Inside a PC Power Supply

After opening a power supply for the very first time (don’t do this with its power cord attached or you will get an electrical shock), you may find yourself quite lost trying to figure out what is what. But you will recognize at least two things you already know: the power supply fan and some heatsinks.

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Figure 6: Inside a PC power supply.

But you should able to recognize very easily the components that belong to the primary and the components that belong to the secondary.

You will find one (on power supplies with a active PFC) or two (on power supplies without PFC) big electrolytic capacitors. Find them and you will find the primary.

Usually PC power supplies have three transformers between two big heatsinks, as you can see on Figure 7. The main transformer is the biggest one. The medium transformer is used to generate the +5VSB output and the smallest transformer is used by the PWM control circuit to isolate the secondary from the primary (this is the transformer labeled as “isolator” on Figures 3 and 4). Several power supplies instead of using a transformer as an isolator uses one or more optocouplers (they look like small integrated circuits), so on power supplies using these components you will probably find only two transformers. We will talk more about this later.

One of the heatsinks belongs to the primary and the other belongs to the secondary.

On the primary heatsink you will find the switching transistors and also the PFC transistors and diode, if your power supply has active PFC. Some manufacturers may choose to use a separated heatsink for the active PFC components, so on power supplies with active PFC you may find two heatsinks on its primary.

On the secondary heatsink you will find several rectifiers. They look like transistors but they have two power diodes inside.

You will also find several smaller electrolytic capacitors and coils that belong to the filtering phase – finding them you will find the secondary.

An easier way to find the secondary and the primary is just following the power supply wires. The output wires will be connected to the secondary while the input wires (the ones coming from the power cord) will be connected to the primary. See Figure 7.

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Figure 7: Locating the primary and the secondary.

Now let’s talk about the components found on each stage of the power supply.

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