position. Notice that connector 2 ( FIG. 7) is common to both circuits, regardless of the slider position. The connector numbers are not shown in actual schematic diagrams.

A simple circuit using an SPDT switch is shown in FIG. 9. R2 and R3 are in parallel with R1, but current will flow through only one resistor at any given time, depending on the position of the slider. In FIG. 9A, current can flow through R1 and R2, but not R3. In FIG. 9B, the situation is reversed. Current can flow through R1 and R3, but not R2.

- --8 Schematic symbol for an SPDT switch.

- --9 An SPDT switch in a parallel circuit.

If you assume the battery puts out 6 V R is 100 ohm, R is 220 CT and R is 4,700 C), the milliammeter will read 87 mA (0.087 ampere) in FIG. 9A, and in FIG. 9B it will read 98 mA (0.98 A).

Some SPDT switches have a center off position. In this case, the slider would be positioned as shown in FIG. 10. Neither circuit 1-2, nor circuit 2-3 is complete, so no current flows through the switch at all.

- --10 An SPDT switch with center off.

An SPDT switch could be used as an SPST unit by just leaving one of the end connectors unused. See FIG. 11.

- --11 Using an SPDT switch as an SPST switch.

DPST switches:

A similar type of switch is the double-pole, single-throw switch (DPS7). The schematic symbol for this device is shown in FIG. 12. Notice that it’s actually two entirely separate (electrically) SPST switches with a common slider. The slider has two separate metal strips to complete two separate circuits with no common connections. Of course, these two SPST switches always work in union. They are either both off, or both on. A DPST switch can be used as an SPST switch by just using one set of contacts. Actually, DPST switches are fairly uncommon. When a DPST function is required, a DPDT switch is usually used.

- --12 Schematic symbol for a DPST (double-pole, single-throw) switch.

DPDT switches:

Double-pole/double-throw (DPDT) switches have six connection terminals (see FIG. 13) and are essentially two SPDT switches with a common slider, just as a DPST switch is essentially two SPST switches with a common slider. Obviously a DPDT switch can be substituted for any of the other switch types (SPST, SPDT, DPST, or a combination of an SPST and an SPD’I) simply by using only the appropriate terminals. By using all six contacts, you can achieve two electrically separate SPDT actions with the throw of a single switch. The schematic symbol for a DPDT switch is shown in FIG. 14. Like the SPDT switch, DPDT switches are often equipped with a center-off position that leaves all six terminals open.

Multi-position switches:

The four basic types of switches discussed above (SPST, SPDT, DPST, and DPD’I) are the most commonly used configurations, but other configurations are some times required in specific circuits. Generally, slide switches are available only up to DP4T (two poles with four positions each). If more poles, or positions are needed, a rotary switch is used. This type of switch gets its name from the fact that a knob is rotated to control the position of the slider(s).

Not connected

- --13 A DPDT (double-pole, double-throw) slide switch. Metal contacts

--- 14 Schematic symbol for a DPDT switch.

The schematic symbol for a rotary switch varies somewhat, of course, de pending on the number of poles and positions in the specific switch. FIG. 15A shows the schematic symbol for an SP12T rotary switch, and FIG. 15B is the symbol for a 3P6T unit. Many other combinations are also possible.

There are two basic varieties of rotary switches. The non-shorting type disconnects the circuit at one position completely before the connection of the next position is made. The other kind of rotary switch is the shorting, or make-before-break type. With this type of switch, in switching from position A to position B, the switch makes contact with both position A and position B for a brief instant, before the connection at position A is broken. In most circuits it doesn’t really matter which type is used, but some specialized circuits require one type or the other.

- --15 Schematic symbols. SP12T rotary switch (A); 3P6T rotary switch (B).

Momentary-contact switches:

Sometimes it’s necessary to open or close a circuit connection only briefly, then return it to its original condition. In many such cases, manually moving the switch back and forth is inconvenient or impractical. For this sort of situation, a momentary- action or momentary-contact switch is used. This kind of switch is loaded with a spring that always returns the slider to a specific rest position, unless it’s manually held in the other position. The rest position can be either normally open (NO) or normally closed (NO), depending upon the requirements of the specific circuit.

Although momentary slide switches and toggle switches are available, this type of switch action is usually in the form of a pushbutton switch. Pushbutton switches can be SPST, SPDT, or DPDT (or, occasionally DPST), but the most common configuration for momentary action switches is SPST. If it’s a momentary action switch, this information will be indicated in the parts list or in a note on the schematic. Notice that a double-throw, momentary-contact switch has one set of contacts that are normally open, and another set of contacts that are normally closed.

Most pushbutton switches are of the momentary action type, but some work as regular switches that can be left in either position. A pushbutton switch that does not have momentary contacts is called a push-on, push-off switch.

Potentiometer switches:

Another type of commonly used switch fits onto the back of a potentiometer. This kind of switch is usually an SPST switch. The switch is controlled by the potentiometer knob. When the potentiometer is in its maximum resistance position, the switch is open (off). But as soon as the control knob is advanced from this extreme position, the switch is clicked shut, turning the controlled circuit on. From then on the potentiometer operates normally.

The potentiometer and the switch will usually be used in the same part of the circuit buy not always. The potentiometer and the switch are mechanically tied together, but electrically distinct. They could be used in two entirely separate circuits, although this arrangement could cause some confusion in operation.

Potentiometer switches are most often used to turn the main power supply of a circuit on and off. For example, an amplifier might have a switch connected to its volume control-potentiometer, so that when the volume is turned all the way down to its minimum setting, the entire circuit is switched off.


In many applications, it might not be practical to use any of the basic manual switches discussed so fan For instance, you might need a circuit that must be switched on when the voltage in another circuit rises above some specified level. Of course, you could watch a voltmeter connected to the second circuit and manually flip a switch in the first circuit at the appropriate moment, but that is obviously a highly impractical and inconvenient approach to the application. Some sort of automatic switching would clearly be more efficient.

An automatic switch of some kind would also be necessary in a remotely con trolled circuit. Often the circuit being switched is not readily accessible. It would be inconvenient at best to run a pair of wires carrying the full power-supply voltage or electrical data over a long distance to a convenient control point. It would be far better to just send a small control voltage over light-duty connecting wires to a remote-controlled electrical switch.

The simplest form of automatic switching in an electrical circuit is through a device called a relay. A relay basically consists of two parts—a coil and a magnetic switch. When an electrical current flows through a coil, a magnetic field is created around it. This magnetic field is proportional to the amount of current flow through the coil. At some specific point, the magnetic field will be strong enough to puff the switch slider from its rest, or de-energized position to its momentary, or energized position. lithe electrical power through the coil drops, the strength of the magnetic field will drop off to zero, releasing the switch slider and allowing it to spring back to the original de-energized position.

The switch section of a relay can be any of the basic switching types discussed in this section (SPST, SPDT, DPST, or DPD’D. For single-throw units the switch contacts might be either normally open or normally closed. Of course, with an SPDT or a DPDT switching arrangement, you have both a normally open and a normally closed contact simultaneously. The schematic symbols for an SPST and an SPDT relay are shown in FIG. 16. Relays are usually identified in schematic diagrams and parts lists by the letter K.

--- 16 Schematic symbols. SPST relay (A); SPDT relay (B).

A relay coil and switching contacts are virtually always used in electrically isolated circuits. That is, the current through one circuit controls the switching of an other circuit. Relays vary greatly in size, depending primarily on the amount of power that they can safely carry. Separate ratings are usually given for the coil and the switch contacts, because they are generally used in separate circuits. Relays range from tiny units intended for 0.5W transistorized equipment to huge megawatt (millions of watts) devices used in industrial power-generating plants. At either extreme, the principle of operation is precisely the same.

Of course, the most important rating for a relay is the voltage required to make the switch contacts move to their energized position. This action is called tripping the relay. Typical trip voltages for relays used in electronic circuits are 6, 12, 24, 48, 117, and 240 V. Both ac and dc types are available.

The controlling voltage through a relay coil should be kept within about ±25% of the rated value. Too large a voltage could burn out the delicate coil windings. On the other hand, too small a control voltage could result in erratic operation of the relay.

Sometimes it might be necessary to drive a relatively high-power circuit with a rather low-power control signal. This application can be handled the type of circuitry shown in FIG. 17. High voltage supply B is operated only when the relay is energized, minimizing power consumption.

Occasionally the available control signal won’t be sufficient to drive a large

- --17 A relay is used so that a low voltage can control a higher-voltage load.

enough relay for the circuit to be controlled. The solution might be to add an additional medium-power relay to act as an intermediate stage, as shown in FIG. 18.

--- 18 A small relay can be used to drive a heavier relay.

The coil winding could self-destruct if the current through it changes suddenly, perhaps due to opening a series switch as shown in FIG. 19A. The voltage drops from a positive voltage to 0 in a fraction of a second. This fast drop causes the magnetic field around the coil to collapse rapidly. This abrupt change in the magnetic field will induce a brief high-voltage spike in the relay. This spike could be high enough to damage or eventually destroy the switch contacts.

- --19 A parallel diode is often used to protect a relay coil from high-voltage spikes, which can occur during switching.

A diode is often placed in parallel with the relay coil to suppress such high- voltage transients. This arrangement is shown in FIG. 19B. With this arrangement, the diode limits the voltage through the relay coil to the power supply voltage (unless, of course, the spike is large enough to damage the diode itself.

A transistor amplifier is often used to drive a high-current relay from the low- current source, such as a small battery. The low-current source limits undue power drain. A typical circuit of this type is shown in FIG. 20.

- --20 A transistor amplifier can be used to drive a high-current relay source.

Some relays are latching relays. One control pulse closes the switch contacts, which will remain closed, even if the control pulse is removed. A separate control pulse opens the switch contacts. Each time the relay is triggered, it latches into the appropriate position until the next trigger signal is received. Not surprisingly, this type of device is called a latching relay.

Fuses and circuit breakers

Another kind of automatic switching is used specifically for circuit protection. The voltage through a circuit can be controlled by the design of the power supply, except for relatively rare transients. But, the current drawn through a circuit depends on the resistance and impedance factors within the load circuit. If the resistance drops because of a short circuit, or some other defect, the current could rapidly rise to a level that can damage or destroy some of the components. What is needed is a way to disconnect the power supply from the load circuit before the current reaches a dangerous level. This is most often done with a special device called a fuse. A fuse is basically just a thin wire that is carefully manufactured so that it will melt when the current passing through it exceeds a specific value.

The schematic symbol for a fuse is shown in FIG. 21, and FIG. 22 illustrates a simplified circuit using a fuse. If the current drawn by the load circuit exceeds the current rating of the fuse for any reason (such as changing the value of R1), the fuse will blow, opening the circuit. No further current will flow through the circuit.

--- 21 Schematic symbol for a fuse.

--- 22 A simple circuit using a fuse.

Fuses are usually enclosed in glass (or sometimes metal) tubes for protection. The fuse wire is very thin, and could easily be damaged. FIG. 23 shows a typical fuse.

--- 23 Basic structure of a fuse.

Sometimes fuses are soldered directly into a circuit, but because once a fuse element has been melted, it must be replaced before the circuit can be reused, this installation method is rather impractical in most applications. For more convenient fuse replacement, some kind of socket is generally used for fuses. Most commonly the fuse is held between a set of spring clips. Another frequently used method is to fit the fuse into a special receptacle with a screw cap.

Frequently replacing fuses can be a nuisance, so sometimes a component called a circuit breaker is used. This switch is a special switch that will automatically open if the current through it exceeds some specific amount. To close the switch again, you just manually push a reset button.

Occasionally transients (brief irregular signals) can cause a fuse to blow, or a circuit breaker to open even if there is no defect in the load circuit at all. But if a new fuse immediately blows, or if the circuit breaker repeatedly opens when it’s reset, it indicates that something is wrong and repairs are needed. Never replace a fuse with a higher rated unit. You could end up blowing some expensive electrical components to protect the fuse, and that certainly doesn’t make much sense.


Motors aren’t switching devices, but they warrant a brief discussion here. Rather than including a separate short section on motors, this section is something of a potpourri. A motor is a transducer, a device that converts one form of energy into another form of energy. Additional transducers will be discussed in sections 19, 22, and 30. In the case of a motor, electrical energy is converted into mechanical energy. That is, an electrical signal can cause something to physically move.

A motor is a practical application of the electromagnetic fields discussed in section 7. There are many different types of motors; some are extremely tiny, and others are huge. Some can only move very small weights, and larger motors can move tons; some run on dc and others run on ac. Regardless of these differences, all motors are basically the same, at least in their operating principles.

An electric current is fed through a set of coils, setting up a strong magnetic field. The attraction of opposite magnetic poles and the repulsion of like magnetic poles results in the mechanical motion of the motor.

A simplified cutaway diagram of a typical motor is shown in FIG. 24. Notice that there are two sets of coils. One is stationary, and is known as the field coil. The other coil, which is known as the armature coil, can freely rotate within the magnetic field of the field coil. The motor shaft is connected directly to the movable armature coil. As the armature coil moves, the motor shaft rotates.

- --24 Simplified cutaway diagram of a typical motor. Field Commutator coil brush; Commutator; Commutator brush

The commutator reverses the polarity of the current with each half rotation of the armature and shaft. This reversal keeps the armature coil constantly in motion. In FIG. 25A, the armature coil is positioned so that its magnetic poles are lined up with the like poles of the field coil. The like magnetic poles repel each other, forcing the armature coil to rotate, as shown in FIG. 25B. At some point, the at traction of unlike poles will take over, pulling the armature into the position shown in FIG. 25C.

- --25 The commutator reverses the polarity of the current with each half-rotation to keep the armature in constant motion.

The commutator reverses the polarity of the current, so once again the like poles of the armature coil and the field coil are lined up. The whole process repeats for the second half rotation ( FIG. 25D), bringing you back to the position shown in FIG. 25A, and a new cycle begins.

Increasing the current through the coils (assuming everything else remains equal) will increase the torque of the motor. That is, it can turn a larger load if a larger current is supplied. Using a given motor to move a load, the heavier the load is, the more current the motor will be forced to draw from the power supply.

Some motors are designed to operate at a constant speed; others will change their rotation speed with changes in the current or voltage applied to them. The size of the load can also affect the motor’s rotation speed. Obviously, heavier weights will slow the motor, because it has to work harder to turn the load. This slowing is especially noticeable if a constant current source is driving the motor. Some motors are specifically designed for the load to control the actual rotation speed.

Direct-current motors and ac motors are generally not interchangeable. Using the wrong type of power source could damage or destroy the motor. Excessive loads can also damage small motors.


A lamp is another common type of transducer. In this case, electrical energy is converted into light energy It isn’t too hard for most people to understand how a simple lamp or light bulb works. Most people are surprised to find out that there are also transducer devices that convert light energy into electrical energy Such devices are covered in section 22.

Incandescent lamps:

The most common and familiar type of lamp is the incandescent lamp. This lamp is the type of light bulb you’re acquainted with and use in your home every day. Similar lamps are used in many electronics circuits. The only real difference is size. An incandescent lamp is a vacuum enclosed glass bulb. The bulb is air tight to maintain the vacuum. Within the bulb is a short length of a special resistive wire. The two ends of this wire are brought out separately to the metallic socket in the bulb.

When a sufficient electrical current at the correct voltage is passed through this thin wire, or filament, it will heat up and start to glow, giving off a great deal of light. The filament must be contained in a vacuum to prevent it from burning out too quickly. When the filament wire burns through .or breaks from some other cause, the bulb must be replaced.

Contrary to popular belief, Edison did not really invent the incandescent lamp. He just came up with the first practical device of this type. He was the first to devise a suitable filament that would last.

Incandescent lamps vary widely in size and shape, as well as power requirements. You can find incandescent lamp bulbs designed to operate at almost any voltage ranging from less than one volt up to several hundred volts.

In electronics work, you generally use only small, low-voltage bulbs. These lamps are often called flashlight bulbs, because they are commonly used in flash lights. Small incandescent lamps are used in many electronics circuits (especially older designs) as indicator devices. Often, the bulb will be painted with translucent paint, or a translucent plastic cap will be placed over the bulb to give the light a specific color, such as red or green.

At one time, small incandescent lamps were just about the only practical choice for low-power indicator devices. Today they are largely being replaced by LEDs (see section 19) because the lamps are bulky and fragile. They are also quite inefficient in terms of power. They generate more heat than light. Because many modern semi conductor components are heat sensitive, this is not just wasteful; it could actually cause harm.

Some other types of lamps are filled with a gas, rather than being evacuated. A fluorescent tube is a common example.

Neon lamps:

A number of electronic circuits (again, mostly older designs) use neon lamps as indicating devices. The construction of a neon lamp is shown in FIG. 26. In a neon lamp, there is air-tight glass bulb, just like in the lamp discussed above, but this bulb does not contain a vacuum; it contains neon gas. Instead of a filament, there are two separated electrodes. If a voltage is connected across these electrodes, the neon gas between them will become ionized and start to glow. Neon glows with a characteristic orange color. A high voltage is normally required to light a neon lamp; typically, close to 100 V is required.

---26 Construction of a neon lamp. Glass envelope

Because of the way the neon lamp suddenly switches on or fires when its thresh old level (minimum operating voltage) is exceeded, this device is often used for triggering and voltage regulation applications, as well as an output indicator.


1. What is the simplest type of switch?

A SPDT switch

B Toggle switch

C Knife switch

D Relay

E None of the above

2. How many circuits can a DPDT switch simultaneously control?

A One

B Two

C Three

D Four

E None of the above

3. Which of the following is not a standard switch type?





E None of the above

4. If R in the circuit shown in FIG. 6 is 2200 ohm., and R2 is 3900 ohm what is the circuit resistance when the switch is closed?

A 2200-ohm

B 3900-ohm

C 6100-ohm

D 1407-ohm

E None of the above

5. What is the term for a switch that disconnects the circuit completely at one position before the connection of the next position is made?

A Rotary

B Shorting

C Non-shorting

D Make-before-break

E None of the above

6. What happens when a momentary action switch is released?

A Nothing

B The switch contacts latch into the new position

C The switch contacts return to their normal rest position

7. What is contained in a relay?

A An RC circuit

B A coil and a fuse

C A coil and a diode

D A coil and a set of switch contacts

E None of the above

8. Why is a diode placed across the coil portion of a relay?

A To increase the current flow

B To speed up the switching

C To protect the relay from high-voltage transients when the magnetic field collapses

D To protect the relay against incorrect polarity

E None of the above

9. What is the name of a device that protects a load circuit from excessive cur rent flow?

A Relay

B Suppression diode

C SPDT switch

D Fuse

E None of the above

10. When should a fuse be replaced with a higher rated unit?

A If it blows

B Never

C When the original value is not available

D When fuses of the original value blow as soon as they are replaced

E None of the above

Article Index

<<    >>

top of page  Article Index   Home

Home | Glossary | Books | Links/Resources
EMC Testing | Environmental Testing | Vibration Testing

Updated: Tuesday, 2014-07-08 22:29 PST