NOTE
Asatisfactory substitute for a differen- tial voltmeter is a reference voltage source and null detector arranged as shown in Figure 5-1.The reference voltage source is adjusted so that the voltage difference between the supply being measured and the reference volt- age will have the required resolution for the measurement being made. The voltage difference will be a function of the null detector that is used. Exam- ples of satisfactory null detectors are:
419A null detector, a dc coupled oscilloscope utilizing differential in- put, or a 50mV meter movement with a 100 division scale. For the latter, a 2mV change in voltage will result in a meter deflection of four divisions.
Figure 5-1.Differential Voltmeter Substitute
Test Setup
Care must be exercised to avoid ground loops and circulating currents when using an electronic null detector in which one input terminal is grounded.
5-5PERFORMANCE TEST
5-6 The following test can be used as an incoming inspection check and appropriate portions of the test can be repeated either to check the operation of the instrument after repairs or for periodic maintenance tests. The tests are performed using a 230V ac, 60 Hz, single phase input power source. If the correct result is not obtained for a particular check, do not adjust any internal controls; proceed to troubleshooting (Paragraph 5-5 1).
5-7CONSTANT VOLTAGE TESTS
5-8 If maximum accuracy is to be obtained in the following measurements, the measuring devices must be connected as close to the output terminals as possible. This is particularly important when measuring the transient response, regulation, or ripple of the power supply. A measurement made across the load includes the impedance of the leads to the load and such lead lengths can easily have an impedance several orders of magnitude greater than the supply impedance, thus invalidating the measurement.
5-9 To avoid mutual coupling effects, each monitoring device must be’ connected to the output terminals by a separate pair of leads. Twisted pairs or shielded two-wire cables should be used to avoid pickup on the measuring leads. The load resistor should be connected across the output terminals as close to the supply as possible. When measuring the constant voltage performance specifications, the current controls should be set well above (at least 10%) the maximum output current which the supply will draw, since the onset of constant current action will cause a drop in output voltage, increased ripple, and other performance changes not properly ascribed to the constant voltage operation of the supply .
5-10 Voltage Output and Voltmeter Accuracy. To check the output voltage, proceed as follows:
a.Connect load resistor (RL) indicated in Figure 5-2across output terminals of supply.
b. Connect differential voltmeter acress
+OUT and -OUT terminals of supply, observing
