Range resistor

In order for electronic transmission systems to be less susceptible to interference from magnetic fields, current is used for the transmission signal instead of voltage. The signal range is 4 to 20 miUiamps. In each circuit or current loop, there can be only one transmitter. There can be more than one receiver, but not an unlimited number. For each receiver, a 250 ohm range resistor is inserted into the current loop, which provides a 1- to 5-volt input to the receiving device. The number of receivers is hmited by the power available from the transmitter. 

High level. Where the source is a process transmitter, the range resistor in the current loop converts the 4-20 milliamp signal into a 1-5 volt signal. The conversion equipment can be unipolar (i.e., capable of processing only positive voltages). 

In addition to the value of the range resistor Ri, other parameters also affect the effective bandwidth of the system. As shown in Fig. 3.16b, the bandwidth improves when the width of the depletion layer (W) increases. This is because a larger value... 

Fig. 3.16 Simulated impedance spectra (Bode plots) for the R(RC) circuit from Fig. 3.12c. The simulation includes a simple model that describes the bandwidth limitation of a (hypothetical) potentiostat. Several parameters are varied (a) the range resistor, Ri, (b) the depletion layer thickness W (c) the sample diameter ) (d) the resistance parallel to the space charge capacitance, Rsc- The series resistance Rs is fixed at 140 Q and Csc is calculated using Csc = (so Af h where Sr is the dielectric constant (25) and the surface area A is given by ll4nD. Note that the simulation in (a) does not show the increase in noise that one expects at less sensitive current ranges...

Thus, the unknown impedance can be determined directly from the value of the range resistor, Rr, and the attenuation factors a and b imposed by the null detector to achieve the null condition. 

The two Safety Channels are basically operational amplifiers (Kiethley 301) with switchable feedback resistors. The feedback resistors (called range resistors) change the sensitivity from... 

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