DIY Variable Power Supply – Transforming AC to DC

This entry is part 2 of 2 in the series DIY Variable Power Supply  

One of the main functions of the power supply is to convert our 120V mains AC electricity into usable DC electricity. This is done using a transformer and a bridge rectifier circuit.

The bridge rectifier is a common circuit utilizing diodes that converts AC power to DC.
The bridge rectifier is a common circuit utilizing diodes that converts AC power to DC.

The Transformer

A step-down transformer is used to convert the dangerous 120V AC electricity into a more manageable 25V.   In order to get the 2A output from my power supply, I needed a beefy transformer.  The Radio Shack 25.2V center tap 2A heavy duty transformer was a perfect choice (Model #273-1512).  This transformer has 3 output leads.  In order to get the full 25V, I used the outer leads and safely taped the middle lead out of the way.

I added a 3A fast acting fuse immediately after the transformer in order to protect the circuit in the event of an accidental direct connection to the 120V mains power.  The fuse holder is a Schurter 5 X 20mm (part # 0031.8211).

The Bridge Rectifier

The bridge rectifier is a very simple circuit that essentially uses 4 diodes to convert the negative portion of the AC wave into a positive wave.

This video does a great job explaining the whole transformation:

In my project, I used 4 1N5822 Schottky diodes.  With normal diodes, you get around a 1.4V voltage drop in a bridge rectifier, while Schottky diodes only have a voltage drop of around 0.6V.

I could have simplified this quite a bit by using an all-in-one bridge rectifier IC but I really wanted to build a traditional version using the 4 diodes.  After some positioning and repositioning, I ended up with the following configuration on my circuit board:

This layout didn't take much space and allowed me to use fairly thick traces to handle the current.
This layout didn’t take much space and allowed me to use fairly thick traces to handle the current.
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1 Comment


  1. Where are the files for building this? I couldn’t find any .zip or so.

    Reply

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