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Figure below shows the operating modes of the three outputs of the power supply. The
operating point of one supply will be either above or below the line RL = RC. This
line represents a load where the output voltage and the output current are equal to the
voltage and current setting. When the load RL is greater than RC, the output voltage
will dominate since the current will be less then the current setting. The power supply
is said to be in constant-voltage mode. The load at point A has a relatively high
resistance value (compared to RC), the output voltage is at the voltage setting, and the
output current is less than the current setting. In this case the power supply is in the
constant-voltage (CV) mode and the current setting acts as a current limit.
When the load RL is less than RC, the output current will dominate since the voltage
will be less than the set voltage. The power supply is said to be in constant-current
mode. The load at point B has a relatively low resistance, the output voltage is less
than the voltage setting, and the output current is at the current setting. The supply is
in constant-current (CC) mode and the voltage setting acts as a voltage limit.
Unregulated State
If the power supply goes into a operation mode that is neither CV nor CC, the power
supply is in unregulated mode, which may occur momentarily. In this mode the output
voltage and current are not predictable. The unregulated condition may be the result
of the input AC voltage below the specifications or caused by non-resistive load. For
example, when the output is programmed for a large voltage step, the output capacitor
or a large capacitive load will charge up at the current limit setting. During the ramp
up of the output voltage the power supply will be in the unregulated mode. During the
transition from CV to CC as the output is shorted, the unregulated state may occur
briefly during the transition.
Unwanted Signals
An ideal power supply has a perfect current or voltage output with no signals across
the terminals or from the terminals to earth ground. The actual power supply has finite
noise across the output terminals and finite noise from either terminal to earth ground.
