2-8 | ni.com
Chapter 2 Basic Functions
When operating in CC mode, the output current is maintained at the programmed current
setpoint. The output voltage V is determined by the equation V = I
S
× R
L
and is less than voltage
limit V
S
. The actual voltage that flows is determined by the current setpoint and the load
resistance and will not necessarily be equal to the programmed value.
For loads that generate transient voltage spikes, V
S
must be set so that the surge voltage does not
reach the voltage limit.
Crossover Point
The RMX programmable power supply switches automatically between CV mode and CC mode
according to the changes in the load. A crossover point is the point at which the mode switches.
For example, when operating in CV mode, if the load changes and the output current reaches the
current limit, the RMX programmable power supply automatically switches to CC mode to
protect the load. Likewise, when operating in CC mode, if the output voltage reaches the voltage
limit, the RMX programmable power supply switches to CV mode.
CV Mode and CC Mode Operation Examples
This section uses a power supply with a rated output voltage of 100 V and a rated output current
of 10 A as an example.
Example 1
A load resistance (R
L
) of 8 Ω is connected to the output terminals of the power supply. The
output voltage and output current are set to 30 V and 5 A, respectively. In this case,
R
C
= 30 V/5 A = 6 Ω. Because 8 Ω is greater than 6 Ω (R
L
> R
C
), the power supply operates in
CV mode. When you want to increase the voltage in CV mode, you can increase the voltage up
to the voltage defined by the following equation: Vs = Is × R
L
. Substituting the values, obtains
Vs = 5 A × 8 Ω = 40 V. If you try to increase the voltage above this point, the crossover point is
reached, and the power supply automatically switches to CC mode. To maintain operations in
CV mode, increase the current limit.
Example 2
Next, a load resistance (R
L
) of 5 Ω is connected to the output terminals of the power supply.
The output voltage and output current are set to 30 V and 5 A, respectively. In this case,
R
C
= 30 V/5 A = 6 Ω. Because 5 Ω is less than 6 Ω (R
L
< R
C
), the power supply operates in
CC mode. When you want to increase the current in CC mode, you can increase the current up
to the current defined by the following equation: I
S
= V
S
/R
L
. Substituting the values, obtains
I
S
= 30 V/5 Ω = 6 A. If you try to increase the current above this point, the crossover point is
reached, and the power supply automatically switches to CV mode. To maintain operations in
CC mode, increase the voltage limit.
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