Agilent Technologies 6621A, Service Manual

Page: 26 / 156

If the output voltage from + V to
- V exceeds the
programmed overvoltage setting (derived from OV REF),
the overvoltage comparator signal (OV COMP) will activate
OV DRIVE and fire the SCR provided that the POV
DISABLE signal is low. The CURRENT COMP signal is
included in the comparison to compensate for the voltage
drop across the current monitoring resistor and permit an
accurate comparison. The POV DISABLE signal is high only
during power on and for a brief time during an overvoltage
reset.
terminals via diode CR356 and transformer T301 to either
notify a remote circuit that the overvoltage circuit was
tripped or alternatively to fire other output boards (up to
eight) by paralleling the external OV lines. The OV TRIP
regardless of the state of the POV DISABLE signal. As
shown in Figure 2-9, OV TRIP is the output of a wired OR
signal can gate and can be activated by either the SENSE
PROTECT signal (as described in paragraph 2 - 41) or by the
can be generated by the fixed OV sensing circuit or by a
remote signal connected to the output's + OV and - OV
terminals.
sensing circuit (U354) continually monitors the voltage
across the output terminals. Because it is biased by the
voltage at the output terminals, it can be activated and
provide protection even when the supply is not connected
to the ac power line.
2-16
signal can activate the OV DRIVE and shut down the supply
Fixed Overvoltage Sensing Circuit - The fixed overvoltage
REMOTE OV TRIP signal. The REMOTE OV TRIP signal
Note that the OV DRIVE signal is also sent to the ± OV