properties of resistors therm is tor characteristics (1)

8/3/2019 Properties of Resistors Therm is Tor Characteristics (1)

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PROPERTIES OF RESISTORS; THERMISTOR CHARACTERISTICS

Introduction:

In this experiment, you will observe the properties of resistors of various types,

measuring the resistance with an ohmmeter. You will study commonly used carbon resistors,individually and connected in series or parallel. The temperature dependence of the resistance of

a coil of metal wire and of a thermistor, a thermally sensitive semiconductor device, will be

measured and studied. You will measure the resistance of your own body under various

conditions and study the implications of the results for shock hazards. Note: The "equivalent

resistance" is the resistance of a combination of resistors. For resistors connected in series (end

to end), the current flowing through each resistor will be the same but the voltage across

individual resistors may be different; the equivalent resistance of the series combination is Rser

= R1 + R2 + .... For resistors connected in parallel (side by side), the voltage across each resistor

will be the same but the current through individual resistors may be different; the equivalent

resistance Rpar of the parallel combination is determined by the relation 1/Rpar = 1/R1 + 1/R2 +....

Many resistors, including the carbon resistors used in this experiment, are color coded to

indicate the resistance, tolerance, etc. Information on the color code is given below.

Equipment:

BK 2806 Digital multimeter used as Autoranging ohmmeter, four-resistor board,resistance coil, thermistor, circuit connection wires, temperature baths, small beaker, ice, ruler,

and thermometer.

Procedure:

An ohmmeter can be used to measure the resistance of an isolated circuit element. A low

voltage (potential difference) from a source inside the meter is applied to the circuit element and

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1

band.

blue

vides these values. 10 %silver

grey

brown

appearance or

the values of T as

8

0

2

4

6

9

5 %

20 %

a b c T

This table provides

given by the

absence of the last

Color Code

red

green

R = [(10 a + b) 10 T]


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the relationship between the voltage and the current which flows is used to determine the

resistance.

The MT 310 Multimeter can be used as an ohmmeter by turning the meter on and setting therotary switch to . Use Autoranging so the meter automatically selects the best resistance range

from ranges of 200 , 2000 , 20 k, 200 k, 2000 k, or 20 M. Make sure you read the meter

correctly and record the range (needed for error estimates) for each measurement. Connect the

leads to and COM. When the leads are touched together, the meter should read near zeroohms (). No adjustment should be required. If you have trouble, ask for help.

WARNING!

Electrical instruments and components each have a maximum current, voltage, and power rating. If these limits are

exceeded, components or instruments can be damaged. Before each measurement with a multimeter, check the

setting of the function (selector) switch and any other switches, and check the connections. Incorrect settings and

connections can result in damage to the meter.

Measurements:

A. Resistance of Carbon Resistors, Series and Parallel Combinations

A.1. Measure the resistance of each of the four carbon resistors on the board by connecting the

ohmmeter leads to the connectors at the ends of the resistor. Record the color code and tolerance

of each resistor. Check your measured values against the color code and tolerance values using

information from the figure. A resistor also has a dissipation rating, expressed in W, determined

by the rate at which it can dissipate thermal energy without an excessive temperature rise. Look

at the page of sample resistors posted in the laboratory; determine the wattage rating for each of

your four resistors based on its size, and record it.

A. 2. Connect any two resistors in series and measure the equivalent resistance Rser. Connect

any two resistors in parallel and measure the equivalent resistance Rpar. Be sure to keep track of

which resistors you have used.

B. Temperature Dependence of Resistance

The resistivity of most materials varies with temperature. For most common metals, this variation is approximately

linear over a fairly wide temperature range, according to the expression

= o{1 + ( - o)} . (1).

Here is the temperature coefficient of resistivity and is the resistivity at some reference temperature, To (usually taken as

20oC). The same expression can be written for the resistance as a function of temperature,

R = Ro{1 + (T - To)} (2)

although the temperature coefficient in (2) will differ slightly from that in (1) as a result of thermal expansion of the material.

Use the small resistance coil (R about 600 near room temperature) connected to thetwin leads with banana connectors to investigate the temperature dependence of resistance. The

coil is made from an alloy having a larger resistance than common conductors such as Al and

Cu.

Measure the resistance of the coil at five different temperatures: a) room temperature;

b) the temperature of hot tap water; c) the temperature of an ice-water mixture (0 oC);

d) the temperature of dry ice in alcohol (-78.5oC); e) the temperature of liquid nitrogen (-

195.8oC). Be sure to record the temperatures.For b) and c) use the small beaker, tap water, and ice as needed.

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For d), and e), you must take your coil and meter to the containers provided on the front benches in the laboratory.

Do not dip much of the insulated leads into the temperature baths.

C. Thermistors The thermistor is made from a thermally-sensitive semiconductor with a large negative temperature

coefficient of resistivity, whose resistance R varies with the absolute temperature T as:

ln R = ln Ro + (1/T - 1/To) (3)

where T is the absolute temperature, To = 298K (25C), Ro is the resistance at temp To ,and is a constant which

is characteristic of the semiconducting material. (Note: ln R is the natural (base e) logarithm of R.) If a thermistoris built into a sensing probe and connected into a circuit which measures the resistance, an indicating meter can be

calibrated directly in C or F. No reference temperature is required. Having relatively high resistance,

thermistors can be used in low current circuits. That eliminates the need for heavy conducting wires on sensitive

measuring equipment. For the same reason thermistors may be used for remote applications and the length of cable

between the probe and the indicating meter is not critical.

Electronic clinical thermometers measure temperature by using a thermistor as the temperature sensing

element. The body temperature is read directly on a calibrated meter. The electronic clinical thermometer has

several advantages over the conventional mercury thermometer. Its response time is about 30 seconds as compared

to about 3 minutes for the mercury thermometer. The electronic thermometer is also easier to read and does not

remain at the measured temperature after use, in contrast to the mercury clinical thermometer which requires

"shaking down".

Measure the resistance of the thermistor for at least 6 temperatures starting with the

temperature of hot tap water. Partially fill the small beaker with hot tap water. Add ice to

produce lower temperatures. Use the thermometer provided to determine the water temperature.

Try to obtain readings at about 40C (104F) and 37C (98.6F) (i.e., in the body temperature

region). Spread the remaining readings over the region below 37oC down to 0C.

D. Body Resistance and Shock Hazard

D. 1. Measure the resistance between your right and left hands with the skin dry. Repeat for

wet skin.

D. 2. Fill the container with water from the tap. Measure the resistance of the tap water by placing

the metal portion of the banana plugs on the meter leads in the water and holding them a few cm

apart. Record the approximate separation of the leads. Repeat the measurement for a different

separation of the leads.

NOTE: The readings for resistance you obtain in this section may vary considerably for small changes in

conditions. Just try to get rough values and describe carefully in the lab report what you did.

Analysis : Do Part C and Part D in lab using lab Computers

Note: The accuracy of resistance readings with the BK 2806 meters is specified as (0.75% of input + 2 counts)for the 200 through 200 kranges, (1% + 2 counts) for the 2000 krange, and (2% + 2 counts) for the 20 M

range. This information may be used to determine experimental errors for your measurements. You must know the

appropriate range and the least count (smallest unit displayed, e.g. 0.1 on the 200 scale which reads 0 to 199.9

, 0.01 kon the 20 kscale which reads 0 to 19.99 k, etc.) of the range in order to determine the error.

A. 1. Determine the experimental error for each resistance measurement. (See above.) Are the

measured resistances of the four carbon resistors within tolerance of the color code values? Are

your measurements within tolerance of the color code values allowing for experimental errors?

From the wattage rating and the measured resistance, calculate the maximum voltage that can

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safely be applied across each resistor. Find also the maximum allowable current through each

resistor.

A. 2. Compare your measured value of Rser and the measured value of Rpar to the corresponding

expected values and their errors from your measured values for R1and R2 .

( For Parts B. and C below, Use lab computers and open Graphic Analysis Program. In computer

plots use R/R = 2% as estimate of errors.)B. Enter your measurements of resistance for the coil, the resistance R and the temperature T in oC

in the table and set vertical error bars to be 2% . Plot R versus T and make a linear fit. Print outthe plot foe each partner.

From Eq (2) which can be rewritten asR = (R0 R0T0 ) + R0T

we see that the slope is equal to R0 ,where R0 is the resistance at T=200C.

Determine and reading off Ro from the plot. (For comparison, for copper is +0.0068 K-1; for

aluminum, +0.0043 K-1; for carbon, -0.0005 K-1. Manganin, an alloy of Cu, Mn, and Ni, has a temperature coefficient

of resistance of only +0.000002 K-1.)

C. a) Enter your measurements of R, resistance for the thermistor, and T in C in the computer

table. Include vertical error bars on R. Plot R versus T . Comment on the shape of the resultingcurve.

C. b) To compare data with Eq.(3).open new columns in the table and define Z=ln(R) ,and another

column A=(1.0/(T+273)). Then plot Z versus A. Print out the plot. Are your points consistent with a

straight line as expected from Eq. (3)?

C. c) From your data in the region of body temperature (37o-40oC) determine the % change in

resistance for a 0.1C change in temperature. How does this compare to the corresponding value for

the coil studied in part B?

Questions for the report:

1) From the value you determined for in part B. b), can you draw any tentativeconclusions about the material from which the coil is made?

2) A few milliamperes (1 mA = 10-3 Amperes) is a serious shock hazard especially in the chest, while 100

mA = 1/10 A can be lethal. Calculate the current through your chest if you held a wire in each hand with

potential differences of 10 V, 100 V, and 1000 V between the wires. Assume both dry and wet

hands and discuss the nature of the shock for these voltages.

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Data Tables for Resistors and Thermistor

A. (1) Individual Resistance Measurement.

R from R measured Rmeas wattage Vmax Imax color code tolerance R

R1

R2

R3

R4

A. (2) Resistance Measurement for combined resistors.

nominal values connection calculated R Rcalc Rmeas Rmeas

of resistors made from nominal values

R1 =R2 =

in series

R1 =

R2 =

in

parallel

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B. Resistance of coil at various temperatures

temperature T R R

room temp

hot tap water

ice-water mix

dry ice in alcohol

liquid nitrogen

C. Measurement of thermistor resistance

temp T R R

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D. (1) Body Resistance

For dry skin R =

For wet skin R =

D. (2) Water Resistance

separation of the leads R R

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properties of resistors therm is tor characteristics (1)

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