This self-heated constant-temperature-delta transistor anemometer is reasonable, rugged, and delicate. It depends on the connection between airspeed (AF) and thermal impedance (ZT = oC/W) of a heated air stream sensor as proven within the method under for a 2N4401 transistor in TO-92:
ZT = ZJ + 1/(SC + KT √AF )
the place:
ZJ = junction-to-case thermal impedance = 44°C/W
SC = still-air case-to-ambient conductivity = 6.4 mW/°C
KT = “King’s Regulation” thermal diffusion fixed = 0.75 mW/°C√fpm
AF = air stream in ft/min
If the transistor junction is held at a relentless temperature differential above ambient (e.g., Dt = 31oC), the facility required to take action shall be a perform of air velocity P = 31/ZT as proven in Determine 1. Notice the annoying non-linearity.
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Determine 1 Energy dissipated versus air stream of TO92 held at a relentless 31oC above ambient (Pw = 31/ZT).
Determine 2 reveals a sensible transportable thermostat circuit to attain and keep this delta-T using a Darlington transistor pair (Q1 and Q2) to compensate for ambient temperature and convert the ensuing nonlinear Pw curve right into a linearized anemometer air stream readout.
Determine 2 Linearized transportable Darlington anemometer schematic.
Right here’s the way it works.
Q1 serves because the self-heated sensor modeled within the Determine 1 math, with Q2 offering ambient temperature compensation. Op-amp A2 runs a suggestions loop that forces the Vbe differential between Q1 and Q2 (and thus the temperature differential between Q1 and ambient) to carry a relentless 31oC. It does this (with the assistance of Darlington present achieve) by forcing Q1’s present draw (I) via R3 to drive Q1’s energy dissipation (Pw) to comply with the Determine 1 curve of heat-vs-air stream. The ensuing voltage developed (IR3) is the idea of the air velocity measurement.
Okay thus far. However how does compensation for Determine 1’s nonlinearity occur?
Effectively, it seems the perform of Q1’s Pw vs collector present, I, isn’t linear both. In reality:
Pw = 5vI – I2R3
That quadratic I2 time period may be very helpful. It’s chargeable for the stunning curve proven in Determine 3.
Determine 3 Q1 energy versus collector present.
The twond-order curvature of Determine 3 is what compensates for the bend in Determine 1. Though the match isn’t excellent, when inverted, offset, and scaled by op-amp A1, the realized output is a calibrated readout (1V = 100fpm) of air velocity that differs from superb by lower than +/- 5% from 0 to 250fpm, as proven in Determine 4.
Determine 4 Darlington anemometer output versus precise airspeed.
The ensuing sensitivity to comparatively sluggish air stream is good for the measurement of cooling-fan forced-air distribution, air infiltration monitoring in HVAC installations, and lots of comparable purposes the place the achieved measurement accuracy and vary are sufficient.
Dynamic response to modifications in airflow is sweet with a Q1 compelled thermal time fixed of about three seconds. Additionally, solid-state sensor sturdiness is best than that of delicate hot-wire sensors.
A element of Determine 2 worthy of point out is Q3, which I embody to preclude the opportunity of the A2 suggestions loop getting “caught” when a transient or different misadventure would possibly trigger R3 voltage drop to exceed 2.5 V. It is a doubtlessly dangerous factor as a result of the Pw vs I curve would go “over-the-top” and invert the I vs Pw suggestions time period from unfavourable to constructive, inflicting A2’s output to latch with the Darlington saturated and keep thus caught for so long as energy is supplied.
If saturation approaches, Q3 conducts and forces A2 to restrict Darlington drive to a secure degree till the transient passes and regular temperature regulation can recuperate.
One other helpful element is “upside-down” regulator U1 which offers not solely essential stability for the 5 V energy rail, but additionally “splits” enter energy and offers an unregulated, however nonetheless helpful, unfavourable rail for the op-amps. This straightforward however helpful trick is described in an earlier Design Thought.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a methods. In all, a complete of 64 submissions have been accepted since his first contribution was revealed in 1974.
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