Laboratory Power Supply From Old ATX

I haven’t have a power supply for lab purposes for a long time ago but sometimes it would have been necessary. Besides the adjustable voltage it is also very useful to limit the output current e.g. in case of testing newly created PCBs. So I decided to make it by myself from available components.

Since I had an unused computer ATX power supply at home, I decided to use it as power source. Usually, these old ATX power supplies ends up in the trash since they have low power (relatively) and they are not usable for new computers. If you don't have one, you can easily get one very cheap from second-hand computer shops. Or just ask your friends whether they have one in the loft. These are very good power source for electrical diy projects.

This way I also don’t have to care much with the case. So I searched for a module, that fits for my expectations:

• Provides variable voltage and current
• Works from 12V input voltage
• Maximum output voltage is at least 24V
• Maximum output current is at least 3A
• And is also relatively cheap.

I have found the ZK-4KX DC-DC Buck-Boost converter module that fits for all of my expectations. Above of that it is mounted with user interfaces also (display, buttons, rotary encoder) so I didn’t have to buy them separately.

It has the following parameters:

• Input voltage: 5 – 30 V
• Output voltage: 0.5 – 30 V
• Output current: 0 – 4 A
• Display resolution: 0.01 V and 0.001 A
• Price is ~ 8 – 10 $

It has many other features and protectionsFor detailed parameters and features see my video and the end of this post.

Above of the DC-DC converter and computer ATX modules we need only some other basic components to have a well usable power supply:

• LED + 1k resistor for indicating the status of ATX unit.
• Simple switch to power on the ATX unit.
• Banana female connectors (2 pairs)
• Aligator clip – banana plug cable.

Besides the adjustable output I also wanted to have a fix +5V output since it is used very commonly.

Take Care!

• Since the ATX power supply works with high voltage, take care that it is unplugged and also wait some time before take it apart! It includes some high voltage capacitors that needs some time to discharge, so don't touch the circuit for some minutes.
• Also take care during soldering that you don't make a short circuit.
• Make sure that you didn't forget to connect the protective earth cable (green-yellow) back to its position.

My computer ATX unit is 300W, but there are a lot of different variants, any of them is suitable for this purpose. It has different output voltage levels, they can be distinguished by the color of the wire:

• Green: We will need it to power on the device by shorting it together with ground.
• Purple: +5V Standby. We will use to to indicate the status of ATX.
• Yellow: +12V. It will be the source power of DC-DC Converter.
• Red: +5V. It will be a fix 5V output for the power supply.

And the following lines are not used, but if you need any of them, just connect its wire to the front plate.

• Grey: +5V Power Ok.
• Orange: +3.3V.
• Blue: -12V.
• White: -5V.

My ATX Power Supply also had an AC output that is not needed so I removed it. Some variants has a switch instead, which is more useful in such projects.

After disassembling I removed all unnecessary cables and the AC Output connector also.

Although there is only a small remaining space inside the ATX unit, with some arrangement I was able to put the whole user interface on one side. After designing the outline of components I’ve cut the holes from the plate, using a jigsaw and a drill.

Since the case looks not so nice, I bought spray paint to have a better look. I have choose metal black color for it.

You have to connect the components in the following way inside the box:

• Power On wire (green) + ground → Switch
• Standby wire (purple) + ground → LED + 1k resistor
• +12V wire (yellow) + ground → Input of ZK-4KX Module
• Output of ZK-4KX Module → Banana female connectors
• +5V wire (red) + ground → Other banana female connectors

Since I removed the AC Output connector and there was a transformer attached on it, I had to assemble the transformer on the case with hot glue.

After assembly the case I powered it on successfully and tried every feature of the power supply.

The only thing I had to do is the calibration as you can see in the video.

Since the measured values by the ZK-4KX Module were not the same as I measured with my multimeter, I recommend to calibrate its parameters before using the power supply. It also provides some protections against overloading the module like over voltage/current/power/temperature. The device will shut down the output if it detects any fault.

By short pressing the SW button, you can change between the following parameters to display in the second line:

• Output current [A]
• Output power [W]
• Output capacity [Ah]
• Elapsed time since power on [h]

By long pressing the SW button, you can change between the following parameters to display in the first line:

• Input voltage [V]
• Output voltage [V]
• Temperature [°C]

To enter parameter set mode, you have to long press the U/I button. You will be able to set the following parameters:

• Normally open [ON/OFF]
• Under voltage [V]
• Over voltage [V]
• Over current [A]
• Over power [W]
• Over temperature [°C]
• Overcapacity [Ah/OFF]
• Timeout [h/OFF]
• Calibration of input voltage [V]
• Calibration of output voltage [V]
• Calibration of output current [A]