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Figure 2. Schematic of the dual, variable power supply ami fixed 15 V supplies.

4013 and 339 control panel LEDs indicate polarity and ammeter range.

Accuracy

Minor inaccuracies introduced by imbalances in the current sensing op amps and calibration errors limit the absolute precision with which the circuits will read out either voltages or currents, Curreni residuals depend upon the tracking accuracy of the positive and negative supplies, and are minimized by splitting DC amplifier duties between the sensors and the follow-up 353 stage. With careful mechanical adjustment of tracking, these reduce to 2 mA or less (apart from other sources of calibration error).

The accuracy of the voltage monitor depends uponi the accuracy of the Calibration results. Power supply monitors provide genera] indications rather than specific measurements of tesi circuit performance. The latter require more precise measurements with the usual array of test bcnch equipment. Therefore, an accuracy of 10** in general monitors is usualK lhxxI enough, and 5rc accuracy is more than most applications require. Careful construction and calibration permit these circuits to better the 5 % mark plus or minus the inherent accuracy of the analog meters themselves (generally 2 to 3%). These figures suggest that the monitor can provide a valuable ser\ ice to the builder and experimenter. In fact, it has already saved me nearh its own cost in components that did not die in test circuits.

Achieving good circuit results depends on careful component selection and calibraiion throughout the supply. Ordinary ham shack-VTVMs and DM Ms are fine, In fact, the circuit requires only 5% tolerance components at its critical points, plus some care. The care contributes to the circuit's fairly low cost. In order to catch all the salient circuit features, let's explore the major circuit divisions one at a time.

Variable Dual Power Supply

Figure 2 shows the dual power supply thai forms the core of the project. The supply is standard in almost every feature i he 18V, two amp transformer provides ample reserve capacity for the supply at peak loads, despite the simple half-wave rectifiers. ITie large filter capacitors minimize ripple to the regulators Tracking u ith thc78G 79G series tour pin regulators is quite simple, compared to others on the market. Moderate si/c heal sinks tor the TO-202 packages will handle the supply's requirements, \ he remaining components are standard data book recommendations.

The 7815/7915 positive and negative fixed regulators provide the voltage for the monitor components. For the small load involved (under 20 mA exclusive of BHDs, which are set at about 10 mA each), Zener regulation might do. but the fixed three pin regulators arc chcap. nearly foolproof, and run cool without heat sinks.

To establish the values of the control com-

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