PC PSU to Variable voltage bench PSU modification, in need of some advice.  - Page 1
Forum section: electronics / beginners

author: booneht
author_rank: Newbie
timestamp: September 18, 2019, 01:42:40 pm
content: 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 buttonProblem 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 andpossiblyhave 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.

author: tunk
author_rank: Super Contributor
timestamp: September 18, 2019, 02:00:40 pm
content: 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-55222248451dAs 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.

author: ledtester
author_rank: Super Contributor
timestamp: September 18, 2019, 02:43:31 pm
content: 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.htmlIf that is the case the switch probably has two poles so you can power the LED from a different voltage.

author: ledtester
author_rank: Super Contributor
timestamp: September 18, 2019, 03:31:15 pm
content: 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=converterHere 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.

author: MosherIV
author_rank: Super Contributor
timestamp: September 18, 2019, 09:28:59 pm
content: HiWelcome 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 videoThe 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

author: janoc
author_rank: Super Contributor
timestamp: September 18, 2019, 09:38:45 pm
content: ^ This! ^ATX supplies make terrible bench supplies - noisy, poor regulation (especially if you are recycling an old supply with half-dead capacitors), no current limiting and limited voltages. And if a newbie pokes around the inside - a lethal hazard. ATX supply is really good only to power motors and similar things that aren't very demanding in terms of power quality but need a lot of current on a bench.Get one of those Ruideng modules and be done with it - that will make for a much better supply than anything you could cobble together from an ATX supply.Or spend some 80-100 bucks and buy a proper linear lab supply. Even second hand from eBay. 0-30V and 3-5A will be plenty, if you can get a dual output that would be great but you don't really need it for starting with the hobby. A good supply is an investment that will last you for a very long time and save you a lot of debugging and magic smoke.BTW,do notrebuild the ATX supply into a plastic case! If the supply gets hot, the plastic could start deforming. Worse, the metal chassis is there to provide some amount of shielding from the huge amounts of EMI an ATX supply otherwise generates. If you remove it, you are going to have noise everywhere. There are some very good reasons why you won't find ATX supplies in plastic cases.

author: booneht
author_rank: Newbie
timestamp: September 19, 2019, 10:27:42 am
content: @tunk, yes, that seems to be the better alternative with higher accuracy, and yes I was going for the 30V/5A model that would cover completely the buck converter capabilities, but some times linking the Ali pages messes up and shows the default model on the store page. As of the potentiometers, I've talked to a guy that used two of those exact converters in another type of project and he said that the 10k are exact match, thus why I picked those, I still will take your advice into consideration.@ledtesterthe converter I've picked goes up to 25V/3A output. However I highly doubt that I will go ever near that, if I ever come to a level when I need that kind of power I would strongly consider a bench PSU of higher quality. I didn't think to power the LED from another stable line at all, if this work it will be a great solution since I will have plenty of free lines and it will do the job perfectly so I can cut off output at my convenience, also thank you for the link to the YT channel, I will give it a look as soon as I get some more free time!@MosherIV, thank you for the feedback. I will definitely read up more on how an actual power supply works and do the job before I decide to proceed with making my own. This is more of a weekend project rather than anything else. I fully understand the danger of messing with electricity despite my illiteracy.@janocI hope the time will come when I have enough knowledge and free time to utilize and properly use a proper bench supply. I also realize that with my level of knowledge I'm building my house from the roof so to speak starting with making my own instead of using the proper, safe bought one. As far as the taking the PSU out, I've considered the temperature problem and the design was to put the circuit on a rail above the acrylic which is already a high-temperature resistant and use 1cm elevating .."bolts" to keep it away from anything else and give it better airflow, however I've completely ignored the fact of "caging" the EMI.Thank you all for the great replies and the patience and time to write them up.

author: ledtester
author_rank: Super Contributor
timestamp: September 19, 2019, 12:27:40 pm
content: You might find some helpful construction techniques in this video:https://youtu.be/RY65YVS4buQ

author: janoc
author_rank: Super Contributor
timestamp: September 19, 2019, 01:23:35 pm
content: Quote from: booneht on September 19, 2019, 10:27:42 amSo instead of using something that is known working and safe you are attempting to build something without knowing what you are doing.How this is somehow a better option escapes me.Power supply is first and foremost a tool. You need to be able to rely on it 100%, even more so when you don't really understand all the intricacies and limitations of it yet. A poor power supply, e.g. something built out of an ATX supply and those cheap crappy Chinese buck converter modules (the LM2577S,. LM2596S type) which are pretty much never able to deliver rated currents without cooking themselves (proper heatsinks cost money), have a lot of ripple and destructive transients on power up will only cause you a ton of grief when trying to debug anything on your bench. For example, I had one such DC-DC module destroy several nRF24L01+ radios because of its unpredictable startup. A rather expensive lesson!Even if you don't buy anything else, do get a decent multimeter and a good lab supply. Those two are indispensable. That's not something where you can save now and "grow into more advanced/expensive stuff later". E.g. a proper adjustable current limiting (which none of those modules or the ATX supply have) will pay for itself in costs saved on things you would have fried otherwise very quickly.Even something cheap ($30) like this will likely do you a better service than the hack you are trying to build:https://www.aliexpress.com/item/4000143173137.htmlFor $50-60 you can have the Korad KA3005P which Dave has reviewed already - that's a very decent low end lab supply.Both of those are likely cheaper than whatever you build once you add up all the material, work and lost time - and the quality and features are not even close.Power supplies used to be beginner projects but those werelinearsupplies and that was at the time, where a basic lab supply has been completely out of the reach of a hobbyist. So we were reusing various salvaged transformers and bolting on voltage regulators. It makes little sense today except as a learning project or if you have special requirements, especially if you have to buy all those parts.