Instructions

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7.9 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.
7.10 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, section 8 ). 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
Enabled, or when the Transient level control key is selected, or (if the slow start circuit is active)
when the source voltage rises above the Dropout Voltage threshold.
It is also possible to use the External Analogue Voltage control mode to generate transients of
any required shape by using an external generator to produce the required waveform. This is
also controlled by the Extern menu, section 8 ). Note that the external signal still passes through
the internal slew rate control circuits, so appropriate Slew Rate settings need to be made.
To end Transient operation (whether internal or external) and return to a constant level press
either the A or B key (in the
LEVEL SELECT section) as required.
7.11 Transient Menu
Press the Trans soft-key on the home screen to enter the Transient menu which gives access to
the controls for the Frequency and Duty Cycle of the internal oscillator and the Slew Rate
settings which apply to all changes in level, however caused.
These parameters can also be changed using the
∆ ADJUST facilities described previously, but to
do this the unit must be on the home screen not the Transient menu.
Note that changes to the transient frequency or duty cycle do not take effect until the end of the
present cycle, at the return to Level A. This is particularly noticeable at very low frequencies.
Disabling and then re-enabling the load input will also immediately start a new cycle.