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a soft entry into the dropout condition – the series voltage drop will reduce as the current starts to
fall, so raising the voltage measured by the load. Batteries may also recover as the load is
reduced, raising the voltage back above the dropout threshold so the load resumes conduction.
There is a possibility of hunting or instability in this operating condition. The front panel lamp will
show yellow and the status line report Dropout when the dropout circuit becomes active.
The Dropout Voltage setting has a special effect in Constant Resistance (CR) mode, when it
provides a starting point for the constant resistance characteristic (see the description in the
‘Application Notes‘ chapter for more detail). The Dropout facility is not available in Constant
Voltage (CV) mode as it conflicts with the basic intention of that mode.
The Dropout Voltage setting is also used as the threshold for the Slow Start circuit (see below).
If the dropout facility is not required, set the Dropout Voltage to 0 Volts. The status line will show
Dropout as a warning whenever this setting is above 0V and no current is being drawn.
Slow Start
The purpose of the slow start circuit is to ramp the demand of the load up slowly from zero to the
final value. The rate of rise is determined by the Slew Rate setting. The ramp starts either when
the Input is Enabled, or when the voltage from the attached source passes the level of the
Dropout Voltage setting. When the Input is Disabled the demand will ramp back down to provide
a slow stop (assuming, of course, that the source voltage remains active).
The setting for this facility is on the Transient menu, so first press the Trans soft-key on the
home screen to enter the Transient menu, and then press the Slow soft-key on that menu to
access a soft-key to alternately enable or disable slow start. While it is enabled, Slow will appear
on the status line of the instrument. Press the Back soft-key to return to the transient menu or
the Home key to return directly to the home screen.
In CP (constant power) mode the slow start facility will almost always be needed, to avoid the
lock-up condition that will occur at low voltages (when attaining the desired power requires a high
current) if the source does not have sufficient current capability to reach the power level
demanded. See the ‘Application Notes’ chapter later for a discussion of some of the implications
of the fact that constant power mode causes the load to act as a negative resistance.
In CR (constant resistance) mode the load will start at the maximum Ohms level of the active
range and ramp down to the final Ohms value. Because the initial resistance is not infinite there
will be an initial current step before the ramp starts when the load becomes active. Note also that
a linear slew in Ohms is not a linear slew in Amps if the source voltage is constant.
In CV (constant voltage) mode the setting will start at the maximum Voltage level of the active
range and ramp down to the final Voltage value. The load will not start to conduct current until the
setting ramps down past the open circuit voltage of the attached source. After this the current will
increase at a rate determined by the characteristics of the source and its effective output
resistance, until the set-point voltage is reached. If the source enters a constant current mode
then the load cannot impose a slow current ramp.
Introduction to Transient Operation
The unit includes the capability of generating load transients, which are intended to help in
testing the transient response of a source. Transient operation is available in all operating modes.
A transient is an alternation between the Level A and Level B settings, with the transition between
the two levels being a straight line (in the controlled variable of the active mode) whose slope is
determined by the Slew Rate setting. Note that the transient is specified by setting the two
absolute levels, not the difference between them (as is the case with some electronic loads).
There is no limitation on which of the two levels is the larger.
Transients can be timed either by an internal oscillator or by an external TTL signal (see the
description of the Extern menu on page 27 below). Transient operation controlled by the internal
oscillator always starts with the Level A setting, including a transition from Level B if required. The
oscillator starts when the last of the three required conditions occurs: either when the input is
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