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How to switch between Constant Voltage (CV) and Constant Current (CC) mode on a DC power supply

Last Updated: Sep 28, 2017 03:53PM PDT
This article primarily applies to DC power supplies that can switch between constant voltage (CV) and constant current (CC) mode automatically.  Many of B&K Precision's DC power supplies are categorized as this type of power supply, and depending on the application it is important and useful to know how to switch between the two modes.

Upon powering a DC power supply of this type, with nothing connected the default mode should be in CV mode.  This means that the power supply will have voltage control of the output power.  These power supplies also allow you to set a current value, which is often referred to as the current limit.  This setting can either be entered in through the supply's interface, or in some cases users must short the positive and negative terminals to adjust the current limit value.

The current limit value is the crossover point between CV and CC mode.  Meaning, if a load connected to the power supply draws or needs to draw more current than the limit, the supply will automatically go into CC mode, giving the supply current control of the power output.

For example, suppose the power supply is set to 5 V and current limit is set to 1 A.  When the load connected draws current less than 1 A, the power supply will maintain voltage control in CV mode.  If the load draws current at 1 A, the supply will automatically switch to CC mode.  In these cases, the current will output at 1 A or at the current limit, while the voltage will decrease down to a value that's dependent on the load.  Note that the voltage drop is always dependent on the load.  Ohm's law (V = IR) must always be satisfied.  For example, if the load is a 5 ohms resistor, than the power supply should be able to supply 5V and 1 A since 5V/1A = 5 ohms according to the formula. (Note:  the power supply may fluctuate between CV and CC mode since this would hit the current limit).  Now, if the resistor was 2 ohms instead, the power supply will switch to CC mode, and the voltage will no longer be 5 V while maintaining 1 A current.  It will instead be approximately 2 V since 1A x 2 ohms = 2 V.

In short, to switch between CV and CC mode, users must configure a current limit at a value in which they want to run CC mode.  Then, based on the load connected, the supply will automatically switch to CC from CV when current limit condition is satisfied.
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