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R&S®Essentials | Power supplies fundamentals

Understanding Constant Voltage & Constant Current

Author: Paul Denisowski, Test & measurement expert

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:

“Overcurrent protection” (or “electronic fuse”): Disable or “switch off” the output if the supply senses that the current threshold is being exceeded. Although turning off the output does protect the load, completely shutting off the power is sometimes undesirable.
“Constant current mode”: Reduces the voltage such that the current falls below the threshold.

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.

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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.

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1. Power supply output is configured to 10V with a current limit of 1.5A. If the load resistance is 10Ω, according to Ohm’s law, the supply output is 1A, which is below the current limit.

2. If resistance is increased to 20Ω, voltage remains constant at 10V, and current decreases to 0.5A.

3. If resistance were decreased to only 5Ω and voltage were to remain constant at 10V, current would increase to 2A, which is above the limit.

4. Instead, the supply would enter constant current mode and would lower the output voltage to only 7.5V so that the output current is limited to the configure maximum value of 1.5A. If load resistance dropped again to only 3Ω, the supply would lower output voltage again to 4.5V to ensure that output current was limited to 1.5A.

5. The voltage remains constant in “constant voltage” mode and the current remains constant in “constant current” mode.

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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.

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Summary

Most DC benchtop power supplies are normally operated in constant voltage (CV) mode:
- User enters the desired output voltage
- Output current depends on load resistance (as per Ohm’s law)
Constant current (CC) mode is used to protect the load from excessive output current.
- User enters the maximum allowable output current
- Supply automatically lowers the output voltage to keep the output current below this limit.
Transition between CV and CC mode occurs based on readback current value.

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