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When studying voltage sources and current sources, the concept of internal resistance often causes confusion. To simplify, remember: when a voltage source is connected to an external load, its internal resistance is in **series** with the load. On the other hand, for a current source, the internal resistance is in **parallel** with the load. The smaller the internal resistance of a voltage source, the better it performs; conversely, the larger the internal resistance of a current source, the more ideal it becomes. But why is that the case? What role does internal resistance play in a power supply? And why does maximum power transfer occur when the internal resistance matches the load?
Let’s break this down.
**First, the basics**
1. A circuit consists of a power supply and a load.
2. It can be divided into two parts: the internal circuit (within the power supply) and the external circuit (the load).
3. Inside the power supply, there's a resistor—this is called the internal resistance.
4. Current flowing through the internal resistance causes energy loss in the form of heat.
5. This energy loss isn't just wasteful—it also leads to temperature rise, which can damage the power supply if not controlled.
6. The internal resistance is essentially the resistance of the conductive material inside the power supply.
**Second, why does matching internal resistance with the load maximize power transfer?**
A power supply has two main roles: as a **power source** or a **signal source**.
- As a power source, we want the internal resistance to be as small as possible to maximize efficiency. For example, generators and transformers are designed with low internal resistance.
- As a signal source, we aim to deliver the maximum signal power to the load, like making a speaker louder.
Now, let’s look at what happens when the load resistance changes:
- When the load resistance is much greater than the internal resistance, the total power decreases, even though the internal resistance consumes less energy.
- When the load resistance is much smaller, the internal resistance consumes more, reducing the power delivered to the load.
- Only when the load resistance equals the internal resistance does the power delivered to the load reach its **maximum**, which is 50% of the total power generated.
**Third, why do voltage sources need low internal resistance, and current sources high internal resistance?**
In circuit analysis, real power supplies are often modeled as either a **voltage source** in series with internal resistance or a **current source** in parallel with internal resistance.
- A **voltage source** maintains a constant terminal voltage, so a lower internal resistance makes it closer to an ideal voltage source.
- A **current source** maintains a constant current, so a higher internal resistance makes it closer to an ideal current source.
For instance:
- If the load resistance is much larger than the internal resistance, the power supply behaves like a voltage source.
- If the load resistance is much smaller, it behaves like a current source.
So, while it's commonly said that "voltage sources need low internal resistance" and "current sources need high internal resistance," a more accurate way to put it is:
- The **smaller** the internal resistance, the closer the actual power supply is to an ideal **voltage source**.
- The **larger** the internal resistance, the closer the actual power supply is to an ideal **current source**.
Understanding these concepts helps in designing circuits and choosing the right type of power supply for different applications.