±4 TO ±18 VOLT, 500 MILLIAMPERE D.C. POWER SUPPLY

DESCRIPTION:  This is a simple, general-purpose power supply intended for laboratory applications that require both a positive and negative current. The circuit uses commonly-available integrated circuit regulators that provide integral thermal overload protection as well as safe-area limiting. These devices also provide regulation characteristics which are better than standard, fixed-voltage, three terminal regulators such as types 7812 and 7912. The circuit features independent controls for each polarity output, as opposed to a "tracking" type supply, in which the positive and negative outputs are simultaneously adjusted with one control. Optionally, the load may be connected exclusive of the "OUTPUT COMMON" terminal to furnish a single output up to 36 volts. Since this supply has no provision for adjustable current limiting, the user is advised to take precautions when connecting sensitive loads such as integrated circuits, which may be damaged from overvoltage or incorrect polarity.

The 500 milliampere output current specification is based on worst-case conditions of minimum voltage settings (±4 volts) and maximum load. Under these conditions, power dissipation, expressed by the formula Pd = Vin-Vout(Iload), will be about 11 watts per regulator, whereas the maximum allowable dissipation for each regulator is 15 watts. For this calculation, Vin is given as:  (transformer RMS secondary voltage @ 1 ampere x 0.707) - Vdiode, which yields approximately 21.5 volts. Under more typical conditions, with the outputs set at ±12 volts, the voltage differential across each regulator will be considerably less. For these conditions, the allowable current may be increased to 1 ampere without difficulty.

The output specification of ±4 to ±18 volts is typical, and subject to component tolerances that include the reference voltage and adjustment terminal current tolerances of the VR1 and VR2 regulators. Because of these tolerances, the minimum and maximum output voltages may vary several per cent. For that reason, the nominal ±4 to ±18 volt range was selected to guarantee the most commonly-required minimum of ±5, and maximum of ±15 volts.

It is recommended that suitable voltmeters are connected to the supply's outputs in order to provide a convenient indication of the settings. For applications requiring fixed voltages, RV1, R3, and R5 may be replaced with an appropriate fixed-value resistors for the POSITIVE OUTPUT, and RV2, R4, and R6 may be replaced for the NEGATIVE OUTPUT. With the potentiometers connected as shown in the schematic, the magnitude of the output voltages increase with clockwise rotation. The output voltages are given as:

The transformer indicated in the Parts Table has two separate primary windings for 110 to 120 volts each. In the schematic, they are shown connected in series for applications using 220 to 240 volt mains. For applications using 110 to 120 volt (North American) mains, the primary windings are connected in parallel, instead.

CONSTRUCTION CONSIDERATIONS:  Note that VR1, the National Semiconductor LM317T positive voltage regulator, has a metal mounting tab which is connected to its center "OUTPUT" pin. To allow optimal heat removal from the device, VR1 is mounted directly to a radiator (Thermalloy type 6300B) without an insulating kit. This causes the radiator to be electrically connected to the device's OUTPUT pin. Accordingly, VR1's radiator must be isolated from all other conductors, including any grounded chassis or enclosure.

Similarly, VR2, the National Semiconductor LM337T negative voltage regulator, has a metal mounting tab connected to its center "INPUT" pin. VR2 is mounted to a separate radiator, and that radiator will be connected to the device's INPUT pin. Accordingly, VR2's radiator must also be isolated from all other conductors, including VR1's radiator, a grounded chassis, or enclosure.

The most important consideration for radiator selection is thermal resistance (Rth). The Thermalloy type 6300B radiators indicated in the Parts Table were chosen for their low thermal resistance of 3.1°C/W. Given the junction-to-case thermal resistance of 4°C/W for VR1 and VR2, the 3.1°C/W thermal resistance of the radiators, and an ambient temperature of 25°C, the predicted junction temperature per regulator, under worst-case conditions (±4 volts at 500 milliamperes), is expressed as: Iout(Vin-Vout)(Rth radiator + Rth junction-to-case) + Tambient. Solving this equation yields a per-regulator junction temperature of about 100°C. The maximum allowable junction temperature for the regulators is 125°C, so the predicted value is well within safe operating limits.

The Thermalloy 6300B radiator dimensions are 2.5"H x 1.65"W x 1"D. They are intended for vertical mounting to a printed circuit board, facilitated with two solderable pins at their bases, resulting in a footprint of 1.65" x 1". The 6300B is furnished with a mounting hole for the regulator.

If printed circuit construction is impractical, ABL type 345AB1000B vertical-fin extrusion radiators may be used instead. These radiators are more massive (1.46"H x 4.73"W x 3.94"D) than the Thermalloy units, providing the advantage of lower thermal resistance (Rth = 1.0°C/W). They may be mounted in either of two orientations, resulting in a footprint of either 4.73" x 3.94" (regulator horizontal) or 1.46" x 4.73" (regulator vertical). The 345AB1000B will require insulated mounting hardware for attachment to a chassis, which may be facilitated with any suitable combination of Nylon screws, washers, and, (for the regulator-vertical orientation), right-angle metal brackets. The ABL units are furnished without holes, so they will require drilling for both the regulators and mounting hardware.

To minimize the thermal resistance between the voltage regulators and the radiators, thermally-conductive compound, such as Wakefield Engineering type 126-2 (or similar) should be applied between the regulators' mounting surfaces and the radiators. The regulators should be attached to the radiators with 6-32 hardware, including a heavy split-ring lockwasher. The mounting screw should be well-torqued to ensure good heat transfer. To allow proper air flow, the radiators should not be enclosed; however, they should be protected from accidental contact with conductors. A perforated metal cage may be used for this purpose.

To ensure good regulation, 150 ohm resistors R1 and R2 should be connected directly to the terminals of their respective regulators. The wires connecting the 4700 microfarad filter capacitors to the regulators should be of a heavy gauge (18 AWG or thicker) and kept to a length of six inches or less. Likewise, the path from the transformer's secondary center tap to the OUTPUT COMMON should consist of heavy-gauge conductors. The wiring from the regulator outputs to the output terminals should also be of a heavy gauge. For printed-circuit construction, the tracks for these connections should be as wide as practical.

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Link to Supplier: http://www.farnell.com/

Note:  Only major items are indicated in the Table. Items such as wire, terminals, circuit board, enclosure, and fasteners are not shown.

March 23, 1998
Updated August 26, 2003
Parts list updated July 1, 2007 for RoHS compliance

Text and images ©1998, 2003, 2007 by Arthur Harrison

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