Test some semiconductor (electronic) devices

For most applications, the ohmmeters used most frequently are the ohmmeter section of a VOM or DMM. The details of the internal cir- cuitry and the method of using the meter will be left primarily for a laboratory exercise. In general, however, the resistance of a resistor can be measured by simply connecting the two leads of the meter across the resistor, as shown in Fig. 3.31. There is no need to be concerned about which lead goes on which end; the result will be the same in either case since resistors offer the same resistance to the flow of charge (current) in either direction. If the VOM is employed, a switch must be set to the proper resistance range, and a nonlinear scale (usually the top scale of the meter) must be properly read to obtain the resistance value. The DMM also requires choosing the best scale setting for the resistance to be measured, but the result appears as a numerical display, with the proper placement of the dec- imal point as determined by the chosen scale. When measuring the resistance of a single resistor, it is usually best to remove the resis- tor from the network before making the measurement. If this is diffi- cult or impossible, at least one end of the resistor must not be con- nected to the network, or the reading may include the effects of the other elements of the system.

If the two leads of the meter are touching in the ohmmeter mode, the resulting resistance is zero. A connection can be checked as shown in Fig. 3.32 by simply hooking up the meter to either side of the connec- tion. If the resistance is zero, the connection is secure. If it is other than zero, the connection could be weak, and, if it is infinite, there is no con- nection at all.

If one wire of a harness is known, a second can be found as shown in Fig. 3.33. Simply connect the end of the known lead to the end of any other lead. When the ohmmeter indicates zero ohms (or very low resistance), the second lead has been identified. The above procedure can also be used to determine the first known lead by simply connect- ing the meter to any wire at one end and then touching all the leads at the other end until a zero-ohm indication is obtained.

FIG. 3.31

Measuring the resistance of a single element.

FIG. 3.32

Checking the continuity of a connection.

FIG. 3.33

G THERMISTORS  83

Preliminary measurements of the condition of some electronic devices such as the diode and transistor can be made using the ohmmeter. The meter can also be used to identify the terminals of such devices.

One important note about the use of any ohmmeter: Never hook up an ohmmeter to a live circuit!

The reading will be meaningless and you may damage the instrument. The ohmmeter section of any meter is designed to pass a small sensing current through the resistance to be measured. A large external current could damage the movement and would certainly throw off the calibra- tion of the instrument. In addition,

never store a VOM or a DMM in the resistance mode.

The two leads of the meter could touch and the small sensing current could drain the internal battery. VOMs should be stored with the selec- tor switch on the highest voltage range, and the selector switch of DMMs should be in the off position.

3.11

THERMISTORS

The thermistor is a two-terminal semiconductor device whose resis- tance, as the name suggests, is temperature sensitive. A representative characteristic appears in Fig. 3.34 with the graphic symbol for the device. Note the nonlinearity of the curve and the drop in resistance from about 5000 to 100 for an increase in temperature from 20°C to 100°C. The decrease in resistance with an increase in temperature indicates a negative temperature coefficient.

The temperature of the device can be changed internally or exter- nally. An increase in current through the device will raise its tempera- ture, causing a drop in its terminal resistance. Any externally applied heat source will result in an increase in its body temperature and a drop in resistance. This type of action (internal or external) lends itself well to control mechanisms. Many different types of thermistors are shown in Fig. 3.35. Materials employed in the manufacture of thermistors include oxides of cobalt, nickel, strontium, and manganese.

Temperature (°C) (a) –100 0 100 200 300 400 10–4 10–2 100 102 104 106 108 R Specif ic resistance ( Ω -cm) (b) T FIG. 3.34

Thermistor: (a) characteristics; (b) symbol.

FIG. 3.35

NTC (negative temperature coefficient) and PTC (positive temperature coefficient) thermistors. (Courtesy of Siemens Components, Inc.)

R 0.1 k Cell resistance 0.1 1.0 10 100 1000 Illumination (foot-candles) (a) (b) 1 k 1 0 k 100 k FIG. 3.36

Photoconductive cell: (a) characteristics; (b) symbol.

FIG. 3.37

Photoconductive cells. (Courtesy of EG&G VACTEC, Inc.)

Note the use of a log scale (to be discussed in Chapter 23) in Fig. 3.34 for the vertical axis. The log scale permits the display of a wider range of specific resistance levels than a linear scale such as the horizontal axis. Note that it extends from 0.0001· cm to 100,000,000· cm over a very short interval. The log scale is used for both the vertical and the horizontal axis of Fig. 3.36.

3.12

PHOTOCONDUCTIVE CELL

The photoconductive cell is a two-terminal semiconductor device whose terminal resistance is determined by the intensity of the incident light on its exposed surface. As the applied illumination increases in intensity, the energy state of the surface electrons and atoms increases, with a resultant increase in the number of “free carriers” and a corre- sponding drop in resistance. A typical set of characteristics and the photoconductive cell’s graphic symbol appear in Fig. 3.36. Note the negative illumination coefficient. Several cadmium sulfide photocon- ductive cells appear in Fig. 3.37.

3.13

VARISTORS

Varistors are voltage-dependent, nonlinear resistors used to suppress high-voltage transients; that is, their characteristics are such as to limit the voltage that can appear across the terminals of a sensitive device or system. A typical set of characteristics appears in Fig. 3.38(a), along with a linear resistance characteristic for comparison purposes. Note that at a particular “firing voltage,” the current rises rapidly but the volt- age is limited to a level just above this firing potential. In other words, the magnitude of the voltage that can appear across this device cannot exceed that level defined by its characteristics. Through proper design techniques this device can therefore limit the voltage appearing across sensitive regions of a network. The current is simply limited by the net- work to which it is connected. A photograph of a number of commer- cial units appears in Fig. 3.38(b).

FIG. 3.38

Varistors available with maximum dc voltage ratings between 18 V and 615 V. (Courtesy of Philips Components, Inc.)

G APPLICATIONS  85

3.14

APPLICATIONS

The following are examples of how resistance can be used to perform a variety of tasks, from heating to measuring the stress or strain on a sup- porting member of a structure. In general, resistance is a component of every electrical or electronic application.