Aim TTi LD300 Fonctionnement page 21

Fault Conditions
The unit detects the following fault conditions:
•
Current above about 92 Amps.
•
Voltage above about 95 Volts.
•
Power in excess of about 350 Watts (that the power limit circuit has not succeeded in
controlling).
•
Excess Heatsink temperature.
•
Excess difference between external and internal voltage sense values.
When any such fault condition occurs, the input is disabled, so the unit will cease to conduct
current, and the INPUT lamp on the front panel turns red. This condition is latched until the
INPUT ENABLE switch is released, when the lamp will turn off, if the fault condition has been
cleared. If the lamp remains red, then the fault is still present and must be either excessive
voltage, high heatsink temperature or incorrect sense connections.
The fault detectors for excess current and power have filter networks with a time-constant of a
few milliseconds to allow brief transients to be handled.
Transient Operation
The unit includes the capability of generating load transients, which are intended to help in
testing the transient response of a source. A transient is a change from one load setting to
another, with a defined slew rate between them. The two load settings are defined by the
LEVEL A and LEVEL B controls - there is no limitation on which of these levels is the larger. Note
that the transient is specified as the two absolute levels, not the difference between them (as is
the case with some electronic loads).
Transient operation is available in all operating modes. The load is designed so that the transition
between the two levels is a straight line whose slope is determined by the setting of the slew rate
controls.
Transients can be timed either by the internal oscillator or by an external TTL signal. It is also
possible to use EXTERNAL VOLTAGE mode to generate transients of any required shape by
making the external generator produce the required waveform. Note that the external signal still
passes through the internal slew rate generator, so appropriate settings need to be made on its
front panel controls.
Transient Frequency and Duty Cycle
The frequency of the internal oscillator is set by a three-position range switch and a single-turn
variable control, which covers a 100:1 range. The duty cycle can be varied over the range 1% to
99% by a second single-turn control. This percentage is the portion of each repetition spent at
the LEVEL B setting, including the transition from LEVEL A to LEVEL B; the transition back to
LEVEL A and the time stable at that setting occupy the remainder of the cycle.
These settings can be read on the meters by switching the METERS switch to the FREQ &
DUTY % position. The frequency reading (on the left hand meter) may be in Hz or kHz as
indicated; the DUTY % reading is always in percent.
Transient operation always starts with the LEVEL A setting. The oscillator starts on the last of
three possible events: either the LEVEL CONTROL switch is moved to the TRANSIENT position,
or the input is enabled or (if the slow start circuit is active) the source voltage rises above the
DROPOUT threshold. The duration of the first LEVEL A condition will be about 3% longer than
the normal value.
Note that the time interval represented by the shortest portion of the cycle must be sufficient for
the transition defined by the slew rate and level setting controls to occur, otherwise the load will
never reach the steady state at the set value. This error condition is discussed below.
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