Aim TTi LD300 Fonctionnement page 18

Constant Current (CI)
In this mode, the LEVEL setting defines a required current and the load seeks to sink this current
regardless of the voltage of the source. This mode is not suitable for use with constant current
sources as the load will either saturate (if the load setting is greater than the supply setting) or
switch off.
If the source cannot supply the current level set then the load will saturate, conducting as much
current as possible with a low voltage across its terminals; the front panel lamp will show orange.
This condition also occurs while the source is switched off. If the current capability of the source
is increased, normal operation will resume.
If the combination of the load current setting and the actual supply voltage exceeds the power
dissipation capability of the load, the current value will be reduced in an attempt to keep the
power at the permitted limit. If this is not possible, or if the maximum rated voltage is exceeded,
the unit will enter the fault state. While the power limiting circuit is active the front panel lamp will
show orange.
The dropout voltage capability is active in this mode; if it is not required set the control to 0V. As
the voltage falls below the dropout threshold the current reduces rapidly to zero. In this state the
front panel lamp shows orange. There is a possibility of instability in the transition region.
Constant Voltage (CV)
In this mode, the LEVEL setting defines a required voltage and the load attempts to sink
whatever current is needed to maintain this voltage. If the applied voltage rises, the resistance of
the load will fall in attempt to sink more current and pull the voltage down to the required level.
This behaviour is that of a shunt regulator (it might be considered as an adjustable Zener diode)
and requires the source to have a high impedance. Because of the active feedback within the
load the slope resistance is extremely low, and this mode is best used with a true current source.
If the source impedance is too low, the current will rise until the maximum power dissipation limit
is reached and the unit will enter the fault state.
The slow start capability can be used, but will result in the load starting to conduct at zero volts;
this is the highest current condition, which is probably not the action required. It may be
necessary, with some sources, to enable the load with the voltage level set above the output
voltage of the source (so no current flows), and then manually decrease the setting until the
required voltage is reached.
Constant Power (CP)
In this mode, the load implements the equation I = W / V. The LEVEL setting defines the required
power, and the unit continuously monitors the source voltage and calculates the current required
to obtain the set power dissipation. As the source voltage falls, the demanded current will rise.
This action simulates the behaviour of many switch-mode power supplies; it is also useful in
checking the characteristics of photovoltaic cells. However, the load is acting as a negative
resistance, which can give rise to stability difficulties with some sources.
If the source enters a current limit, the load will saturate with a low terminal voltage while
attempting to reach the required power setting by increasing the current. This condition is not
recoverable without disabling the load and allowing the source to recover. With most sources this
current limited condition will occur at switch-on, so it will normally be necessary to use the slow
start facility in constant power mode.
The dropout voltage capability is active in this mode; if it is not required set the control to 0V.
Constant Conductance (CG)
In this mode, the load implements the equation: I = V * G. The unit continuously measures the
source voltage and calculates the current required to simulate the required conductance. For any
given conductance setting, the load current is directly proportional to the applied voltage.
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