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Voltage and Current Limits
These limits specify values of source voltage and load current which will cause the load to
automatically disable its input if the actual measured voltage or current exceeds the set limit. This
is not an independent hardware trip, but a simple comparison against the meter measurements.
To access the Limits menu, press the Limits soft-key on the home screen. This menu also gives
access to the facility which allows the unit to dissipate up to 600 Watts under intermittent
operation, which is described below.
The V / I soft-key toggles the menu between setting up the two limits, allowing a numeric value
for each to be entered in the usual way. Pressing the None soft-key (or entering a value of zero)
disables that particular limit. Press the Back soft-key or the
Home key to return to the home
screen.
While a value is specified for either limit, Lim appears on the status line of the display. Then, if
either of the limits is exceeded a fault message is displayed and the load input is disabled.
Short Term or Intermittent Operation at up to 600 Watts
For situations where the load is only required for short term or intermittent testing of a source, it
is possible for the unit to dissipate up to 600W subject to time and duty cycle limitations.
To enter this mode of operation press the Limits soft-key on the home screen and then press the
600W soft-key. A warning message is displayed outlining the limitations described here. Press
the Confirm soft-key to accept the warning and enter the high dissipation mode. The fan will run
at full speed for as long as this mode is enabled. Press the Back soft-key to return to the home
screen.
In the
METERS panel of the home screen, below the calculated power value, the display now
shows a read-out of percentage of the permitted accumulated energy limit. This is calculated
from the product of actual power dissipation and elapsed time and is an indication of the
temperature stress being imposed on the power stage of the load. When this value reaches
100% a warning message is shown on the screen. To avoid unexpected behaviour the load input
is not immediately disabled, however the user must either manually switch off the source or the
load, or at least reduce the dissipation by switching to a lower level setting, otherwise the load will
automatically disable its input ten seconds after the warning is shown. If this trip is allowed to
occur then the input cannot be re-enabled for 60 seconds.
Once the high power condition is removed (or set to a low level) the percentage of limit count will
decrease, giving the user an indication of when it is safe to return to high power operation.
Provided the fault trip did not occur, it is possible to re-enable the input without waiting for the
value to return fully to zero, but the subsequent period of high power operation will be
correspondingly reduced.
This mode is primarily intended for use in situations where periods of high power operation for up
to about one minute are interspersed with periods at zero power for at least twice as long.
However it is also possible to configure transient operation between a high power condition and a
much lower power condition, subject to limitations on the repetition rate. Whilst in this mode, the
transient oscillator Frequency is limited to 1Hz or less, and the Slew Rate is limited to 0·1% of the
maximum available in normal operation. Within these restrictions the load can be set up as
required; if the average power is less than 200 Watts then the thermal duty cycle limitation is
satisfied and the configuration can be used continuously.
Important: it should be recognised that significantly exceeding the recommended duty cycle can
raise the junction temperatures of the power FETs beyond their rating and possibly reduce their
lifetime. Also, repeated rapid changes in dissipation can result in stress fatigue of both the bond
wires and the junction to mounting base interface, so avoid using external voltage control to
implement large dissipation changes at high frequencies. Successful use of this 600 Watt
capability requires the user to exercise a reasonable degree of caution.
To return to normal operation, access the Limits menu again and press the 400W soft-key.
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