8 - Manual setup
Load Modes
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SMU4000 Series Instruction Manual
Load Modes
The selected load mode determines how the current drawn by the SMU varies with the applied
voltage.
When set to a load (or sink) mode, the power dissipating stage in the SMU 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.
The SMUs fast digital feedback loop is used to offer other operating modes in which the current
does depend on the applied voltage in a known way, providing the additional choices of
Constant Power and Constant Resistance characteristics.
The SMU load modes can only be used in one single quadrant, with positive external source
voltages and negative load currents.
When sinking high currents, it is always recommended to use the 4-wire terminal configuration
to sense the voltage directly at the external source terminals.
Dropout voltage
Resistive discharge with voltage dropout.
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 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.
To help avoid dropout oscillations, a 5% hysteresis is applied by the SMU on the limit. As such
the external source voltage has to increase to above 5% of the dropout level in order to conduct
current once more.
If the dropout facility is not required, set the Dropout Voltage to 0 Volts.
Load mode selection
LC
Load Current
The current is the Level setting, independent of voltage.
LP
Load Power
Implements I = W / V where W is the Level setting.
LR
Load Resistance
Implements I = V / R where R is the Level setting.
The following sections give a brief description of the way each mode is implemented and give
some guidance of the effect that has on the application of the load.