2.4 OVERVOLTAGE CROWBAR ADJUSTMENT
An overvoltage crowbar
circuit
is provided
to
protect the load
against
an excessive voltage
level. Such a
condition
can result from an internal
malfunction
of
the supply
or
simply
by the
operator inadvertantly
misadjusting
the voltage control.
When the
crowbar
is activated an SCR that is
electrically
connected across the
output
ter·
minals
of
the supply is triggered. This effectively places a short
cirucit
across the
output
ter-
minals.
With the
front
panel overvoltage
control
set
fully
clockwise
the overvoltage trip
point
is
ap-
proximately
20% higher than the maximum rated
output
of
the supply. To set the
trip
point
to
a lower level, proceed as follows:
(1)
Remove the load from the supply.
(2)
Set the meter
function
switch
to
the
volts
position.
(3)
Adjust
the voltage
control
to
the desired
trip
point
as indicated on the panel
meter.
(4)
Turn the crowbar control
slowly
counter-clockwise
until
the
output
voltage as
indicated on the panel meter suddenly collapses
to
approximately 1 volt and
the current mode
indicator
lights.
(5)
Turn the voltage control counter-clockwise.
(6)
Reset the
crowbar
by turning the AC
switch
OFF and then back ON.
(7)
Adjust
the voltage control
to
the proper operating level.
NOTE:
Although
Source A and Source B have independent OV trip settings,
it
is
im-
portant
to
note
that
if
either one operates,
it
will
short both
output
terminals
simultaneously.
-4-
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