George F. Atkinson
University of Waterloo Waterloo. Ontario Canada
II
Student Potentiometer Using a Zener Diode
In a recent paper, Battino1 described simple student potentiometers. Our potentiometer costs more than Battino's, but remains less expensive than a commercial model. I n addition, it illustrates the simplification of circuitry achieved by use of a zener diode power source. The circuit of the potentiometer in use in our lahoratory is shown in the figure. A range of 0 to 1 v, with a sensitivity of 1 mv, was desired. The Instrulab Evenvolt unit Model 201 was used as a voltage source because it was available, and does not represent a preference over similar units. The Helipot Model A 100-ohm potentiometer was found to be wound of fairly coarse wire, and gave detectable discrete jumps of voltage in the finished instrument, each corresponding to about mv. This represents the limit of resolu-
Figure 1. Potentiometer circuit. RI. selected (see t e x t ) ; R q Helipot A (10 turn, 100 ohm with 10-turn dioll; RZ, 1000 ohm; 2,lnrtrulab 201 Evenvolt unit (1.030 v a t 10 m a ) ; SIand Szr Automatic Electric s o t d o g no. DD-32, "quick-acting" push keys; S3,DPDT; #A, Beckman 074-6579 microammeter (100-0-100pa. 530 ohms).
'BATTINO, R., J. CHEM. EDUC.,42, 211(1965).
656 / Journol of Cherniccd Edumtion
tion, and lies within our specification. Initial adjustment of the supply loop of the circuit was done by selecting a wire-wound resistor for R1 to make the voltage drop across the Helipot exactly 1 v. Alternative methods would be the use of a screwdriver-adjustable small potentiometer (about 5 ohms) or the adjustment of the trimming screw on the zener source itself. The circuit was built on a sheet of acrylic plastic to expose the components. This was supported a t a suitable angle for reading the meter on a frame of Meccano girders to which the zener unit was bolted. The instrument was calibrated, and its sensitivity tested, by connection to a Leeds and Northrup 8691 millivolt potentiometer operating as a voltage source. Starting from balance, a movement of the voltage setting either of the new instrument or of the testing potentiometer produced a detectable meter deflection for mv. An imbalance of 1 mv showed a current of approximately 1 pa on the meter. As the use of zener sources in instruments is coming to be standard practice, we feel that this potentiometer serves as a useful introduction for the student to the circuit changes thus introduced. Suitable arrangements for range switching, etc., could adapt the basic circuit to various lab uses. One modification of interest from the standpoint of instrumentation teaching would be provision for a low resistance galvanometer to rcplace the more rugged hut less sensitive microammeter.
