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 circulation sensor as proven within the components 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 Legislation” thermal diffusion fixed = 0.75 mW/°C√fpm
AF = air circulation in ft/min
If the transistor junction is held at a continuing temperature differential above ambient (e.g., Dt = 31oC), the facility required to take action can be a operate of air pace P = 31/ZT as proven in Determine 1. Observe the annoying non-linearity.
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Determine 1 Energy dissipated versus air circulation of TO92 held at a continuing 31oC above ambient (Pw = 31/ZT).
Determine 2 reveals a sensible transportable thermostat circuit to realize and preserve 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 circulation 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 continuing 31oC. It does this (with the assistance of Darlington present acquire) by forcing Q1’s present draw (I) by means of R3 to drive Q1’s energy dissipation (Pw) to comply with the Determine 1 curve of heat-vs-air circulation. The ensuing voltage developed (IR3) is the premise of the air pace measurement.
Okay to this point. However how does compensation for Determine 1’s nonlinearity occur?
Nicely, it seems the operate of Q1’s Pw vs collector present, I, isn’t linear both. In reality:
Pw = 5vI – I2R3
That quadratic I2 time period could be very helpful. It’s chargeable for the beautiful 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 good, when inverted, offset, and scaled by op-amp A1, the realized output is a calibrated readout (1V = 100fpm) of air pace that differs from splendid 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 gradual air circulation is good for the measurement of cooling-fan forced-air distribution, air infiltration monitoring in HVAC installations, and plenty of comparable purposes the place the achieved measurement accuracy and vary are ample.
Dynamic response to modifications in airflow is sweet with a Q1 pressured thermal time fixed of about three seconds. Additionally, solid-state sensor sturdiness is healthier than that of delicate hot-wire sensors.
A element of Determine 2 worthy of point out is Q3, which I embrace 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. This can be a doubtlessly unhealthy 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 detrimental 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 protected stage till the transient passes and regular temperature regulation can get better.
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, detrimental rail for the op-amps. This easy however useful 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 printed in 1974.
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