FIGURE 5.18Thermal sensing techniqu

FIGURE 5.18
Thermal sensing technique. (Courtesy of Fluke Corp. Reproduced with Permission-)
it into a DC voltage level. The precision rectifier acts as an ideal diode with no forward voltage drop. An AID converter digitizes the output. A microprocessor makes any necessary corrections for calculating rms values and for any necessary correcting factors. This type of input section assumes that the input signal is a pure sine wave (and the correction factor is 1.11 X Vavg as discussed in Chapter 1).
True ims converters use one of three ac-to-dc conversion methods. These are: (1) thermal sensing (Fig. 5.18); (2) log/antilog analog computation (Fig. 5.19); and (3) digital sampling (Fig. 5.20). In the thermal-sensing converter the output of the range amplifier is applied to a resistor that heats the base of the transistor on the left side of the rms sensor. The transistor’s collector current is increased as a result, and this unbalances the differential amplifier depicted above the two sensor transistors. The differential amplifier supplies a dc current through the resistor on the right side of the sensor. The heat in this resistor increases the collector current in the right transistor and restores the balance of
the differential amplifier. The dc voltage drop across the right-hand resistor is proportional to the true rms ac voltage drop across the left-hand rsis1sr and is digitized by the