Aim TTi LD300 Fonctionnement page 20

The front panel lamp will show orange when the dropout limit circuit becomes active. A detector
on the current monitor output might also be used to detect the operation of the dropout voltage
circuit and trigger an external timer.
The dropout voltage facility is not active in Constant Voltage (CV) mode as it is not necessary – if
the source voltage falls below the LEVEL setting then the load does not conduct any current. As
described above, the Dropout voltage setting has a special effect in Constant Resistance mode.
Slow Start
The purpose of the slow start circuit is to detect the initiation of voltage from the attached source
and to ramp the demand of the load up from zero to the final value. The rate of rise is determined
by the settings of the SLEW RATE controls. The reference voltage for the voltage detection
comparator is the setting of the DROPOUT voltage control.
Note that in Constant Resistance (CR) mode the load will start at (nearly) zero Ohms and ramp
up to the final Ohms value. This means that the load initially conducts maximum current; this is
often not the effect required, in which case Constant Conductance (CG) mode will be more
appropriate.
In Constant Power (CP) mode the slow start facility will usually be needed because of the lock-up
condition that can occur at low voltages, if the source does not have sufficient current capability
to reach the power level demanded. Calculate the voltage at which the supply has sufficient
current capability to achieve the power level setting, and set the dropout voltage to a little above
that level.
Input Enable
Once the required operating mode and level settings are established, depress the INPUT
ENABLE switch.
Input Enable Indicator
The multi-colour lamp above the INPUT ENABLE switch indicates one of four conditions:
Lamp Off
Green
Orange
Red
Power Limit
The unit continuously monitors the internal power dissipation and switches the fans to a higher
speed above about 230 Watts.
If the dissipation rises above about 335 Watts, a power limit circuit will come into operation and
attempt to reduce the load current to control the dissipation. The unit is then effectively operating
in constant power mode, which will change the stability conditions. If the power limit circuit fails to
keep the power at a reasonable level (because the source voltage rises as the current falls, for
example), the fault latch will be tripped and the load will cease to conduct.
Input not enabled.
Input enabled and operating normally.
Input enabled, but the power stages are saturated. Occurs if the source is
switched off. Also indicates if the current is below the expected value because of
operation of the Voltage Dropout or Power Limit circuits.
A fault condition has occurred and the input has been disabled. This condition will
latch until the Input Enable switch is released. The lamp will continue to show red,
even with the input disabled, if the fault condition persists.
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