General Measurement Techniques - HP 6236B Operating And Service Manual

5-8 General Measurement Techniques

5-9 Connecting Measuring Devices. To achieve valid
results when measuring the load effect, PARD (ripple and
noise), and transient recovery time of the supply,
measuring devices must be connected as close to the
output terminals as possible. A measurement made across
the load includes the impedance of the leads to the load.
The impedance of the load leads can easily.be several
orders of magnitude greater than the supply impedance
and thus invalidate the measurement. To avoid mutual
coupling effects, each measuring device must be
connected directly to the output terminals by separate
pairs of leads.
5-10 When measurements are made at the front panel
terminals, the monitoring leads must be connected at
point A, as shown in Figure 5-1, and not at point B.
Connecting the measuring device at point B would result
in a measurement that includes the resistance of the leads
between the output terminals and the point of connection.
Figure 5-1. Front Panel Terminal Connections
5-11 Selecting Load Resistors. Power supply specifica-
tions are checked with a full load resistance connected
across the supply output. 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 rated at 20 volts and 0.5 amperes would require a
load resistance of 40 ohms at the rated output voltage. The
wattage rating of this resistor would have to be at least 10
watts.
5.12 Either a fixed or variable resistor (rheostat) can
be used as the load resistance. Using a rheostat (alone or
in series with a fixed resistor) is often more convenient
than using fixed resistors as loads because the latter
may be more difficult to obtain in the exact resistance
required. A supplier of rheostats appropriate for testing
these supplies is listed in Table 5-1.
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 resistor and the output
current calculated by dividing the voltage across the
the sampling resistor by its ohmic value. The total
resistance of the series combination should be equal to
the full load resistance as determined in the preceding
paragraphs. Of course, if the value of the sampling
resistor is very low when compared to the full load
resistance, the value of the sampling resistor may be
ignored. The meter shunt recommended in Table 5-1, for
example, has a resistance of only 1 milliohm and can be
neglected when calculating the load resistance of the
supply.
5-14 Figure 5-2 shows a four terminal meter shunt.
The load current through a shunt must be fed to the
extremes of the wire leading to the resistor while the
sampling connec- tions are made as close as possible to
the resistance portion itself.
Figure 5-2. Current Sampling Resistor Connections
All instructions in this section apply to Models
6236B and 6237B unless otherwise indiated.
5-15 Rated Output, Tracking, Meter Accuracy,
and Current Limit
5-16 To check that all supplies will furnish their maxi-
mum rated output voltage and current, that the ±20V
outputs track each other, that the front panel meters are
accurate, and that the current limit circuits function,
proceed as follows:
Voltmeter Accuracy
a. With no loads connected: energize the supply, con-
nect a digital voltmeter between the +6V terminal (+18V
in Model 6237B) and common (COM), and set the +6V
(+18V) VOLTAGE control so that the DVM indication is as
near as possible to 6 volts (18 volts).
b. Set the METER switch to the +6V (+18V) range
and check the front panel voltmeter indication. It should be
within 4% of the DVM indication.
c. Set the TRACKING RATIO control to the FIXED
position, and check the +20V and -20V ranges of the
panel voltmeter similarly by connecting the DVM to each of
these outputs in turn, setting the ±20V VOLTAGE control
5-2
NOTE