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Next determine RB (which is the parallel equivalent of R1,R2) and then R1 and R2 as shown. These values are not “real” so far as preferred value resistors are concerned and so need to be rationalized. They represent only limit (worst case) values with plenty of scope to change them provided adherence to three basic rules: 1) R1 and R2 can be changed as long as the relative ratios are maintained. 2) R1 or R2 can be changed as long as this only results in reducing the ratio of R1/(R1+R2). 3) Do not reduce the ratio so low that the voltage across R1 is lower than the minimum operating voltage of the ZXCT1010. The first thing to note is that R2 is quite large at 38.88MΩ. While such high values are not impossible to obtain, they are much less common. Beyond 10MΩ it gets more and more difficult to obtain many types of resistors. Therefore, R2 can be 3.9MΩ for example, which results in an R1 value of 557kΩ (or the nearest lower preferred value of 510kΩ within the E24 resistor series).
