edited by SHELDON H. COHEN
inflation fighter/ Low-Cost, High-Voltage, Two-Terminal, Constanr-Current Source for Student Use in Electrophoresis Experiments J. J. Meyer lnstitut de Topologie et de Oynamique des S y s t h e s UniversitC Paris VII associC au C.N.R.S. 1. rue Guy de la Brosse, 75005-PARIS, France
SDS (sodium dodecy1sulfate)-polyacrylamidegel electrophoresis is a widely used technique for the separation of proteins. I t is well known that the most effective separation is obtained with an apparatus powered by a constant-current source. The equivalent resistance of an electrophoresis slab gel apparatus carrying a current of about 10 mA commonly ranges from a few ohms to about 10 kQ. The increase of the gel slab resistance during the experiment requires a current source regulating within a wide voltage range. Tube regulated constant voltage supplies capable of 50 mA DC output a t voltages between zero and 250 V are still available in most laboratories. Taking advantage of this, we designed an inexpensive, two-terminal, constant-current regulator acting as a variable impedance inserted between the output of the high voltage regulator and the load. basic all^,',^ this circuit uses two PNP and NPN current sources (TI, T j ) connected so as to regulate each other's reference (CRe, Dl; CR4, D2). To meet the high voltage requirements we designed the current regulator with two heat-sink-mounted low-power transistors in a series stack (TI, T2: TQ,T4). The voltage dividers CR1, Rs, R4 (CRs, R?, Rs) maintain an average voltage of no more than 50-55 V across each transistor in the stack when the potential difference across the regulator is 120 V. Since the regulating current source is operated from transistor T I (T3),it is necessary that the collector-to-emitter voltage of T I (T3) be maintained for small voltage differences across the regulator. This is accomplished by two 1.24 V band gap reference diodes (CRI, CRd holding the collector of TI (Ti) a t about 2 V from the emitter of T I (Ta),if at least 100 PA flows through the reference diodes. The voltage divider carries
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Marcus, J.. "Sourcebook o f Electronic Circuits," McGraw-Hill, York, 1968, p. 171. Stock, J. T., J. CHEM.EDUC., 46, 858 (1969).
New
Washburn University Topeka.Kansas66621
about 0:8 mA a t maximum input voltage. Capacitor CI acrbss transistor T3 prevents instability in the circuit. The output currents (10 or 30 mA) are fixed hy resistors R1 (R6) or R2 (R4. Typical regulation of a 10 mA current is better than 1%for load changes from a few ohms to 10 kQ with the DC voltage source set at 120 V. The minimum and maximum voltages allowed across the current source for effective regulation are respectively 20 and 120 V.
Cost Saving Tips for Chemical Reagents Joseph M. Stefanko Central High School 8516 W. Lincoln Avenue West Allis, WI 53227
As a high school chemistry teacher I have become very concerned with budget expenditures. Our school budget is based on the number of students enrolled, and with declining enrollments, our budget is decreasing even though inflation has increased our supply costs. Therefore, we have been actively looking for other sources of supplies. In many experiments, it is not necessary to use reagent grade chemicals. Here are some of the adjustments we have made. 1. Marble chips are used to produce CO2. Nursery supply
stores and some hardware stores sell a white decorative stone. These are marble chips of good quality. 2. Elemental sulfur is needed in a number of experiments. Sulfur is available in fifty pound bags from industrial suppliers. This is good quality sulfur and can he purchased a t about 20% of the cost from chemical supply houses. (Contents should be transfered to safe containers for storage.) 3. Sodium chloride is needed in such experiments as the production of hydrogen chloride gas. The salt sold for the recharging of water softener resins is of good quality and is available in many grocery stores, especially those Methods and details of how you can save money by building your own equipment are ovtlined inthis feature. If you have made something which is particularly useful or money saving, please share it with our readers.
Design of two terminal constant current regulator
Volume 60
Number 2
February 1983
143
