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The temperature coefficient of a Zener is determined by placing the instrument in a temperature-controlled pressure-tight enclosure and varying the external temperature. Output voltages are measured while monitoring the internal temperature indicated by the built-in thermistor. Figure 1 shows the result of a typical measurement.
| Figure 1. |
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The Zener output voltage, U, can be expressed as a function of time, t, and thermistor resistance, R, by
U(t, R) = U0 [1 + k(t – t0) +  R(R –
R0)]
where k is a constant drift rate and R is a constant coefficient relating the relative change in output voltage to the thermistor resistance; U0, t0 and R0 are constants.
Figure 2 summarizes the results. It shows the relative temperature coefficients (TC) of fifteen 732B instruments as a function both of the thermistor resistance and of the external temperature immediately surrounding the instrument. In this plot, every point for which the 1.018 V uncertainty bar (red) does not intersect the vertical axis (which corresponds to a TC of the 1.018 V output of zero) represents an instrument having a statistically significant TC (at the 1  level) for the 1.018 V output. Similarly, every point for which the 10 V uncertainty bar (blue) does not intersect the horizontal axis (which corresponds to a TC of the 10 V output of zero) represents an instrument having a statistically significant TC for the 10 V output.
| Figure 2. |
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Details of the TC measurements are given in the following reference:
 Witt, T.J., Measurements of the temperature dependence of the output voltages of some Zener diode based electronic voltage standards, IEE Proc.-Sci. Meas. Technol., 1998, 145, 154-158.
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Summary
