Honeywell AUTOMATIC CONTROL User Manual

ENGINEERING MANUAL OF AUTOMATION CONTROL
ELECTRONIC CONTROL FUNDAMENTALS
124
Another material used in RTD sensors is platinum. It is
linear in response and stable over time. In some applications
a short length of wire is used to provide a nominal resistance
of 100 ohms. However, with a lo w resistance value, the
temper atur e indica tion can be ef fected b y element self hea ting
and sensor lead wire resistance . Additionall y, due to the small
amount of resistance change of the element, additional
amplification m ust be used to incr ease the signal le vel.
To use the desir able char acter istics of pla tinum and minimiz e
any offset, one man ufactur ing technique de posits a film of
platinum in a lad der pa tter n on an insula ting base . A laser
tr imming method (F ig. 4) then b ur ns away a por tion of the metal
to calibr ate the sensor , pr oviding a r esistance of 1000 ohms a t
74F. This pla tinum film sensor pr ovides a high r esistance-to-
temper atur e relationship. With its high r esistance , the sensor is
relatively immune to self-hea ting and sensor lead wire resistance
offsets. In ad dition, the sensor is an e xtr emely low-mass de vice
and r esponds quic kly to chang es in temper atur e. RTD elements
of this type ar e common. Ear ly thin f ilm pla tinum R TDs dr ifted
due to their high surf ace-to-volume r atio which made them
sensitive to contamina tion. Impr oved packaging and f ilm
isolation ha ve elimina ted these pr oblems r esulting in incr eased
use of pla tinum R TDs over wir e wound and NTC ther mistor s.
LADDER NETWORK OF
METALLIC FILM RESISTOR
LASER TRIM (INDICATEDBY
GAPS IN LADDER NETWORK)
CONNECTION PADS
C3098
Fig. 4. Platinum Element RTD Sensor.
Solid-State Resistance Temperature Devices
Figur e 5 shows examples of solid-sta te r esistance temper atur e
sensor s having negative and positi ve temper atur e coefficients.
Thermistor s are negative temper atur e coefficient sensor s
typicall y enclosed in very small cases (similar to a glass diode
or small tr ansistor) and pr ovide quick response. As the
temperature increases, the resistance of a thermistor decreases
(Fig. 6). Selection of a thermistor sensor must consider the
highly nonlinear temperature/resistance characteristic.
POSITIVE RTD
THERMISTORS
C3077
Fig. 6. Resistance vs Temperature Relationship
for Solid-State Sensors.
Positive temper atur e coefficient solid-sta te temper atur e
sensors may have relatively high r esistance values a t room
temper atur e. As the temper atur e incr eases, the r esistance of the
sensor incr eases (Fig. 6). Some solid-sta te sensor s have near
perfect linear c har acter istics over their usa ble temper atur e rang e.
TEMPERATURE (DEGREES)
o
F
30
40 50 60 70
80
90
100
110
0
10
20
30
40
RESISTANCE (OHMS)
20K OHM AT
77
o
F (25
o
C)
80K
70K
60K
50K
40K
30K
20K
10K
o
C
TEMPERATURE (°F)
POSITIVE RTD RESISTANCE ( )
050100 200150
M15034
POSITIVE RTD
20K OHM NTC THERMISTOR
Fig. 5. Solid-State Temperature Sensors.

Honeywell AUTOMATIC CONTROL Specifications

General

Brand	Honeywell
Model	AUTOMATIC CONTROL
Category	Controller
Language	English