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
1. Constant Voltage (CV) Mode: Voltage Fixed, Current Follows Load
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?
- 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.
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:
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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;
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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;
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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;
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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.
Common Q&A: Solve Your Practical Puzzles Directly
Q1: Is the power supply broken if the actual output does not match the set value?
Q2: Why does the output voltage fluctuate slightly (±0.1V)?
Q3: Why does the voltage drop to nearly 0 when the load is short-circuited?
Q4: What determines the regulation accuracy of the power supply?
- 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.