QUESTIONS & ANSWERS
■ FIGURE 9
temperature (75-180° F) can be easily
monitored by strapping a sensor on
to the outlet water copper pipe. The
required temperature setting could be
as simple as a potentiometer with
experimentation and an external mechanical thermometer for determining
the proper setting. The actuating
motor is 24 VAC and rotates quite
slowly, and is probably best interfaced
by a contact closure. The system
temperature as a whole also responds
quite slowly, so the temperature sampling rate could be quite infrequent.
An override input to force the valve
to full open (or close) would also
be quite convenient. Can you
please suggest a starting point for
an experimental circuit? Can this
be easily accomplished PIC-less?
— Tim Bates
is fixed at five seconds by R10. The
resistance of the thermistor is 100K at
25° C (about 77° F) and 12K at 80° C
(about 176° F). The corresponding
set voltages at R2 will be 2.1 VDC
and 0.3 VDC. When the water
temperature is higher than the set
temperature, the resistance of the
thermistor is low, pulling pin 5 of
IC1B low and turning on RLY2. When
the water temperature is lower, the
resistance of the thermistor is high,
allowing pin 2 of IC1A to go high,
turning on RLY1. You may want to
swap the connection to the motor
valve, depending on your configuration. The G3VM-41GR5 solid-state
relay is rated 300 mA max; if your
motor valve requires more current,
other relays are available, such as
Mouser 849-CPC1979J, rated 600
VAC, 3A. The parts list is Figure 10.
You will want to adjust the on
and off times of the motor such that
the temperature stabilizes quickly
but does not oscillate around the set
temperature. There is a dead time
(hysteresis) determined by R4 of
about 1° F where the motor is not
turning CW or CCW. NV
AIn Figure 9, Q1 is an
adjustable constant
current source charging
C1 through D2. The 555
output will be high, allowing the
motor to run during the charge
time. The range of charge time is
very short to five seconds. The
discharge time (motor not running)
TEMPERATURE CONTROL PARTS LIST
PART DESCRIPTION
■ IC1 Dual comparator
■ IC2 555 timer
■ Q1 PNP, 60V, 100 ma
■ D1, D2 Gen-purp. diode, 50V 10 ma
■ RLY1, RLY2 Solid-state relay, 40V, 300 ma
■ C1 3. 3 μF, 50V, film
■ R1, R3 470K, 1/4W, 5%
■ R2 100K potientometer, linear
■ R4, R7 1K, 1/4W, 5%
■ R5 Thermistor, 100K
■ R6, R8 10K, 1/4W, 5%
■ R9 500K potientometer, linear
■ R10 2.2M, 1/4W, 5%
■ R11 470 ohm, 1/4W, 5%
■ FIGURE 10
MOUSER P/N
512-LM393N
512-LM555CN
512-2N3906TFR
512-1N4148T26A
653-G3VM-41GR5
871-B32529D5335K189
271-470K-RC
317-2090F-100K
271-1K-RC
871-B57560G104F
271-10K-RC
317-2090F-500K
271-2.2M-RC
271-470-RC
September 2008 29
