How do the C.C. and C.V. modes on a laboratory dc power supply work?

Many customers ask a common question in practice: Why don’t the actual output voltage and current of a DC regulated power supply match the set values? It’s a confusing problem for both beginners and even some practitioners, and the answer lies in the core working principles of DC regulated power supplies—a combination of Ohm’s Law and automatic CV/CC mode switching. This article explains the regulation logic in a professional yet easy-to-understand way, breaking down the key mechanisms without obscure jargon, so everyone from electronics beginners to R&D engineers can grasp it.

The Basic Rule: Ohm’s Law is the Fundamental

First, we need to clarify a basic fact: A DC regulated power supply cannot fix both voltage and current at the set values at the same time, and this is determined by Ohm’s Law (U=I×R). The output current of the power supply is not an independent setting, but jointly determined by the output voltage and the connected load resistance (I=U/R); similarly, the output voltage will change with the load resistance when the current is fixed.

The power supply’s core job is to maintain one fixed parameter (either voltage or current) according to the load change, and let the other parameter adapt to the load automatically. This is the root cause of the "set value and actual output inconsistency" problem, and it is not a fault of the power supply.

Two Core Working Modes: CV and CC

DC regulated power supplies achieve stable output through two automatic switching working modes—Constant Voltage (CV) Mode and Constant Current (CC) Mode. The switching between the two modes is fully automatic and based on the load resistance, with no manual operation required.

1. Constant Voltage (CV) Mode: Voltage Fixed, Current Follows Load

CV mode is the most commonly used working mode. When the load resistance is relatively large, the power supply will automatically run in this mode, with the output voltage locked at the set value, and the output current changing with the load resistance (I=U/R).

Simple Example: Set the power supply to 10V, connect a 20Ω load, and the actual output current is 0.5A (10V/20Ω); replace the load with 10Ω, and the current will automatically become 1A (10V/10Ω). The voltage remains 10V, and the current changes with the load—this is the normal working state of CV mode.

2. Constant Current (CC) Mode: Current Fixed, Voltage Follows Load

When the load resistance is too small (e.g., the load is close to a short circuit), the output current will quickly reach the current limit value set by the power supply, and the power supply will automatically switch to CC mode. At this time, the output current is locked at the set limit value, and the output voltage changes with the load resistance (U=I×R).

Simple Example: Set the power supply’s current limit to 1A, connect a 10Ω load, and the actual output voltage is 10V (1A×10Ω); replace the load with 5Ω, and the voltage will automatically drop to 5V (1A×5Ω). The current remains 1A, and the voltage adapts to the load—this is the normal working state of CC mode.

How Do the Two Modes Switch Automatically?

The power supply judges the load resistance through an internal diode OR network to realize seamless switching between CV and CC modes:

When load resistance R > Uset/Iset: Actual current < set current limit → CV mode works;
When load resistance R < Uset/Iset: Actual current = set current limit → CC mode starts.

In short, the load resistance is the "switch" for the power supply’s working mode, and the whole process is completed in an instant inside the power supply.

The Internal "Regulation System" of the Power Supply

For those who want to understand the professional principle, the power supply’s precise control of voltage and current relies on a closed-loop control system of sampling → comparison → adjustment. It is like the "brain + eyes + hands" of the power supply, and the core consists of four key modules:

Reference Voltage Source: The "standard ruler" of the power supply, providing a stable fixed voltage (e.g., TL431 chip) that is not affected by temperature and load, as the reference for all regulation;
Sampling Circuit: The "eyes" of the power supply, including voltage and current sampling, real-time detecting the actual output parameters and feeding them back to the control core;
Error Amplifier: The "brain" of the power supply, comparing the sampled actual signal with the reference signal, calculating the error and amplifying it to generate a control command;
Regulator Tube (Power Tube): The "hands" of the power supply, adjusting its conduction degree according to the control command, and finally realizing the precise regulation of output voltage and current.

This set of systems runs continuously, ensuring that the power supply can quickly respond to load changes and maintain the stability of the fixed parameter (voltage or current).

Common Q&A: Solve Your Practical Puzzles Directly

Q1: Is the power supply broken if the actual output does not match the set value?

A: Definitely not. This is the normal working characteristic of DC regulated power supplies. The power supply can only fix one parameter (voltage or current), and the other is determined by the load—this is the design logic to protect the load and the power supply itself.

Q2: Why does the output voltage fluctuate slightly (±0.1V)?

A: This is a normal phenomenon. On the one hand, the load resistance will change slightly with temperature; on the other hand, the internal sampling circuit and reference source of the power supply have tiny errors, and the power supply will perform real-time fine-tuning, resulting in slight fluctuations that do not affect use.

Q3: Why does the voltage drop to nearly 0 when the load is short-circuited?

A: This is the power supply’s overcurrent protection mechanism. When the load is short-circuited, the resistance approaches 0, and the current will surge without voltage regulation. The power supply will immediately switch to CC mode, lower the output voltage to limit the current to the set safe value, and avoid burning the power supply or load.

Q4: What determines the regulation accuracy of the power supply?

A: It mainly depends on three core components: the precision of the reference voltage source , the precision of the sampling resistor (reducing sampling errors), and the gain of the error amplifier (the higher the gain, the more sensitive the regulation).

The key to understanding why the DC regulated power supply’s output does not match the set value is to remember two points:

Ohm’s Law is the fundamental: The output parameters are constrained by U=I×R, and voltage and current cannot be fixed at the same time;
CV/CC automatic switching: The power supply locks voltage or current according to the load resistance, and the other parameter adapts automatically.

For beginners, there is no need to worry about the "inconsistency" of the set value and actual output—it is just the power supply working as designed. For professional practitioners, understanding the internal closed-loop control and mode switching logic can help with more reasonable parameter setting and fault troubleshooting.

In fact, the "flexibility" of the DC regulated power supply’s output is its biggest advantage, which can adapt to various load changes and protect the connected electronic equipment, making it an indispensable basic device in electronics research, production and maintenance.

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