Among the many many strategies out there for air circulate detection, self-heated thermal circulate sensors are easy, low-cost, rugged, and delicate. They depend on the connection between airspeed (AF) and thermal impedance (ZT = oC/W) of a heated sensor as proven within the empirical thermal impedance method beneath. It quantitatively relates junction temperature rise, energy dissipation, and air circulate velocity for a self-heated 2N4401 transistor within the conventional TO-92 package deal:
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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 circulate in ft/min
Determine 1 exhibits junction temperature vs air circulate predicted by the above expression for transistor energy dissipation of 320mW and air circulate velocity from zero (stagnant air) to 1000fpm (~11mph). Word that sensitivity is sweet even for very sluggish air speeds, e.g., the 50fpm (~1/2mph) level indicated.
Determine 1 TO92 junction temperature rise versus air velocity.
Determine 2 exhibits the right way to translate Determine 1’s math right into a sensible circuit, utilizing to advertvantage what’s usually considered a disadvantage of the traditional Darlington topology.
Determine 2 The flowrate sensor circuit.
Q1 performs the function of Determine 1’s self-heated sensor, and its tempco converts junction temperature into voltage at -1.5mV/oC. The LM10 200mV reference A1 regulates Q1 present to 0.2V/R3 = 67mA and thereby Q1’s energy dissipation to a relentless 67mA * 4.8V = 320mW. The ensuing junction temperature delta, as proven by Determine 1, supplies the airspeed readout because it falls from 64oC at 0fpm, to 25oC at 1000fpm, with a corresponding rise in junction voltage resulting from Q1’s Vbe tempco, from 0.654V @ 0fpm to 0.713V @ 1000fpm.
In fact, these numbers are relative to ambient temperature and their correct interpretation subsequently will depend on correct compensation for adjustments in ambient. That’s the place the Darlington connection and its “drawback” are available.
Since its invention by Sidney Darlington in 1953, the Darlington pair has been a well-liked topology due to the benefit supplied by the cascaded present positive aspects of the 2 transistors multiplying collectively. In the meantime, and normally considered a drawback of the Darlington, is that the “on voltages” (e.g., Vbe) of the pair unavoidably sum collectively. This Design Concept, against this, makes that unhealthy factor right into a good factor.
Each Vbe1 and Vbe2 comprise temperature-dependent parts proportional to self-heating (which is delicate to airspeed) and to ambient temperature (which isn’t). However, as a result of Q2’s energy dissipation is so very small (~1mW), its corresponding self-heating is << 1oC and may subsequently be safely ignored, making Vbe2 precisely dependent solely on ambient temperature and never airspeed.
Thus, the sign at Q2’s base is a reference that the R1-R2 voltage divider inputs to comparator A2, that tracks and cancels the results of ambient temperature change on Q1. The R1/R2 ratio accommodates the upper -2mV/oC tempco of Q2 versus Q1’s -1.5mV/oC (which ends from the Darlington present acquire and Q2’s consequent 150x decrease collector present) making A2’s differential comparability impartial of ambient temperature and influenced solely by airspeed.
Word that Q1’s web junction temperature (rise + ambient) stays beneath the 2N4401’s max rated 150oC in ambient temperatures as scorching as 700C, even in zero air circulate.
Bias resistor R4 supplies a voltage offset that cancels Q2’s decrease Vbe and units the air circulate threshold setpoint. The 220k proven units a 50fpm setpoint, however totally different circulate charges will be chosen with a easy change to R4; greater R4 = greater flowrate setpoint.
Determine 2’s Darlington-based circuit is strong and energy-efficient. Its whole energy draw is lower than 400mW.
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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