ELECTRONIC DESIGN • January 22, 2001より引用
W. Stephen Woodward
University of North Carolina, Venable Hall, CB3300, Chapel Hill, NC 27599-3300; e-mail: woodward@unc.edu
CIRCLE 522
Explicit airflow detection is essential in many applications. High
power-density electronics are
liable to overheat and self-destruct
when cooling-fan failures go unnoticed. Heating and air-conditioning
systems often incorporate multipoint
monitoring of ventilation-duct flow.
Clean-room air-handling systems with
undetected dirty, blocked air filters can
ruin process yield. Laboratory fume
hoods can contain volatile solvents or
toxic reagents, making adequate air
turn-over critical to safety.
In these and similar scenarios, the
consequences of undetected airflow
interruption can range from the merely
expensive to the frankly dangerous.
Therefore, it becomes necessary to use
some reliable means for airflow detection. Usually, either a mechanical pressure-actuated vane switch or one of the
various types of heat-transfer-based airflow sensors is employed.
An advantage of thermal sensors is
that they contain no moving parts. But
they often require several watts of heating input to run hot enough to overcome ambient temperature variations.
The detector described here is a power
thrifty member of the thermal genre. It
employs an ambient-compensated airflow-detection scheme based on differential heating of a series-connected
transistor pair (Fig. 1).
In operation, 200-mV reference regulator A1 maintains a constant Q1/Q2
current drive equal to 40 mA (i.e., 200
mV/R1). Since the two transistors pass
the same current, their relative power
dissipations are determined solely by
their respective VCE voltages. For the circuit constants shown, these power levels work out to 4 V × 40 mA = 160 mW
for Q1 and 0.75 V × 40 mA = 30 mW
for Q2. The 130-mW heat-flow difference leads to a temperature difference
determined by the heat-dissipation-ver
sus-airspeed characteristics of the
2N4401’s plastic TO-92 package. The
TO-92’s thermal-impedance-versus
airspeed characteristic is well approximated by the simple equation shown in
Figure 2:
ZT = ZJ + 1/(SC + KT√AF )
where:
ZT = “total immersion” junction-tocase thermal impedance = 44°C/W
SC = still-air case-to-ambient conductivity = 6.4 mW/°C
KT = “King’s Law” thermal diffusion
constant = 750 µW/°C−√fpm
AF = airspeed in ft./min.
Therefore, the Q1/Q2 temperature
differential ranges from 130 mW ×
200°C/W = 26°C at 0 fpm (zero flow),
to 130 mW × 75°C/W = 10°C at 1200
fpm (the 14-mph breeze found at the
output face of a typical 100-cfm cooling
fan). This flow-dependent temperature
differential gives rise to a flow-dependent VBE differential via the 2N4401’s
typical-transistor VBE temperature coefficient of −2 mV/°C. Comparator A2
