HP A Series Operating And Service Manual page 21

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 be¬
tween its two vertical input terminals, thus ignoring the
effects of any common mode signal produced by the
difference in the ac potential between the power supply
case and scope case. Before using a differential 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. If
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 manufacturer's instructions until proper common
mode rejection is attained.
5-30 Ripple and/or noise output measurement pro¬
cedures are given in the following steps. If a high fre¬
quency noise measurement is desired, an oscilloscope
with sufficient bandwidth {20MHzj must be used. To
measure the ripple/noise output, proceed as follows:
a. Connect oscilloscope or RMS voltmeter as
shown in Figures 5-3B or 5-3C.
b. Connect input power and observe oscilloscope.
c. The observed ripple should be less than 1 mVrms
and 2mV p-p.
5-31 Load Transient Recovery
Definition: The time "X" for output voltage re¬
covery to within "Y" millivolts of the nominal
output voltage following a "Z" amp step change
in load current — where:
"X" = SOjisec, "Y" = 15mV, and "Z" is the
specified load current change, equal to half of
the current rating of the supply. The nominal out¬
put voltage is defined as the DC level half way be¬
tween the static output voltage before and after
the imposed load change.
5-32 Transient recovery time may be measured at any
input line voltage combined with any output voltage
and load current within rating.
5-33 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 oscillo¬
scopes, and the arc energy occurring during switching
action completely masks the display with a noise burst.
Transistor load switching devices are expensive if reason¬
ably rapid load current changes are to be acheived.
5-34 A mercury-wetted relay, as connected in the load
switching circuit of Figure 5-4 should be used for load¬
ing and unloading the supply. When this load switch is
connected to a 60Hz 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. This relay may also be used
with a 50Hz ac input.
5-35 The maximum load ratings listed in Figure 5-4
must be observed in order to preserve the mercury-wetted
relay contacts. Switching of larger load currents can be
accomplished with mercury pool relays; with this tech¬
nique fast rise times can still be obtained, but the large
inertia of mercury pool relays limits the maximum
repetition rate of load switching and makes the clear
display of the transient recovery characteristic on an
oscilloscope more difficult.
POWER SUPPLY
UNDER TEST
OSCILLOSCOPE
y_
ir\
h\p
•-MV
CONTACT PROTECTION > ^T
NETWORK ?1N0TE 4)
' iNOTE 3) ^
NOTES:
1. 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;
SAMPS, 500V, aSOW (NOT
2500W ).
2. USE MERCURY RELAY
CLARE TYPE HGP 1002
OR W.E. TYPE 276B.
3. SELECT CONTACT PRO-
TEaiON NETW(3RK
ACCORDING TO MERCURY
RELAY MANUFACTURERS
INSTRUCTIONS,
4. EACH Rt is equal TO
TWICE THE NORMAL FULL
LOAD RESISTANCE
USED IN PREVIOUS TESTS.
Figure 5-4. Transient Recovery Time, Test Setup
O-pt
5-5