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5-33 The test setups illustrated in Figures 5-4A
and 5-4B are generally not acceptable for measur-
ing spikes; a differential oscilloscope is necessary.
Furthermore, the measurement concept of Figure
5-4C must be modified if accurate spike measure-
ment is to be achieved:
AC
ACC
GNO
50 fl.
TERMINATION
POWER SUPPLY
T·CONNECTOR
n
OSCILLOSCOPE
~, : o•~:
I:I
>r,::::m
-- --
~
1 INPUT
,---i-,,• I-AC
~~!I:
VERTICAL
~ o
QIU
~on .
I
.________.~
INP VT
T-CONNECTOR
50!1
TERMINATION
Figure 5-6.
Noise Spikes Test Setup
ACC
GNO
1.
As shown in Figure 5-6, two coax cables,
must be substituted for the shielded two-wire ca-
ble. In addition a high voltage protection network
(Figure 5-5) as described in paragraph 5-31 should
be connected in series with the power supply out-
put.
2.
Impedance matching resistors must be
included to eliminate standing waves and cable
ringing, and the capacitors must be connected to
block the DC current path.
3.
The length of the test leads outside the
coax is critical and must be kept as short as pos-
sible; the blocking capacitor and the impedance
matching resistor should be connected directly
from the inner conductor of the cable to the power
supply terminals.
4.
Notice that the shields of the power
supply end of the two coax cables are not con-
nected to the power supply ground, since such a
connection would give rise to a ground current
path through the coax shield, resulting in an er-
roneous measurement.
5.
The measured noise spike values must
be doubled, since the impedance matching resis-
tors constitute a 2-to-l attenuator.
6.
The noise spikes observed on the oscil-
loscope should be less than 2. 5mV p-p.
5-34 The circuit of Figure 5-6 can also be used
for the normal measurement of low frequency
ripple and noise; simply remove the four terminat-
ing resistors and the blocking capacitors and sub-
stitute a higher gain vertical plug-in in place of
the wide-band plug-in required for spike measure-
ments.
Notice that with these changes, Figure
5-6 becomes a two-cable version of Figure 5-4C.
5-7
5-35 TRANSIENT RECOVERY TIME
Definition: The time "X" for output
voltage recovery to within "Y" mil-
livolts of the nominal output volt-
age following a "Z" amp step change
in load current - where:
"Y" is specified separately for each
model but is generally of the same
order as the load regulation speci-
fication.
The nominal output volt-
age is defined as the DC level half
way between the static output volt-
age before and after the imposed
load change, and
"Z" is the specified load current
change, normally equal to the full
load current rating of the supply,
5-36 Transient recovery time may be measured at
any input line voltage combined with any output
voltage and load current within rating.
5-37 Reasonable care must be taken in switching
the load resistance on and off.
A
hand-operated
switch in series with the load is not adequate,
since the resulting one-shot displays are difficult
to observe on most oscilloscopes, and the arc
energyoccuringduring switching action complete-
ly masks the display with a noise burst. Transis-
tor load switching devices are expensive if rea-
sonably rapid load current changes are to be a-
chieved.
A mercury-wetted relay, as connected in the
load switching circuit of Figure 5-7 should be
used for loading and unloading the supply.
When
this load switch is connected to a 60
Hz
AC input,
the mercury-wetted relay will open and close 60
times per second.
Adjustment of the 25K control
permits adjustment of the duty cycle of the load
current switching and reduction in jitter of the
oscilloscope display,
5-38 To check the transient recovery time, pro-
ceed as follows:
a.
Connect test setup as shown in Fig-
ure 5-7.
b.
Set the VOLTAGE thumbwheel switches
for 90Vdc.
c.
Close the line switch on the repetitive
load switch setup.
d.
Set the oscilloscope for internal sync
and lock on either the positive or negative load
transient spike.
e.
Set the vertical input of the oscilloscope
for AC coupling so that small DC level changes in
the output voltage of the power supply will not
cause the display to shift.
f.
Adjust the vertical centering on the
scope so that the tail ends
of
the no load and full
load waveforms are symmetrically displaced about
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