Build Your Own 5V, 1A Adjustable SMPS Using a Dead Computer ATX Power Supply




Switch Mode Power Supply (SMPS) is an indispensable part of any electronic design. It is used to convert mains high-voltage AC to low voltage DC, and it does it by first converting the mains AC to high voltage DC, then switching the high voltage DC to generate the desired voltage. We have already made a few SMPS circuits earlier, like this 5V 2A SMPS circuit and 12V 1A TNY268 SMPS circuit. We even did build our own SMPS transformer that could be used in our SMPS designs along with the driver IC.

Design Considerations for 5V 1A Power Supply blank

You may not notice it but most of the household products like mobile charger, laptop charger, Wi-Fi Routers, require a switching mode power supply to operate, and most of those are 5V one. So with that in mind, in this article, we will show you how you can build a 5V, 1A SMPS circuit by salvaging parts from an old throwaway PC ATX power supply. 

Warning: Working with AC mains needs prior skills and supervision. Do not open an old SMPS or try building a new one without experience. Be careful around charged capacitors and live wires. You have been warned, proceed with caution, and take expert guidance wherever needed.  

Design Considerations for 5V 1A Power Supply

Before we continue further, let’s clear out some of the basic design consideration and protection features.

Why should you build an SMPS circuit from a computer power supply?

For me it’s cheap, then again cheap is a very expensive word, it’s literally free. You may ask how so? Just talk to your local PC service shops, they will give it to you for free at least that was the case for me. Also, ask your friends if they have any of those broken ones lying around.

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Building/procuring the transformer for the circuit is the most crucial part of any SMPS design, but this method completely avoids this step by salvaging the transformer, also it comes with a very good learning experience if you are an electronic junky like me. My ATX power supply after salvaging the required parts are shown below.

Design Considerations for 5V 1A Power Supply

With this design, you can add a potentiometer and vary the output voltage a little. that may come in handy in some cases and the most interesting thing about the circuit is that it’s made with very generic parts so if something blows up finding and replacing them is a very easy task.

SMPS circuits function differently in different conditions, if you are building this circuit knowing the actual input-output characteristic can help you to debug the circuit if you find any problem with it.

Input Voltage:

Since the input voltage of the standard PC PSU is 220V, our salvaged circuit also operates on that voltage. But with my current table setup, I will try to operate the circuit with an 85V input voltage as well.

Output voltage:

The output voltage of the circuit is 5V with 1A of current rating, which means this circuit can handle a power of 5W. This circuit operates under constant voltage mode, so the output voltage should stay pretty much the same irrespective of load current.

Output Ripple:

The transformer in this circuit is made by a professional manufacturer so we can expect a low ripple. Since its construction in a dotted board, we can expect a little more ripple than usual.

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Protection Features:

In general, there are many protection circuits SMPS designs but our circuit is made from an old PC PSU, so we can add or subtract protection features as per the requirement of our final application. You can also check out the following protection circuits we build earlier.

I am going to use this circuit to power my IoT projects. So I decided to go with a minimum protection feature which is a fusible resistor at the input, and an overvoltage protection circuit at the output section.

So, to summarize, the AC mains voltage for our power supply would be 220V AC, the output voltage will be 5V DC with 1A of maximum output current. We will try to make the output ripple voltage as low as we can and we have an input fusible resistor with an output overvoltage protection circuit. 

Components Required for 5V 1A SMPS Circuit

Sl.No Parts Type Quantity Part In Schematic
1 4.7R Resistor 1 R1
2 39R Resistor 1 R10
3 56R,1W Resistor 1 R9
4 100R Resistor 2 R7, R6
5 220R Resistor 1 R5
6 100K Resistor 1 R2
7 560K, 1W Resistor 2 R3, R4
8 1N4007 Diode 4 D2,D3,D4,D5
9 UF4007 Diode 1 D6
10 1N5819 Diode 1 D1
11 1N4148 Diode 1 D7
12 103,50V Capacitor   C4
13 102, 1KV Capacitor 2 C3
14 10uF,400V Capacitor 1 C1
15 100uF,16V Capacitor 1 C6
16 470uF Capacitor 2 C7, C8
17 222pF,50V Capacitor 1 C5
18 3.3uH, 2.66A Inductor 1 L2
19 2SC945 Transistor 1 T1
20 C5353 Transistor 1 Q1
21 PC817 Optocoupler 1 OK1
22 TL431CLP Voltage Reference 1 VR1
23 10K Trim Pot 1 R11
24 Screw Terminal 5mm 2 S1, S2
25 1N5908 Diode 1 D9
26 Transformer From PC PSU 1 TR1
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5V 1A SMPS Circuit Diagram

The below image shows the schematics of 5V 1A SMPS Power supply that we will build in this tutorial.​




5V 1A SMPS Circuit Diagram

I built the circuit on a breadboard and it looked like this when completed.

Let’s understand the circuit by breaking it down to many functional blocks and let’s understand each block.

The Fusible Resistor:

First, we have R1 which serves two purposes. First, it acts as a fusible resistor. Second, it acts as a current limiting resistor.

The Bridge Rectifier & the Filter:

Next, we have 1N4007 diodes, D2, D3, D4, D5, four of which form the bridge rectifier, along with a 10uF filter capacitor to convert AC to DC.

Please note that I have removed the PI filter because I am not going to be using this power supply other than charging a battery, if you intend to use this other way, an EMI filter is must, you can always pull it out from the same power supply. If you are not sure what is PI filter or how it works, you can check out the linked article. You can also check out other designs to reduce EMI in SMPS circuit that we have discussed earlier.




Source: Build Your Own 5V, 1A Adjustable SMPS Using a Dead Computer ATX Power Supply




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