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# How does internal resistance affect a power supply?

2023-03-18【Archive】

When studying a current source and a voltage source, internal resistance of a power source often confuses many people. Just remember that when a voltage source is connected to an external load, internal resistance is considered to be in series with external load. ;When a current source is connected to an external load, internal resistance is considered to be connected in parallel with external load. When using it, smaller internal resistance of voltage source, better, and internal resistance greater than current source, better! Do not understand why? How does internal resistance affect a power supply? Why does internal resistance have to match external load in order to achieve maximum power output?

** 1. Basic concepts **

1. The circuit consists of a power source and a load;

2. The circuit is divided into two parts, an internal circuit and an external circuit, and power circuit is an internal circuit;

3. When current flows through internal circuit of power supply, resistance also arises, which is called internal resistance;

4. The current also consumes electrical energy to generate heat at internal resistance;

5. As a power supply, consumption of internal resistance is not only a kind of waste, but also causes temperature of power supply itself to rise, and in serious cases, this may damage power supply!

6. The internal resistance of power supply is actual resistance of the conductor!

** 2. Only when internal resistance matches external load, output power of power supply can reach maximum power. Why? **

1. The power supply performs two functions: one is source of energy for load, which we call power supply; other is source of information about load, which we call "signal". source"; as a power supply, we hope smaller internal resistance of power supply, better, that is, consumption of internal resistance is small, and output is high, that is, efficiency must be high; for example, power supply system, generator and transformer used as a power source , should have a small internal resistance.As a signal source, we require output signal power to be as high as possible, for example, we want sound of speaker to be "loud".

2. When is output power of signal source highest, that is, loudspeaker is loudest?

(1) When internal resistance of signal source is constant, when load resistance is greater than internal resistance and becomes larger and larger, although energy consumed by internal resistance is less than energy of load, total signal power decreases, and signal received by load decreases. If power should decrease, this is a "subtraction function" from a mathematical point of view;

(2) When internal resistance of signal source is constant, when load resistance is less than internal resistance and becomes smaller and smaller, although total signal power increases, energy consumed by internal resistance is greater than that of load, and signal power received by load, decreases, from a mathematical point of view it is an "increasing function";

(3) When internal resistance of signal source is constant, only when load resistance is equal to internal resistance, energy consumed by internal resistance is equal to energy of load, and signal power received by load is largest, which is 50% of total signal power, from a mathematical point of view, this is the "maximum value".

** 3. The smaller internal resistance required by voltage source, better, and larger internal resistance required by current source, better! **

1.When analyzing and solving complex circuits, we need to equate a real power supply circuit with an ideal power supply, main parameters of which are internal resistance and electromotive force, and there are two idealized power supplies:

(1) A single voltage source with a constant voltage across terminals in a circuit, also called a constant voltage source; (2) One current source with a constant current in circuit, also called a constant voltage source. DC This is not equivalent to directly replacing actual power supply in circuit with an ideal power supply; actual power supply can be equivalently converted to a voltage source connected in series with internal resistance, or, equivalently, to a current source connected in parallel with resistance; after equivalent changes, make connection between elements of circuit simple in series-parallel, thereby turning complex circuit into a simple one.

2. How to understand "an ideal constant voltage source and a constant current source"? We all know actual power source:

(1) there is internal resistance;

(2) There is a constant electromotive force;

(3) In a circuit, terminal voltage and terminal current vary with load.

3. Under what circumstances can an actual power supply be equivalent to an "ideal constant voltage and constant current supply"?

(1) When load resistance of circuit is much larger than internal resistance, or internal resistance is small and can be neglected, it can be considered as a voltage source with a constant voltage at terminals;

(2) When load resistance of circuit is much less than internal resistance, or when internal resistance reaches infinity, it can be considered as a current source with constant current at terminals; For example, in Triode Amplifier Circuit, collector current has nothing to do with magnitude of load and can be considered as a current source.

5. It is inappropriate to say that "the smaller internal resistance required by voltage source, better, and greater internal resistance required by current source" should be said:

(1) The actual power supply with lower internal resistance is closer to DC voltage source;

(2) The actual power supply with higher internal resistance is closer to DC supply;

(3) In other words, a constant voltage source can be understood as an actual power source with lower internal resistance;

(4) In other words, a constant current supply can be understood as an actual power supply, with more internal resistance, the better.

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