HP 6236A Operating And Service Manual page 19

c. Repeat for the remaining supply outputs.
OSCILLOSCOPE
RT
(NOTE 4)
NOTES:
I. THIS DRAWING SHOWS A
SUGGESTED METHOD OF
BUILDING A LOAD SWITCH.
HOWEVER. OTHER
METHODS COULD BE USED;
SUCH AS A TRANSISTOR
SWITCHING NETWORK.
MAXIMUM LOAD RATINGS
OF LOAD SWITCH ARE:
5AMPS.500V, 250W (NOT
2500W
l.
2. USE MERCURY RELAY
CLARE TYPE HGP 1002
OR W.E. TYPE 276B.
3. SELECT CONTACT PRO-
TECTION NETWORK
ACCORDING TO MERCURY
RELAY MANUFACTURERS
INSTRUCTIONS
4. EACH R
T
IS EQUAL TO
TWICE THE NORMAL FULL
LOAD RESISTANCE (2 XRL)
USED IN PREVIOUS TESTS.
CONTACT PROTECTION
NETWORK
r - (NOTE 3)------
r - - - - - - - - -
I
N.C.
I
I
I
I
-,
I
:t :
I N.C.
I
I I
I
I
I
I
I
I
I
I
I
REPETITIVE
I
~~~ ~W~~_(!::02.E~)
__
.J
+
POWER SUPPLY
UNDER TEST
Figure 5-5. Load Transient Recovery Time, Test Setup
5-29 Load Transient Recovery Time
Definition: The time
"x"
for output voltage
recovery to within "Y" millivolts of the nominal output
voltage following a "Z" amp step change in load current,
where: "X" equals 50J,lsec, "Y" equals 15mV, and
"z"
is the specified load current change, equal to half of the cur-
rent rating of the supply. The nominal output voltage is
defined as the dc level halfway between the static output
voltage before and after the imposed load change.
5-30 Measurement Techniques. 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 re-
sulting one-shot displays are difficult to observe on most
oscilloscopes and the arc energy occurring during switching
completely masks the display with a noise burst. Transistor
load switching devices are expensive if reasonably rapid load
current changes are to be achieved. Instead, a mercury-
wetted relay should be used for loading and unloading the
supply. Connect it in the load switching circuit shown in
Figure 5-5. When this load switch is connected to a 60 Hz
ac input, the mercury-wetted relay will open and close 60
times per second. The 25K control adjusts the duty cycle
of the load current switching to reduce jitter in the oscillo-
scope display. This relay may also be used with a 50 Hz
ac input.
5-25 Either a twisted pair or, preferably, a shielded
two-wire cable should be used to connect the output termin-
als of the power supply to the vertical input terminals of
the scope. When using a twisted pair, care must be taken
that one of the two wires is connected to the grounded in-
put terminal of the oscilloscope to ensure that the supply
output is safely grounded. When using shielded two-wire,
it is essential for the shield to be connected to ground at
one end only to prevent ground current flowing through
this shield from inducing a signal in the shielded leads.
5-27 In most cases, the single-ended scope method of
Figure 5-4B will be adequate to eliminate non-real compon-
ents of ripple so that a satisfactory measurement may be
obtained. However, in more stubborn cases (or if high
frequency noise up to 20 MHz must be measured), it may
be necessary to use a differential scope with floating input
as shown in Figure 5-4C. If desired, two single-conductor
shielded cables may be substituted in place of the shielded
two-wire cable with equal success. Because of its common
mode rejection, a differential oscilloscope displays only the
difference in signal between its two vertical input terminals,
thus ignoring the effects of any common mode signal pro-
duced by the difference in the ac potential between the
power supply case and scope case. Before using a differen-
tial input scope in this manner, however, it is imperative
that the common mode rejection capability of the scope'
be verified by shorting together its two input leads
at
the
power supply and observing the trace on the CRT.
it
this
trace is a straight line, then the scope is properly ignoring
any common mode signal present. If this trace is not a
straight line, then the scope is not rejecting the ground
signal-and must be realigned in accordance with the manu-
facturer's instructions until proper common mode rejection
is attained.
5-26 To verify that the oscilloscope is not displaying
ripple that is induced in the leads or picked up from the
grounds, the (+) scope lead should be shorted to the (-)
scope lead at the power supply terminals. The ripple value
obtained when the leads are shorted should be subtracted
from the actual ripple-measurement.
5-28 Measurement Procedure. To measure the ripple
and noise on each supply output, follow the steps below,
If a high frequency noise measurement is desired, an oscil-
loscope with sufficient bandwidth (20 MHz) must be used.
Ripple and noise measurements can be made at any input
ac line voltage combined with any dc output voltage and
load current within rating.
a. Connect an oscilloscope or rms voltmeter across an
output of the supply as shown in Figures 5-4B or 5-4C.
b. Energize the supply and observe the oscilloscope or
meter indication. The ripple and noise should not be
greater than 0.35mV rms or 1.5mV peak-to-peak.
5-5