Understanding the Internal Resistance of Power Supplies

When learning about current and voltage sources, the issue of internal resistance in the power supply often confuses many people, only remembering that when the voltage source is connected to an external load, the internal resistance is considered to be in series with the external load; When connecting a current source to an external load, it is considered that the internal resistance is in parallel with the external load. When using it, it is required that the internal resistance of the voltage source be as small as possible, and the internal resistance of the current source be as large as possible!Today qihe smt pick and place machine sharing the Internal Resistance of Power Supplies .

Don’t understand why? What is the impact of internal resistance on the power supply? Why is it necessary to match the internal resistance with the external load in order to achieve maximum power output?

Basic concepts

1. The circuit is composed of a power supply and a load;
2. The circuit is divided into two parts: an internal circuit and an external circuit, with the power circuit being the internal circuit;smt pick and place machine
3. When current passes through the internal circuit of the power supply, there is also a resistance, which is called an internal resistance;
4. The current also consumes electrical energy and generates heat on the internal resistance;
5. As a power supply, the consumption of internal resistance is not only a waste, but also can cause the temperature of the power supply itself to rise, and in severe cases, it can damage the power supply!
6. The internal resistance of the power supply is the actual conductor resistance!

The maximum power output can only be achieved by matching the internal resistance with the external load. Why?

There are two types of functions of power supply: firstly, it serves as the energy source for the load, which is what we call an electric power supply; The second is as the information source of the load, which is what we call a “signal source”; As an electric power source, we hope that the smaller the internal resistance of the power source, the better, that is, the smaller the internal resistance consumption and the higher the output, that is, the higher the efficiency;

For example, in power supply systems, generators, transformers, etc. used as power sources should have low internal resistance. As a signal source, we require the output signal power to be as high as possible, for example, we want the sound of the speaker to be “loud”;smt pick and place machine

When is the maximum output power of the signal source, i.e. the loudest speaker?

When the internal resistance of the signal source is constant, and the resistance of the load is greater than the internal resistance, and becomes larger and larger, although the energy consumed by the internal resistance is smaller than the load, the total signal power decreases, and the signal power obtained by the load needs to decrease. In mathematical terms, it is called a “subtraction function”;

When the internal resistance of the signal source is constant, and the resistance of the load is smaller than the internal resistance, and as it decreases, although the total signal power increases, the energy consumed by the internal resistance is greater than that of the load, and the signal power obtained by the load will decrease, which is a “increasing function” in mathematical terms;

When the internal resistance of the signal source is constant, only when the resistance of the load is equal to the internal resistance, can the energy consumed by the internal resistance be equal to the energy of the load. The maximum signal power obtained by the load is 50% of the total signal power, which is mathematically referred to as the “maximum value”;smt pick and place machine

The voltage source requires a smaller internal resistance as possible, while the current source requires a larger internal resistance as possible!

When analyzing and solving complex circuits, we need to equivalently convert the actual power supply circuit into an ideal power supply; The main parameters of the actual power supply are internal resistance and electromotive force; There are two types of idealized power sources:

(1) One type is a voltage source, which has a constant terminal voltage in the circuit, also known as a constant voltage source;

(2) One type is a current source, which has a constant current in the circuit, also known as a constant current source; It is not equivalent to directly replace the actual power supply in the circuit with an ideal power supply; The actual power supply can be equivalently transformed into a voltage source connected in series with an internal resistance, or equivalent to a current source connected in parallel with a resistance; Through equivalent changes, the relationship between the components of the circuit is transformed into a single series parallel relationship, thereby transforming complex circuits into simple circuits;

How to understand “an ideal constant voltage source, constant current source”? We all know the actual power supply:

(1) There is internal resistance;

(2) Having a constant electromotive force;

(3) The terminal voltage and current in the circuit vary with the load;

Under what circumstances can the actual power supply be equivalently transformed into an “ideal constant voltage source, constant current source”?

(1) When the load resistance of the circuit is much larger than the internal resistance, or when the internal resistance is much smaller, it can be ignored without timing and can be seen as a voltage source with a constant terminal voltage;smt pick and place machine

(2) When the load resistance of a circuit is much smaller than the internal resistance, or when the internal resistance is as large as infinity, it can be seen as a constant current source at the end; For example, in a transistor amplification circuit, the collector current is independent of the load size and can be regarded as a current source;

The statement that ‘voltage sources require a smaller internal resistance as the best, while current sources require a larger internal resistance as the best’ is inappropriate. It should be said as follows:

(1) The smaller the internal resistance, the closer the actual power source is to a constant voltage source;

(2) The larger the internal resistance, the closer the actual power source is to a constant current source;

(3) Alternatively, a constant voltage source can be understood as an actual power source with a smaller internal resistance as better;

(4) Alternatively, a constant current source can be understood as an actual power source with a larger internal resistance as better;

Read more: Understanding the Internal Resistance of Power Supplies

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