Williams 08 - NSC Application Notes.pdf - Książki - neo666x

Circuitry for Inexpensive

Relative Humidity

Measurement

Of all common environmental parameters, humidity is per-

haps the least understood and most difficult to measure. The

most common electronic humidity detection methods, albeit

highly accurate, are not obvious and tend to be expensive

and complex (See Box). Accurate humidity measurement is

vital to a number of diverse areas, including food processing,

paper and lumber production, pollution monitoring and

chemical manufacturing. Despite these and other applica-

tions, little design oriented material has appeared on circuitry

to measure humidity. This is primarily due to the small num-

ber of transducers available and a generally accepted notion

that they are difficult and expensive to signal condition.

Although not as accurate as other methods, the sensor

described by the response curve ( Figure 1 ) is inexpensive

and provides a direct readout of relative humidity. The curve

reveals a close exponential relationship between the sensor

and relative humidity spanning almost 4 decades of resis-

tance. Linearization of this curve may be accomplished by

taking the logarithm of the resistance value and utilizing

breakpoint approximation techniques to minimize the re-

sidual non-linearities. A further consideration in signal condi-

tioning is that the manufacturer specifies that no significant

DC current component may pass through the sensor. This

device must be excited with an unbiased AC waveform to

preclude detrimental electrochemical migration. In addition,

it has a 0.36 RH unit/˚C positive temperature coefficient. The

sensor is a chemically treated styrene copolymer which has

a surface layer whose resistivity varies with relative humidity.

Because the humidity sensitive portion of the sensor is at its

surface, time response is reasonably rapid and is on the

order of seconds.

National Semiconductor

Application Note 256

August 1981

00871301

FIGURE 1. Phys-Chemical Research Corp. Model

PCRC-55 Humidity Sensor

A block diagram of the concept chosen to instrument the

sensor appears in Figure 2 . An amplitude stabilized square

wave which is symmetrical about zero volts is used to pro-

vide a precision alternating current through the sensor, sat-

isfying the requirement for a zero DC component drive. The

current through the sensor is fed into a current sensitive (e.g.

the input is at virtual ground) logarithmic amplifier, which

linearizes sensor response. The output of the logarithmic

amplifier is scaled, rectified and filtered to provide a DC

output which represents relative humidity. Residual

non-linearity due to the sensors non-logarithmic response

below RH = 40% is compensated by breakpoint techniques

in this final stage.

00871302

FIGURE 2.

The detailed circuitry appears in Figure 3 . It is worth noting

that the entire function described in Figure 2 requires a small

number of inexpensive ICs. This is accomplished by novel

circuitry approaches, especially in the design of the logarith-

mic amplifier. The stabilized symmetrical square wave is

generated by A1, 1 ⁄ 4 of an LF347 quad amplifier. A1 is set up

in a positive feedback configuration, causing it to oscillate.

The output of A1 is current limited and clamped to ground for

either polarity output by the LM334 current source diode

bridge combination. The LM334 is programmed by the 15

across the 120

resistor string to stabilize at about

± 8V. Each time A1’s output changes state the charging

current into the 0.002 µF capacitor reverses, causing the

amplifier to switch again when the capacitor reaches a

threshold established by the 120 Ω –1.5 k Ω divider (wave-

forms, Figure 4 ). This circuit’s output is buffered by the A1

follower. The amplitude stability of the waveform is depen-

dent upon the +0.33%/˚C temperature coefficient of the

LM334. This T.C. has been intentionally designed into the

LM334 so that it may be used in temperature sensing and

compensation applications. Here, the negative 0.3%/˚C tem-

Ω

–1.5 k

Ω

resistor to current limit at about 5 mA. This forces the voltage

AN008713

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perature dependence of the humidity sensor is reduced by

more than an order of magnitude by the LM334’s T.C. and

thermally induced inaccuracy in the humidity sensor’s re-

sponse drops out as an error term. In practice, the LM334

should be mounted in proximity to the humidity sensor. The

residual −0.03%/˚C temperature coefficient is negligibly

small compared to the sensors ± 1% accuracy specification.

The output square wave is used to drive current through the

sensor and into the summing junction of another 1 ⁄ 4 of A1,

which is connected as a logarithmic amplifier. On negative

cycles of the input waveform the transistor (Q1) in the feed-

back loop provides logarithmic response, due to the well

known relationship between V BE and collector current in

transistors. During positive excursions of the input waveform

the diode provides feedback to the amplifier’s summing junc-

tion. In this manner the summing junction always remains at

virtual ground while the input current is expressed in loga-

rithmic form by the negative going square wave at the tran-

sistor emitter. Since the summing junction is always at

ground potential the sensor sees the required symmetrical

drive (waveforms, Figure 5 ).

00871303

FIGURE 3.

The output of this stage is fed to another 1 ⁄ 4 of A1. This

amplifier is used to sum in the 40% RH trim and provide

adjustable gain to set the 100% RH trim. The output is

filtered to DC and routed to one half of A2, an LF353, which

unloads the filter and provides additional gain and the final

output.

The other 1 ⁄ 2 of A2 is used to compensate the sensor depar-

ture from logarithmic conformity below 40% RH ( Figure 1 ).

This is accomplished by changing the gain of the output

amplifier for RH readings below 40%. The input to the output

amplifier is sensed by the breakpoint amplifier. When this

input goes below RH = 40% (about 0.36V at the output

amplifiers “+” terminal) the breakpoint amplifier swings posi-

tive. This turns on the 2N2222A, causing the required gain

change to occur at the output amplifier. For RH values above

40% the transistor is off and the circuits linearizing function is

determined solely by the logarithmic amplifier.

In logarithmic configurations such as this, Q1’s DC operating

point will vary wildly with temperature and the circuit nor-

mally requires careful attention to temperature compensa-

tion, resulting in the expense associated with logarithmic

amplifiers. Here, A3, an LM389 audio amplifier IC which also

contains three discrete transistors, is used in an unorthodox

configuration to eliminate all temperature compensation re-

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quirements. In addition, the cost of the log function is re-

duced by an order of magnitude compared to available ICs

and modules. Q3 functions as a chip temperature sensor

while Q2 serves as a heater. The amplifier senses the tem-

perature dependent V BE of Q3 and drives Q2 to servo the

chip temperature to the set-point established by the

10 k

divider string. The LM329 reference ensures

power supply independence of the temperature control. Q1

operates in this tightly controlled thermal environment (typi-

cally 50˚C) and is immune to ambient temperature shifts.

The LM340L 12V regulator ensures safe operation of the

LM389, a 12V device. The zener at the base of Q2 prevents

servo lock-up during circuit start-up. Because of the small

size of the chip, warm-up is quick and power consumption

low. Figure 6 shows the thermal servo’s performance for a

step function of 7˚C change in set-point. The step is shown in

trace A while the LM389 output appears in trace B. The

output responds almost instantaneously and complete set-

tling to the new set-point occurs within 100 ms.

To adjust this circuit, ground the base of Q2, apply circuit

power and measure the collector potential of Q3, at known

room temperature. Next, calculate what Q3’s collector po-

tential will be at 50˚C, allowing −2.2 mV/˚C. Select the 1k

value to yield a voltage close to the calculated 50˚C potential

at the LM389’s negative input. This can be a fairly loose trim,

as the exact chip temperature is unimportant so long as it is

stable. Finally, unground Q2’s base and the circuit will servo.

This may be functionally checked by reading Q3’s collector

voltage and noting stability within 100 µV (0.05˚C) while

blowing on A3.

To calibrate the circuit for RH, place a 35 k

Ω

–1 k

Ω

00871304

FIGURE 4.

Ω

resistor in the

sensor position and trim the 150 k

Ω

pot for an output of 10V.

Next, substitute an 8 M

resistor for the sensor and trim the

10k potentiometer for an output of 4V. Repeat this procedure

until the adjustments do not interfere with each other. Finally,

substitute a 60 M

Ω

resistor for the sensor and select the

nominal 40 k

value in the breakpoint amplifier for a reading

of RH = 24%. It may be necessary to select the 1.5 M

Ω

00871305

Ω

value to minimize “hop” at the circuit output when the break-

point is activated. The circuit is now calibrated and will read

ambient relative humidity when the PCRC-55 sensor is con-

nected.

FIGURE 5.

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Some of the more common ways of expressing humidity

related information include wet bulb temperature, dew point

and frost point. Wet bulb temperature refers to the minimum

temperature reached by a wetted thermometer bulb in a

stream of air. The dew point is the point at which water

saturation occurs in air. It is evidenced by water condensa-

tion. When temperatures below 0˚C are required to produce

this phenomenon it is called the frost point.

Other measurements and ways of expressing humidity exist

and are useful in a variety of applications. For additional

information consult the bibliography.

Bibliography

“Humidity Sensors”—brochure describing Models

PCRC-11 and PCRC-55 Relative Humidity Sensors.

Phys-Chemical Research Corp.; New York.

00871306

“Humidity Measurement”— Instrumentation Technology ;

reprint P. R. Wiederhold. Available from General Eastern

Corp.; Watertown, Mass.

FIGURE 6.

Humidity

Humidity is simply water gas. In air the humidity may vary

from zero percent for 90˚F dry air to as much as 4.5 percent

for heavily water laden air at 90˚F. The amount of water air

will hold is dependent upon temperature. Relative humidity is

an expression denoting the ratio of water vapor in the air to

the amount possible in saturated air at the same tempera-

ture.

“Handbook of Transducers for Electronic Measuring

Systems”—Norton, Harry N.; Prentice Hall, Inc.; 1969.

“Electric Hygrometers”—Wexler, A.; NBS Circular 586;

NBS Washington, D.C. 1957.

“An elegant 6-IC circuit gauges relative

humidity”—Williams, James M.; EDN Magazine , June

5, 1980.

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT

DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL

COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant

into the body, or (b) support or sustain life, and

whose failure to perform when properly used in

accordance with instructions for use provided in the

labeling, can be reasonably expected to result in a

significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform

can be reasonably expected to cause the failure of

the life support device or system, or to affect

its

safety or effectiveness.

National Semiconductor

Corporation

Americas

Email: support@nsc.com

National Semiconductor

Europe

National Semiconductor

Asia Pacific Customer

Response Group

Tel: 65-2544466

Fax: 65-2504466

Email: ap.support@nsc.com

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Japan Ltd.

Tel: 81-3-5639-7560

Fax: 81-3-5639-7507

Fax: +49 (0) 180-530 85 86

Email: europe.support@nsc.com

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www.national.com

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

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