What is the difference between CV and CC power supply?
The following will explain the difference between constant voltage and constant current modes in benchtop DC power supplies.
The user of a benchtop DC power supply normally starts by configuring the output voltage, and in most cases the supply operates in so-called constant voltage mode because the supply delivers the same, or a constant, voltage regardless of the attached load.
The output current is determined by the resistance of the attached load, as per Ohm’s law – it is not directly configured or specified by the user. Nevertheless, most DC benchtop power supplies do have a user-configurable “current” setting, which is described below.
How to avoid excessive current?
Modern benchtop power supplies typically monitor the output and can determine in real time how much current is being drawn by the load. Excessively and/or unexpectedly high current can occur when load resistance decreases. Since high current often will damage or destroy the load, it is important to find a way to prevent this situation from occurring.
There are two main ways that power supplies do this:
What is the constant current mode?
The current value configured on a power supply sets the maximum allowable current or current limit – it doesn’t control or set the output current below this threshold. Constant current mode keeps the output current below a maximum value by reducing the output voltage.
In many cases, the power supply will produce some type of visual indication when it enters constant current mode, such as changing the display color or displaying a “CC” icon. Constant current mode is entered and exited based on the readback current value, it is not a button or setting that can be toggled by the user.
Constant voltage, constant current, constant resistance
Depending on the application, it is important to keep either the voltage, current or resistance fixed. This is achieved in the CV, CC and CR modes.
In constant voltage (CV) mode, the output voltage is kept constant at the set level, e.g. 5 V, while the current varies as a function of the DUT’s behavior and within the set current limits. All Rohde&Schwarz power supplies can also be operated in constant current mode (CC), where each channel can be separately configured. In this mode, a constant preset current flows to the DUT, and the voltage varies, i.e. the output voltage is reduced or increased to keep the current at the set value. When the power supply operates as an electronic load, constant resistance mode (CR) is also available. In this mode, the power supply behaves like a constant, user-settable resistance over the entire load range. This makes it possible to simulate battery discharge behavior with a constant load resistance.
If the DUT load current is low and the current drawn is lower than the set current limit, the power supply will by default operate in CV mode. The voltage is regulated to a constant value, and the current varies as a function of the load. If the load current is high and the load attempts to draw current above the set current limit, the power supply will by default limit the current to the set value and operate in CC mode. The current is regulated and the voltage is determined by the load.