© 2023
11
WHY5640 TEMPERATURE CONTROLLER
When calculating component values, keep in mind these
points:
• As k
T
becomes larger, choosing component values
becomes more difficult because larger C
L
values are
required.
• Keep R
L
as small as possible. Higher values of R
L
are more noisy, and values above 4 MΩ will impact
temperature control stability.
• As R
L
becomes smaller, C
L
must be larger. Use a non-
polarized capacitor for C
L
.
Use Equation 1 to calculate the Proportional Gain, k
P
k
P
= 4
( )
amps / volt
R
L
R
G
(1)
Use Equation 2 to calculate the Integrator Time Constant, t
C
t
C
=
( )
seconds
R
G
C
L
4
(2)
Use Equation 3 to calculate the relationship between k
P
, t
C
,
and k
T
.
k
Ƭ
= k
P
t
C
amp•seconds / volt
(3)
Use Equations 4, 5 , and 6 to relate k
T
to component values.
k
Ƭ
= C
L
R
L
amp•seconds / volt
(4)
(5)
(6)
STEP 5
Fine Tuning R
G
, R
L
, and C
L
The R
G
, R
L
, and C
L
component values can be fine-tuned
experimentally. Start with component values from
and operate the temperature controller system to determine
if the load temperature settling time is satisfactory. If it is not,
then follow these steps to fine-tune the component values.
1. Short C
L
to remove the integrator term.
2. Increase the proportional gain k
P
by increasing R
L
until the temperature begins to oscillate; this is the
Critical Gain value of the system. Measure the period
of the oscillation in seconds.
3. Decrease R
L
by half.
4. Use Equation 2 to calculate R
G
and C
L
so that the
value of t
C
is slightly greater than the oscillation
period measured above.
R
G
=
( )
ohms
4R
L
k
P
R
L
Gain
[ k
P
]
Sensor Type/
Thermal Load Speed
1.0 MΩ
5
Thermistor / Fast
Thermistor / Slow
2.0 MΩ
20
20
1.8 MΩ
50
50
RTD / Fast
RTD / Slow
2.2 MΩ
100
2.0 MΩ
AD590 or LM335 / Slow
AD590 or LM335 / Fast
4.0 MΩ
R
G
800 kΩ
400 kΩ
144 kΩ
88 kΩ
400 kΩ
320 kΩ
Integrator Time Constant
[ t
C
, seconds ]
C
L
15 µF
47 µF
15 µF
47 µF
10 µF
15 µF
3
4.5
1
0.53
4.5
1
Table 5. Recommended Gain and Integrator Values
R
G
=
( )
ohms
4t
C
C
L
