Resistors, standard

The procedure is as follows. In switch position 1 and while repeatedly depressing tap key K, the variable resistor R, is adjusted once for each measurement to zero current through galvanometer G, so that the emf of the standard cell C8t (Weston 1.01832 V) becomes accurately compensated over the constant resistor Rj. Next, in switch position 2 the unknown emf of cell Cx is... 

In the direct-reading instruments the emf of the cell is led through an (operational) amplifier across a standard high resistor yielding a current that is measured by a milliammeter calibrated to be read in pH units or millovolts. So, while the null-point system provides a truly potentiometric (non-faradaic) measurement where the off-balance adjustment remains limited to an interrupted temporary current draw-off, the direct-reading system represents an amperometric measurement where a continuous steady-state current draw-off takes place as long as the meter is switched on. In fact, the latter is a deflection method as a pointer indicates the pH units or millivolts by its deflection on the meter scale. 

In principle, a standard cell is not required as the standard high resistor, provided that its resistance is stable, has taken over the calibration role. However, it is clear that the current draw-off will be larger in both its... 

For controlled-potential coulometry the voltage drop over a standard resistor is measured as a function of time by means of a voltage-to-frequency converter the output signal consists of a time-variant and integrally increasing number of counts (e.g., 10 counts mV-1), which by means of an operational amplifier-capacitor yields the current-time curve and integral158. 

Proper calibration of the DSC instruments is crucial. The basis of the enthalpy calibration is generally the enthalpy of fusion of a standard material [21,22], but electrical calibration is an alternative. A resistor is placed in or attached to the calorimeter cell and heat peaks are produced by electrical means just before and after a comparable effect caused by the sample. The different heat transfer conditions during calibration and measurement put limits on the improvement. DSCs are usually limited to temperatures from liquid nitrogen to 873 K, but recent instrumentation with maximum temperatures close to 1800 K is now commercially available. The accuracy of these instruments depends heavily on the instrumentation, on the calibration procedures, on the type of measurements to be performed, on the temperature regime and on the... 

Edward Weston (1850-1936) was a giant in the history of electrical measuring instruments. In the held of measurement, he developed three important components the standard cell, the manganin resistor and the electrical indicating instrument. 

The function of a simple three-electrode system can be understood from Fig. 18b.2a. The variable voltage source, Vs, is placed between the working and the counter electrodes so that the electrochemical reaction can take place on both electrodes at the applied excitation potential. The current response flowing during a redox reaction is monitored by the voltmeter across a standard resistor, Rt. Since we are only interested in... 

Figure 12. Plot of the Fe/ Fe ratio of an Fe standard, analyzed at 200 to 600 ppb concentrations, relative to the average Fe/ Fe of bracketing 400 ppb an Fe standard, versus the measured Fe volts (10 fl resistor). The measured Fe isotope composition varies relative to Fe concentration, which reflects differences in instrumental mass bias as a function of concentration. Data were taken over a 24 hour period using the University of Wisconsin-Madison Micromass IsoProbe.

Most of the devices used by PSpice can include temperature effects in the model. Most of the semiconductor models provided by Oread include temperature dependence. By default, the passive devices such as resistors, capacitors, and inductors do not include temperature dependence. To make these items include temperature effects, you will need to create models that include temperature effects. The temperature dependence of resistors is discussed in Section 4.G.I. In this section, we will show only how the I-V characteristic of a 1N5401 diode is affected by temperature. The D1N5401 diode model already includes temperature effects so we will not need to modify the model. We will use the standard resistor, which does not include temperature effects. We will continue with the circuit of Section 4.B ... 

The mean value of the distribution is 1000 and the standard deviation, a, is 12.5. A 1 kQ, 5% resistor will have values from 950 Q to 1050 Q. Deviations of 4o achieve this spread. This distribution shows us that almost all of the resistors are within plus or minus three standard deviations from the nominal value, 3[Pg.505]

The PSpice Gaussian distribution specifies the nominal value and the standard deviation. All part distributions are limited to 4a since the probability of finding a resistor outside this range is extremely small. The model below describes a 5% resistor with a Gaussian distribution ... 

The simulation says that the maximum value is. 5188, which is less than the expected value. Remember that for the resistor with the 5% Gaussian distribution, the standard deviation was 1.25%, and the absolute limits on the distribution were 4o = 5%. In the Worst Case analysis, a device with a Gaussian distribution is varied by only 3cr. Had we calculated the maximum value with a 3.75% resistor variation, we would have come up with a maximum gain of 0.51875, which agrees with the PSpice result. To obtain the worst case limits, I prefer to use the uniform distribution. Type CTRL-F4 to close the output file and display the schematic. 

The output is similar to the previous simulation except that the deviations from the nominal value are smaller. Only one run did not pass the specification. The run had a lower deviation of 0.0115, which corresponds to a gain of 0.5 - 0.0115 = 0.4885. Smaller deviations should be expected since, in the Gaussian distribution, the bulk of the resistors were within plus or minus one standard deviation. From the results of the two previous simulations, we conclude that the tolerance distribution has a large impact on the Monte Carlo results. 

Remember that in a Gaussian distribution, the tolerance specified is the standard deviation a, and the full distribution extends 4ct. Change both resistors to RIOgauss ... 

One inconvenience associated with using operational amplifiers is the dual positive and negative power supplies that are frequently required. For convenience, and to limit the power source of our electronic load box to one power supply, we will include the following circuit in our electronic load. The circuit takes a 15 V DC signal and converts it to a -11 V DC signal. The schematic for this circuit is shown in Fig. 5.9. Note that the values shown are actual lab values of circuit. Standard resistor values are shown in parentheses. 

---