Electronics > Beginners
PC PSU to Variable voltage bench PSU modification, in need of some advice.
booneht:
Hello everyone, newbie here. Recently I've taken some interest in small electronics as a hobby and of course one of the first things I've found is the EEV YT channel and this forum. I've always tinkered with electronics but only with the most basic stuff. I would change a capacitor or something obviously blown, make a new charger from an old one adding a voltage divider with some resistors and stuff. Nothing sensational, I just liked spending some time with this stuff on a weekend.
However recently my interest has peaked and I'm set on educating myself a bit more about basic electricity and electronics and start experimenting with my own small circuits projects, arduinos and so on. Hopefully, one day my very own Nixie clock.
As the title suggest, I've decided to create myself a bench power supply so I don't have to rework old power adapters I have laying around constantly and jump start my hobby, hopefully not literally. My lack of knowledge however ran me in to some problems that I hope to get advice for from you guys.
First off, before I proceed with my questions, here is a link to a "schematic" so to speak of how I imagine the PSU will be wired and all the parts I intend to use.
• Basic plan of the PSU
• Boost/Buck Converter
• Digital Volt/Ammeter for front panel
• Potentiometer to for the front panel with which I will replaces the ones on the converter
• Possible output on/off button
Problem 1 : The PSU I intend to use. Unfortunately I've removed all the labels from it because I used it in an acrylic PC build and I didn't want any not even dreaming of using it for what am I about to now. All I know is, it's an old, very decent Fortron 350W PSU with Passive PFC. However I have no knowledge of the current on the different lines of the PSU and I've scoured the internet to find anything more but without a model number it looks like a mission impossible. Will this prove a problem since I'm using it only to power small circuits and the converter is limited to 3A, which I doubt will ever use? Also, will the passive PFC be a problem?
Problem 2 : As I hope it can be seen from the "schematic" I'm not set on how to wire it properly. I will mainly use the 20 pin connector and isolate the rest without cutting them off completely, in the events that I decide to add more non-variable outlets to the front. I've seen DYI supplies that use only one live wire+gnd and I've seen some that connect all the free live wires with same voltage in one. For example, the 12V rail from the 20pin and the 2 from the 4pin. Which is the better solution?
Problem 3 : I want to add a neat button to the front panel that will cut off the power to the banana plugs so I can, for example, cut off the power, set the exact voltage and then put power on. I however lack the knowledge how to pick a suitable button that will do the job and possibly have an LED indication working. Should I just pick a button as the one I linked with LED rated for 24V, will this just result in a dimmer light while the voltage is set to something lower, for example 3-5V?
Problem 4 : PSU's Ground wire. I intend to remove the old PSU case and use an acrylic-based material to create a new case for the bench PSU. The PSU circuit + the Passive PFC coil and the cooling in the back, the rest on the front panel. However, I have no idea where to connect the ground wire in this situation that would be otherwise connected to the metal case of the PSU. If it matters, I live in an old "USSR" era apartment building that has only 2 wire 220V installation. All the wall sockets are connected to only 2 wires in the main with a bridge between "earth" and "neutral".
I apologize that I came straight in as a new user with this wall of text asking for so much information, and I will greatly appreciate any information provided.
tunk:
You may want to get this V/A-meter for better resolution (if not better accuracy):
https://www.aliexpress.com/item/32963879295.html?spm=a2g0o.detail.1000060.1.3960324196Xffv&gps-id=pcDetailBottomMoreThisSeller&scm=1007.13339.128609.0&scm_id=1007.13339.128609.0&scm-url=1007.13339.128609.0&pvid=2462ee13-103d-4626-bdaa-55222248451d
As for the potentiometers I would first get the buck/boost converter and experiment a bit.
I recently tested a buck converter where 5k was more suitable.
Edit: Instead of the V/A-meter above, you may consider a 30V/5A model.
ledtester:
Meters: I agree that having mA or higher resolution on your ammeter is very helpful. mV resolution is hardly needed, but knowing current to the mA will save you from having to hook up another multimeter.
Btw - on Aliexpress I would check out the "GWUNW Official Store" for their panel meters. I think they are the original supplier of a lot of the panel meters you see on ebay/aliexpress and you can view the entire breadth of their product line which includes RS232/TTL-enabled and isolated meters.
Problem 2: How much current will the buck-boost converter draw? I would probably just tie all of the +12s from the PS together and an equal number of GNDs.
Problem 3: It's not easy to tell, but from this aliexpress listing it appears that there is are separate terminals for the LED:
https://www.aliexpress.com/item/32799185441.html
If that is the case the switch probably has two poles so you can power the LED from a different voltage.
ledtester:
As for your selection of buck-boost converter... the youtube channel "Robojax" has load tested several of these kinds of converters:
https://www.youtube.com/channel/UCkcBSig_Iu4ZnAIeCeG1TVg/search?query=converter
Here are some tests of a LM2596-based buck converter:
The videos will give you an idea of what kind of performance those small modules have.
MosherIV:
Hi
Welcome to the forum.
The best advice for converting an old PC power supply to a bench power supply is do not do it.
PC power supplies do not have current limit. The 12V rail will deliver the full rated current, probably between 5A to 10A if you connect it to a low resistance. If you build a circuit and it is wrong, the PC PSU will just deliver what ever power into you circuit and burn it.
As you have noted, they do not have an output on/off switch. Using a normal switch means the poor switch need to handle the full voltage and current, meaning a high power switch is needed.
Most bench PSUs use a trick, they use high power transistor (note NOT a FET) and then put a switch between the base and the base drive, this switch can then be a small low current rated switch, the output can be easily turned on/off by turning the base current on/off.
Doing variable voltage (CV) and current control (CC) is not that easy. You have at least decided to let a seperate smps do some of that for you. The easiest option is to get the module featured in this eev video
The best option is use an old laptop psu, try to get one with more than 20V if you can, most only give 19V.
Start with simple linear psu. Try looking at the classic LM317 variable voltage regulator.
Do not try to build psu that give more than 1A while you are learning/discovering, you will find it frustrating finding components to do that. Most electronic circuits do not need that amount of current when starting out.
Have fun ;)
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