LINEAR LTC2991 User Guide

System Monitor with Instrumentation-Grade Accuracy Used to Measure Relative Humidity Leo Chen

The LTC2991 is designed to measure supply voltages, currents and temperatures on large circuit boards when used in system monitor applications. It is also capable of delivering ±1°C accuracy when using a 1-cent MMBT3904 transistor as a temperature sensor, making it suitable for many instrumentation applications. Temperature is the most measured physical parameter, with sensor selection a function of accuracy requirements, durability, cost and compatibility with the medium being measured. An inexpensive NPN transistor is an ideal sensor for applications calling for disposable sensors, or those that require a large number of sensors.

A PSYCHROMETER: NOT NEARLY AS −6 3. Figure 1 shows the LTC2991-based psy-

•( WET= )−

OMINOUS AS IT SOUNDS

chrometer. The two transistors provide

A psychrometer is a type of hygrometer,

WET= the wet bulb and dry bulb tempera-

a device that measures relative humidity.

ture readings when connected to the

It uses two thermometers, one dry (dry ED ESWB DRY −( WET=

appropriate inputs of the LTC2991.

bulb) and one covered in a fabric satu-

The equations include atmospheric pres-

rated with distilled water (wet bulb). Air is DRY=

sure as a variable, which is determined

passed over both thermometers, either by

ED here via a Novasensor NPP301-100

a fan or by swinging the instrument, as in HUMIDITY

a “sling psychrometer.” A psychrometric chart is then used to calculate humidity

from the dry and wet bulb temperatures. VCC

Alternatively, a number of equations exist 0.1µF

WET BULB:

for this purpose. The following equa-

COVERED WITH DAMP FABRIC

tions are used in testing this circuit.

V1 PWM

WET = wet bulb temperature in Celcius V2

V3 SCL DC590B

DRY = dry bulb temperature in Celcius

Q2 V4 LTC2991 SDA

Quik Eval™ II

P = pressure in k Pa

V5 DEMONSTRATION

DRY BULB

CIRCUIT

V6 ADR0 V7 ADR1

V8 ADR2

Figure 1. Simple psychrometer using the LTC2991

Q1, Q2: MMBT3904

PC WITH

Quik Eval II PRESSURE SENSOR SOFTWARE

–IN NOVASENSOR NPP301S8

(INCLUDING PSYCHROMETER EASTER EGG)

28 | January 2012 : LT Journal of Analog Innovation