I not too long ago contributed a design for a easy platinum resistance temperature detector (PRTD) resistance two-wire 4-20mA transmitter circuit illustrated in Determine 1.
Determine 1 A two-wire, 4 to twenty mA present loop PRTD transmitter with 500 µA fixed present sensor excitation. R1 and R2 are 0.1% tolerance, voltage reference is a 2.5-V LM4040x25
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Analog to digital conversion of the 4 to twenty mA Io studying is likewise easy and easy (a 250-Ω shunt resistor on the enter of a 0 to 5-V ADC of ample decision and precision will do properly) and getting from there to Rprtd is a simple chore in software program (Io in milliamps):
PRTD resistance = R1(Io/Ix – 1) = 20Io – 10
The ultimate step from there to a linear temperature measurement could be nearly equally simple, due to Callendar Van Dusen (CVD) math, apart from one annoying element. The well-known CVD polynomial is organized to calculate PRTD resistance from temperature. Sadly, what we want is temperature from resistance!
Fortuitously, one other basic algebraic expression can experience to our rescue: The Quadratic System (QF).
Mixing vigorously CVD and QF and defining two constants:
u = 0.0039083 RPRTD@0oC
w = -0.0000005775 RPRTD@0oC
and one new variable,
x = RPRTD@0oC – PRTD
results in an easy polynomial that can instantly calculate a PRTD temperature from PRTD resistance that’s linear to inside ±0.05oC over a temperature vary spanning -80oC to +850oC.
ToC= (-u + (u2 – 4wx)1/2)/(2w)
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
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