Build a Cheap Power Supply for Your FPV Drone Workbench and LiPo Charger

Flying an FPV drone requires a reliable power supply, especially when charging multiple LiPo batteries simultaneously. Dedicated power supplies can be expensive, so I’ll show you how I made a cheap DIY power supply using a PC or server PSU. This guide covers most things you need to know, from necessary equipment to configuration and safety tips.

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Also check out our previous hack on converting an 1000W Server PSU for LiPo chargers.

Understand the Risks

The PSU operates on mains voltage and contains large capacitors that can store lethal amounts of electrical charge. Building your own power supply requires a high level of electrical knowledge. Always unplug the PSU from the main power and wait at least 30 minutes for capacitors to discharge before accessing the inside. Wear rubber gloves and protective gear. We do not recommend beginners do this. If you decide to follow instructions on this page, do so at your own risk.

Finding a Spare PSU

You can find second-hand server power supplies cheaply, often for as little as $10 on eBay. I found a 400W PC PSU that was labeled “faulty” but tested fine. The entire project, including connectors and switches, can cost as little as $15.

Voltage and Current Requirements

Ensure your PSU provides a 12V output with sufficient current. Mine is rated at 12V, 17A, which is ideal. Modern PSUs typically provide higher current on the 12V rail, while older units focus on 5V.

Cable Color Coding

PC PSUs are ideal for DIY benchtop power supplies, offering decent amp rating at 3.3V, 5V, and 12V outputs. Some older PSU might provide negative voltage (like the one I am using), but it’s not very common nowadays.

Here’s the standard ATX color coding:

Color coding for functions:

• Power Good – This status wire goes high to 5V when the PSU is plugged into the mains and when the main switch is ON. You can make a visible power indicator using this wire with an LED and a resistor.
• Power On – Grounding this wire causes the PSU to “wake up” and start delivering power on the 3.3V, 5V, and 12V rails.
• 3.3V Sense – This wire needs to be soldered with the Orange 3.3V wire as it provides voltage feedback information for the PSU. It’s usually crimped together with the orange wire in the largest connector.

Some PSUs also have a 12V sense wire that you will need to solder with the Yellow 12V. Sometimes, the sense wire color is the same as the corresponding output line and uses thinner gauge wires.

Keep in mind that the above table represents the standard ATX color coding. Some manufacturers, like DELL, may use different coding. Voltage values or functions should be written on the main PSU board. If not, confirm voltages with a multimeter. You can also check the pin-out on your main lead.

Tools and Materials Needed

• Drill, wire cutters, wire strippers, utility knife, third hand
• Soldering iron (at least 60W) – the big clumps of twisted wires and large metal sockets require a lot of heat
• Heat shrink tubes
• Banana plugs and binding posts
• Dummy load resistors (10 ohms, 10W for 5V line)
• Optional: On/Off switch, USB socket, LEDs for status indicators. Instead of using banana sockets, you may just solder golden 4mm connectors to wires if you decide to leave them hanging outside

Let’s Start Building

The PSU was labelled “faulty”, so I first had to test if that’s true.

Check If Spare PSU Is Working

I grounded the green “power on” wire, and the internal fan started spinning. Additionally, I soldered a 55W automotive light bulb to the 12V rail just to check if it was delivering power.

NOTE: Most PSUs require some initial load to function properly. You can do this by soldering a cement 10-ohm 10W dummy load resistor to the 5V line. Remember that this resistor will get HOT! Even if you can measure voltages on all the outputs, some PSUs might not be able to deliver the amount of current they are supposed to unless some load is present on the 5V line. Make sure you find out how your unit works.

After confirming my unit functions properly, I disconnected the power but left the bulb connected to the PSU to discharge possible current stored inside. I also left it like this overnight just in case.

Installing Terminals and Sockets

After opening the case, I started planning the connections and wiring. It seemed that I had a lot of free space inside this unit, so I decided to place the terminals on the front. Originally, all the wires passed through a big hole from inside to outside of the case, but instead, I am going to install a switch in this hole. I also decided to cut a hole in one of the vents to easily mount a USB socket.

I then separated all the wires by color. You also need to separate some ground wires for the power on switch and additional hardware you intend to mount, like USB sockets and the “power good” LED for instance. I got a switch that has a built-in 12V LED as a power-up indicator, and I used a yellow wire for this.

Mounting Banana Sockets and Wiring

The banana sockets I am using require drilling eight 8mm holes for installation.

Drilling these holes is pretty simple:

1. Preparation: Put masking tape on the PSU to avoid leaving any metal dust or scrap that may cause a short circuit. If possible, remove all electronics from the case first.
2. Drilling: Start drilling with a small drill bit and work your way up. I started with a 2mm bit, then progressed to 4mm, and finally 8mm.
3. Support: Try not to drill through components inside. A little tip: put something hard (e.g., a wood block) under the hole you are drilling. However, if you have already removed all the electronics from the case, it would be even better.

As you can see, my drilling isn’t perfect, but it will get the job done just fine.

Wiring the Banana Sockets

Place the banana sockets in the holes and measure how long you need to run the wires.

Strip about 2cm of wire to twist them easily, fill the clump with solder, and then cut about 1cm from the end. I used 14AWG wire I had laying around to connect between ground posts and soldered ground wires to the leftmost one. Do the same with other wires.

Remember to twist the brown wire together with the orange ones as it measures losses in the circuit.

I used the same method to solder both 12V posts – 14AWG wire between posts and soldered the PSU’s yellow wires to the closest one.

Soldering banana sockets with those wires requires a large amount of heat; my 60W soldering iron was barely enough to heat it up properly. So, if you think your 25W iron would do the trick, good luck with that. Been there, done that :)

Adding a USB Socket

The USB socket is optional. It will be used for charging any USB devices like your smartphones.

1. Using 5V Standby: Using the purple 5V standby wire enables you to have some limited current (2A in my case) available even when the entire PSU is in standby mode.
2. Salvaging a Socket: I salvaged the female connector socket from a USB extension cord. Strip the wires; you only need red (+) and black (GND). Solder them to the corresponding PSU wires, heat shrink, and mount the socket in the chassis. Use some hot glue to hold it in place.

Finished Interior of the PSU

Testing with LiPo Charger

Test the PSU with a LiPo charger, still looking for one? Check out my charger recommendations here: https://oscarliang.com/choose-lipo-battery-charger-power-supply/#Top-LiPo-Charger-Recommendations

Noisy Fan?

PSUs are noisy, especially old ones with dirty fans :) You can simply change out the fan to a newer, quieter one.

Nevertheless, these PSUs are designed to pump hot air out of your PC/Server. They don’t need to spin that fast when they are not inside another case. Those fans are typically plugged into the 12V line; you could try to lower their power and rotation speed by powering them with 5V.

It’s probably safer to leave the fans as they are. But if you decide to modify them, make sure you test and check for temperature during load. For units that run warm, you can also add some legs on the bottom to bring in extra airflow.

Summary

Label the output sockets, and you have a powerful, cheap power supply for your LiPo charger and other projects. Happy hacking!