![]() Necessary cookies are absolutely essential for the website to function properly. Flow metering tutorial – Part 1: Understanding the fundamentals.Transistor and FVCs make linear anemometer.In all, a total of 64 submissions have been accepted since his first contribution was published in 1974. Stephen Woodward’s relationship with EDN’s DI column goes back quite a ways. The 220k shown sets a 50fpm setpoint, but different flow rates can be selected with a simple change to R4 higher R4 = higher flowrate setpoint ( Figure 3).įigure 3 The flow rate setpoint for different values of R4.įigure 2’s Darlington-based circuit is robust and energy-efficient. Note that Q1’s net junction temperature (rise + ambient) remains below the 2N4401’s max rated 150 oC in ambient temperatures as hot as 70 0C, even in zero air flow.īias resistor R4 provides a voltage offset that cancels Q2’s lower Vbe and sets the air flow threshold setpoint. The R1/R2 ratio accommodates the higher -2mV/ oC tempco of Q2 versus Q1’s -1.5mV/ oC (which results from the Darlington current gain and Q2’s consequent 150x lower collector current) making A2’s differential comparison independent of ambient temperature and influenced only by airspeed. Thus, the signal at Q2’s base is a reference that the R1-R2 voltage divider inputs to comparator A2, that tracks and cancels the effects of ambient temperature change on Q1. But, because Q2’s power dissipation is so very small (~1mW), its corresponding self-heating is << 1 oC and can therefore be safely ignored, making Vbe2 accurately dependent only on ambient temperature and not airspeed. This Design Idea, by contrast, makes that bad thing into a good thing.īoth Vbe1 and Vbe2 contain temperature-dependent components proportional to self-heating (which is sensitive to airspeed) and to ambient temperature (which isn’t). Meanwhile, and usually thought of as a disadvantage of the Darlington, is that the “on voltages” (e.g., Vbe) of the pair unavoidably sum together. Since its invention by Sidney Darlington in 1953, the Darlington pair has been a popular topology because of the advantage provided by the cascaded current gains of the two transistors multiplying together. That’s where the Darlington connection and its “disadvantage” come in. ![]() Of course, these numbers are relative to ambient temperature and their accurate interpretation therefore depends on accurate compensation for changes in ambient. ![]() The resulting junction temperature delta, as shown by Figure 1, provides the airspeed readout as it falls from 64 oC at 0fpm, to 25 oC at 1000fpm, with a corresponding rise in junction voltage due to Q1’s Vbe tempco, from 0.654V 0fpm to 0.713V 1000fpm. The LM10 200mV reference A1 regulates Q1 current to 0.2V/R3 = 67mA and thereby Q1’s power dissipation to a constant 67mA * 4.8V = 320mW. Q1 plays the role of Figure 1’s self-heated sensor, and its tempco converts junction temperature into voltage at -1.5mV/ oC. Note that sensitivity is good even for very slow air speeds, e.g., the 50fpm (~1/2mph) point indicated.įigure 1 TO92 junction temperature rise versus air speed.įigure 2 shows how to translate Figure 1’s math into a practical circuit, using to advantage what’s often thought of as a disadvantage of the classic Darlington topology. Figure 1 shows junction temperature vs air flow predicted by the above expression for transistor power dissipation of 320mW and air flow speed from zero (stagnant air) to 1000fpm (~11mph). ![]()
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