HP A Series User Manual

5*6 PERFORMANCE TEST
5-7 The following test can be used as an incoming
inspection check and appropriate portions of the test
can be repeated to check the operation of the instru¬
ment after repairs. The tests are performed using the
nominal 120VAC, 60Hz, input power for the unit, if
the correct result is not obtained for a particular check,
proceed to troubleshooting (Paragraph 5-37).
5-12 Either a fixed or variable resistor (rheostat) can
be used as the load resistance. A rheostat is very useful
when measuring the output current of the supply and
also is a convenience if the user is testing a number of
modular power supplies of different ratings. A rheostat
of the type recommended in Figure 5-1 is adequate for
any supply covered by this manual, if a fixed resistor
is used for output current measurements, its tolerance
must be accounted for in evaluating the test results.
5-8 Measurement Techniques
5-9 Connecting Monitoring Device. For the following
Constant Voltage measurements, the measuring device
must be connected across the rear sensing terminals of
the supply in order to achieve valid indications. A mea¬
surement made across the load includes the impedance
of the leads to the load and such lead lengths can easily
have an impedance that is greater than the supply im¬
pedance, thus invalidating the measurement. To avoid
mutual coupling effects, each monitoring device must
be connected directly to the sensing terminals by
separate pairs of leads.
5-10 Avoid Current Limiting. When measuring the
constant voltage performance specifications, the current
limit point should be set at least 3% above the maxi¬
mum output current which the supply will draw, since
the onset of current limiting action will cause a drop
in output voltage, increased ripple, and other changes
not properly ascribed to the constant voltage operation
of the supply. The current limit potentiometer is fac¬
tory set to its proper value when the supply is provid¬
ing nominal output voltage. If an output voltage other
than nominal is selected, read Paragraph 3-8 and then
proceed to the current limit adjustment procedure.
Paragraph 5-72.
5-11 Selecting A Load Resistor. Constant voltage
specifications are checked with a "full” load resistance
connected across the supply. The resistance and
wattage of the load resistor, therefore, must permit
operation of the supply at its rated output voltage and
current. For example, a supply that is rated at 4 ± 0.5
volts and 4 amperes would require a load resistance of
1 ohm at the nominal output voltage (4V). If the out¬
put voltage were set to one of the adjustment limits
(4.5V or 3.5V), the load resistance would have to in¬
crease or decrease accordingly, to allow the unit to
provide its rated current of 4 amperes. The wattage
rating of the 1 ohm resistor would be 4 watts, minimum.
5-13 Output Current Measurements. For accurate out¬
put current measurements a current sampling resistor
should be inserted between the load resistor and the
output of the supply. An accurate voltmeter is then
placed across the sampling resistance and the output
current at any time can be calculated by dividing the
voltage across the sampling resistor by its ohmic value.
The total resistance of the series combination (sampling
resistor and load resistor) should be equal to the full
load resistance as determined in the preceding para¬
graphs. Of course, if the value of the sampling resistance
is very low when compared to the full load resistance,
the value of the sampling resistance may be ignored.
The meter shunt recommended in Table 1-1, for
example, has a resistance of only 5 milliohms and can
be neglected when calculating the load resistance of the
supply.
5-14 Figure 5-1 shows a four terminal meter shunt.
The load current is fed to the extremes of the wire lead¬
ing to the resistor while the sampling terminals are lo¬
cated as close as possible to the resistance portion itself.
Figure 5-1. Current Sampling Resistor Connections
5-15 Rated Output
5-16 Voltage. To ensure that the supply will furnish
the maximum rated output voltage, proceed as follows:
a. Connect digital voltmeter across +S and -S
terminals of supply observing correct polarity.
5-2

HP A Series Specifications

General

Brand	HP
Model	A Series
Category	Power Supply
Language	English