Some modern LiPo chargers are able to charge multiple battery packs at once. However they require large power supply to meet the current demand. Dedicated PSU’s are costly, so in this post we will show you how to make a cheap DIY PC/server PSU for chargers and workbench.
Article written by Konrad and edited by Oscar.
Also check out our previous hack on converting an 1000W Server PSU for LiPo chargers.
Understand The Risk
The PSU we are working with in this project operates on mains voltage, it is also equipped with large capacitors that can store possibly lethal amount of electrical charge. If you decide to build your own power supply, you are doing so at your own risk. This project requires high level of electrical knowledge, we DO NOT recommend beginners to do it.
If you need to access inside of the PSU, always make sure it’s unplugged from main power and wait at least half an hour for the capacitors to discharge. Avoid touching the inside of PSU with bare hands, always wear rubber gloves and protective gear.
Searching For Spare PSU
You can get 2nd hand server power supplies surprisingly cheap for the kind of power they provide. Luckily I found this spare 400W PC PSU from work, that has “faulty” written on it (in Polish, later on I tested it and it’s working fine).
You can get them from old PC’s and servers. An used server PSU only cost around US$10 on ebay. The entire project could cost as little as $15 or less, including the PSU and other parts such as connectors and swtiches.
Voltage and Current Requirement
There is some requirement though: we need a PSU that allows 12V output with large enough current. Mine has an output rated for 12V, 17A so that’s good.
In general older PSU’s tend to be able to output higher current on 5V rails, while modern units deliver more on 12V. All units should be properly labelled, check the sticker for information regarding current capabilities of your device.
Cable Color Coding
PC PSUs are ideal candidates for DIY benchtop power supplies, and for amateur electronics projects. They can deliver decent amount of current at 3.3V, 5V, 12V. They can even also provide small current at negative voltages such as -5V (only on old PSUs, not present anymore on new models) and -12V.
The ATX standard uses the following color coding to differentiate power rails:
|White (only old PSU)||-5V|
Color coding for functions:
- Power Good – a status wire, it goes high to 5V when PSU is plugged into the mains, and when the main switch is ON. You could make a visible power indicator using this wire with a LED and a resistor
- Power On – grounding this wire causes PSU to “wake up” and start delivering power on 3.3V, 5V and 12V
- 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 orange wire in largest connector
Some PSU’s also have a 12V sense wire as well, you will need to solder it with Yellow 12V. Sometimes the sense wire color is the same as the corresponding output line, and it just uses thinner gauge wires.
Keep in mind the above table is the standard ATX color coding, some manufacturers may use different coding, like DELL for example. 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 You Need
- basic tools: drill, wire cutters, wire strippers, utility knife, third hand etc
- soldering iron – at least 60W. You will be dealing with big clumps of twisted wires and large metal sockets that require a lot of heat
- different sizes of heatshrink tubes
- banana plugs and binding posts, different colors – I used red for 12V, blue for 5V, white for 3.3V and black for grounds
- dummy load resistors – some units requires loads on 5V line, some don’t, but since those are cheap components, you can get them just in case. They are useful for testing also. For connecting to 5V, 10ohms 10W should be enough
- On/Off Switch
- USB socket (new or salvage from a USB extension cord)
- LED’s and resistors 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 I have has “faulty?” written on it, so I first had to test it 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 10ohm 10W dummy load resistor to the 5V line. Remember that this resistor will get HOT! Even if you are able to 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 5v line. Make sure you find out how your unit works.
After I’ve made sure my unit functions properly, I disconnected the power but left the bulb connected with 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 have 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 power on switch and additional hardware you intend to mount, like USB sockets and “power good” LED for instance. I got a switch that has a built-in 12V LED as 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 are pretty simple:
- Put masking tape on, cover the internal of PSU to avoid leaving any metal dust or scrap as it may cause short circuit; If possible it would be preferable to remove all electronics from the case first
- Start drilling with a small drill bit and work your way up. I started with 2mm one, then 4mm and 8mm
- Try not to drill through components inside :) A little tip: put something hard (e.g. wood block) under the hole you are drilling, but if you have already removed all the electronics from the case it would even be better
As you can see, my drilling isn’t perfect, but will get the job done just fine.
Place the banana sockets in the holes and measure how long you need to run the wires. I stripped about 2cm of wire to be able to twist them easily, filled 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 leftmost one. Do the same with other wires.
Remember to twist the brown wire together with orange ones as it measures losses in the circuit.
I used the same way to solder both 12V posts – 14AWG wire between posts and soldered PSUs yellow wires to the closest one.
Soldering banana sockets with those wires requires 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 smart phones.
Using purple 5V standby wire enables you to have some limited current (2A in my case) available even when entire PSU is in standby mode.
I salvaged the female connector socket from some USB extension cord. Strip the wires, you only need red (+) and black (GND). Solder them with corresponding PSU wires, heatshrink and mount socket in the chassis. Use some hot glue to hold it in place.
Finished interior of the PSU
Testing with iSDT SC608 Charger
- Got the SC-608 150W LiPo Charger From BG, works great :)
- Check out my review of it’s bigger brother SC-620 500W Charger
PSU’s are noisy, especially old ones with dirty fans :) You can simply change out the fan to a newer, quieter one.
Nevertheless, these PSU is 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 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 decided to modify it, make sure you test and check for temperature during load. For units that runs warm, you can also add some legs on the bottom to bring in extra air flow.
Just add some labels to output sockets and now you have a cheap and powerful source for your LiPo charger that also supplies you with different voltage levels for your other projects.
Author: Konrad Stepanajtys If you like this article, please subscribe to his Youtube Channel to show some love :) Happy hacking!
The largest bonus in using PC PSUs as bench power supply is the possibility to get PSU with nice power output basically for free by cannibalizing it from discontinued office PC. However this is also their great weakness. PC PSUs are supposed to work in certain power threshold. Which is especially true for cheap office PC PSUs. They use the same input voltage rail to power both voltage DC converters without proper output voltage regulation. Thus when load on 12V rail increases, PSU begin to raise voltage on DC converter input which unnecessary raise also 3.3V and 5V voltages above acceptable level and quite a much together with gradually increasing ripple. Not a big deal when only 12V voltage is used. However when something is powered also from 3.3V and 5V, these may require separate DC regulator or connected circuits will burn. And these PSUs usually lack functional OCP and OVP – on overload they may torch themselves. Not always all output power is available for 12V rail, split like 70% for 12V and 30% for 5V/3.3V may present. Insult on injury – fan is noisy.
How can i know if my pc psu needs the dummy load on 5v?
I want to use a Computer Power Suppliy (PSU) for connecting my 12V -> 110V Inverter. Both are rated 300W. After soldering a 12V lamp to the 5V output it only delivers about 30W on the 12V. If I connect the inverter, the voltage drops and the PSU shuts down.
Is the PSU too weak or did I made a mistake?
Where did You bought this psu? It has written “Uszkodzony?” and it means “Broken?” in polish :D
“Author: Konrad Stepanajtys
If you like this article, please subscribe to his Youtube Channel to show some love :)
Just a note about the ATX Power Good pin (Grey wire):
It doesn’t simply goes to 5V when the PSU is plugged in and the power switch is on. The PSU starts up when switching the power on, but it needs some time to stabilize all output voltages. The Power Good is a signal for the motherboard that the PSU is ready to take the load.
It is also a good diagnostic possibility when you have power related problems. It happens that all voltages read perfect values with no or moderate loads yet the PSU fails when the load increases. In this case it’s worth to monitor the Power Good pin: it should read 5V all the time. If it drops, or changes voltage – even just a few times – it’s quite likely that something is wrong with your PSU.
Good article! I followed the guide, and my PSU was working for a while, but then it randomly turned itself off. It started again when I reset it, and it stayed working for the rest of the day. Then today, it switched itself off after 10 minutes of charging, and will not come back on when I reset it. I did forget to join the sense wire, do you think something might have blown? Any ideas for how to fix it? Thanks
first try to find all sense wires (on most units it’s just the brown 3.3V wire) and connect them to corresponding rails, I don’t think anything has blown because of that. Also try to put some load on 5v rail – even an automotive 12V bulb should be enough and check with that plugged in.
My unit is stable without that dummy load, but 12V line is sagging to 11.3V, and it raises to 11.8V as soon as I plug the bulb in to 5V. I will solder the resistor in later.