TWO extremely compact, well-regulated, small d.c. laboratory power supplies, selling for $90, have recently been added to the Hewlett-Packard (Harrison Div.) power-supply line.
These are Models 6215A and 6217A, rated at 25 V, 400 mA and 50 V, 200 mA, respectively. Reliable yet low-cost, these "hand-size" battery substitutes have performance features ideal for circuit development, component evaluation, and other laboratory applications. The power ratings are well suited for breadboarding, where the circuit designer most often needs a sizable number of power sources with higher output ratings. An interlocking feature for stacking several units vertically has been incorporated into the molded, impact-resistant case, thus minimizing the bench space required.
Underlying the new supplies is a design philosophy of achieving a low selling price while omitting only a few of the less often used operating features, such as remote sensing and remote programming. Like the higher priced, higher wattage supplies in the manufacturer's line, the 6215A and the 6217A employ all silicon circuitry. They are capable of 0.01% line regulation and load regulation, have less than 200 µV r.m.s. ripple, and feature an output voltage fully adjustable down to zero. Selection of voltage or current metering is provided by means of a front-panel slide switch.
Low output impedance is the hallmark of high-quality power-supply design; these supplies exhibit less than 0.03 ohm from d.c. to 1 kHz. An output impedance of less than 3 ohms for frequencies as high as 1 MHz assures satisfactory regulation with many types of active loads such as oscillators and amplifiers. Closely allied with low output impedance is fast transient recovery time-the time required for the output voltage to return to within a level approximating the normal d.c. output following a sudden change in load current.
(below) The simplified diagram shows the basic feedback circuit principle used.
The a.c. input, after passing through a power transformer, is rectified and filtered. The series regulator, by feedback action, alters its voltage drop to keep the regulated d.c. output voltage constant in spite of changes in the load, unregulated d.c., or other disturbances.
The comparison amplifier continuously monitors the difference between the voltage across the front-panel voltage control R and the output voltage. If these voltages are not equal, the comparison amplifier produces an amplified error signal. The error signal is of such a magnitude and polarity as to change the conduction of the series regulator, thereby changing the current through the load resistor until the output voltage equals the voltage across the voltage control.
Drift-free performance is assured by the temperature-compensated zener diode and the high-gain feedback amplifier. The total change in output voltage for eight hours (after 30 minutes warmup ) at a constant ambient is less than 0.1% plus 5 millivolts, and the temperature coefficient is less than 0.02% plus 1 millivolt output change per degree centigrade change in the ambient temperature.
Current limiting and shortcircuit protection are derived by permitting the driver stage of the comparison amplifier to sense the voltage developed by the load current flowing through R_L. This resistor is an internal potentiometer which is adjusted so that at a predetermined level of load current, the series regulator will be inhibited from increasing its conduction. The output current then remains within 5% of this value for any further reductions of load resistance, including a short circuit, under which the unit may be operated for an indefinite period of time.
In addition to overload immunity, other precautionary design features include reverse-polarity diodes across the input terminals and series regulator.
These diodes protect supply components from the effects of any reverse voltage accidentally applied across the output terminals and, with series or parallel connection of another power supply, permit turning on one supply before the other. To safeguard the low-level input stage against damage due to large signals associated with rapid manipulation of the output controls, clamping diodes of opposite polarity limit the input voltage to the feedback amplifier to less than one volt.
To facilitate the use of these supplies in systems applications ( for example, as offset sources for multichannel recorders), a rack-mounting kit is available for installing three supplies in a 3.5" high standard 19" rack space.
High-quality components are used in these supplies. The molded case and new product techniques contribute to savings which make possible the low selling price. The result is a precision power supply which sells for about the same price as the components used if they are purchased on a small-quantity basis.
(source: Electronics World, Oct. 1967)
Also see: Seco Model 260 Transistor Analyzer