14
Initial Operation
This instrument provides a controllable DC load (a power sink) intended for testing all forms of
DC power supply including batteries, photo-voltaic cells, fuel cells, turbines and generators as
well as electronic power supply units.
Organisation of this manual
The paragraphs below are intended to briefly introduce the particular features of this instrument
and the terminology used in this manual. More technical details are given in later chapters of the
manual.
The next chapter describes the general operation of the front panel and its display, followed by
full instructions for setting each parameter. A short chapter then describes the Analogue Remote
Control facilities, including level selection by a logic level signal.
Following that there is a chapter giving some application notes and implementation details, which
gives more information on some practical difficulties which may occasionally be encountered in
each operating mode, together with some advice on mitigating strategies.
Finally the digital remote control interfaces and command set of the LD400P programmable
version of the instrument are covered.
Load modes
The power dissipating stage in this load is fundamentally an adjustable current sink, which
conducts a current that does not depend on the voltage presently applied from the source being
investigated. This is known as Constant Current operation.
An analogue multiplier is used to offer other operating modes in which the current does depend
on the applied voltage in a known way, providing a choice of Constant Power, Constant
Resistance, or Constant Conductance characteristics. A fifth mode, Constant Voltage, operates in
a completely different manner to adjust the current to whatever value is needed to obtain the
desired voltage from the source.
Constant and Transient Operation
The load offers two independent level settings, referred to as Level A and Level B. Two keys
marked A and B in the
LEVEL SELECT
area of the front panel allow the choice of which level is
active.
Transient changes in the magnitude of the load are generated by switching between the two
levels. The transition between the two is a straight line at a slew rate that is specified by the user.
The switching between the two levels can be controlled either by an internal transient oscillator,
which has adjustable frequency and duty cycle, or an external logic (TTL level) signal.
There is no restriction on which of the two levels is the larger.
Dropout voltage
The primary purpose of the dropout facility is to protect batteries from being excessively
discharged. When the source voltage falls below the Dropout threshold voltage setting, the load
will reduce the current it draws, eventually to zero. This is a dynamic limit, not a latched state, so
if the source voltage recovers above the threshold (as batteries often do) then the load will
conduct current again.
Slow Start
The slow start facility causes the current taken by the load to rise gently, at the rate determined
by the slew rate setting, when the load is enabled or when the source voltage rises above the
Dropout Voltage threshold setting. It also causes the current to fall at the same rate when the
load input is disabled. This facility is particularly useful in Constant Power mode, to avoid a
latch-up condition when the source is started; see the ‘Application Notes’ chapter for details.
