converting pc power supply to bench power supply! - Page 1
Forum section: electronics / beginners

author: Lui
author_rank: Contributor
timestamp: April 04, 2013, 10:31:40 pm
content: Hi everyone!I am new to electronics!is it a good idea to convert a pc power supply to a bench power supply?your thoughts will be appreciated.thanksLui

author: Fsck
author_rank: Super Contributor
timestamp: April 04, 2013, 10:50:17 pm
content: Good for ridiculous amounts of current, which can be a downside. Kind of noisy though, but if you don't need excellent regulation, can serve as a cheap alternative to linear lab power supplies.So I guess, it is a good idea, but I wouldn't live with only an ATX bench power supply to use in a lab.

author: mariush
author_rank: Super Contributor
timestamp: April 04, 2013, 10:56:53 pm
content: Use the forum's search feature next time:https://www.eevblog.com/forum/reviews/computer-power-supply-conversion/msg209304/#msg209304https://www.eevblog.com/forum/chat/benchtop-supply-converted-from-an-old-atx-pc-supply/msg160135/#msg160135https://www.eevblog.com/forum/testgear/atx-to-psu-no-negative/msg199541/https://www.eevblog.com/forum/projects/using-computer-psu's/msg197595/https://www.eevblog.com/forum/projects/atx-psu-conversion-%28noob-question%29/msg152074/#msg152074https://www.eevblog.com/forum/beginners/questions-about-atx-bench-power-supply-build/msg178031/#msg178031https://www.eevblog.com/forum/beginners/yet-another-atx-power-supply-idea/msg42006/#msg42006

author: Lui
author_rank: Contributor
timestamp: April 06, 2013, 03:27:30 pm
content: thanks for the comment and yes very useful links!by the look of it I think it is better to use a wall wart power supply! and forget the PC-PSU

author: CraigHB
author_rank: Regular Contributor
timestamp: April 06, 2013, 09:15:42 pm
content: I'm not sure I know what you mean by a wall wart power supply, but in general, I've seen some of the poorest power outputs come from a wall wart. They're typically noisy as hell with a lot of regulation error.There's really no substitue for a proper benchtop DC power supply. You can find basic ones pretty cheap if you shop around. Granted, the cheap ones are cheap, but the ones I've bought have been well calibrated and clean, just a power supply after all.

author: Lui
author_rank: Contributor
timestamp: April 06, 2013, 09:54:34 pm
content: Hi CraigHBI am new to electronics and do not really know which power supply to use! (I need a cheap one though)I did a little search and I read somewhere that PC-PSU are not good for bench power supply due to high Amperage! and they suggested to use wall wart power supply!!I was not sure what to use!now I think is better to take your advice and find a DC power supply.thanks

author: mariush
author_rank: Super Contributor
timestamp: April 06, 2013, 11:13:31 pm
content: You need a power supply that checks at least two conditions :* regulated voltage* voltage as stable as possible (very little ripple)* isolation* optionally, possibility to limit currentRegulated voltage means that the voltage won't change with the amount of current you take from the power supply.regulation :A computer power supply will be relatively regulated, as in the standard requires that the voltage must be +/- 5% around the voltage used.A wall wart is usually not regulated - for example, a 7.5v DC wall wart may output 9-10v with just a led or your microcontroller connected to it, and may go down to 6-7v when go pull more current than it's designed.It is possible to use a linear regulator to generate a lower voltage from the voltage generated by a computer power supply or a wall wart, and you'll get regulated voltage but a bit of ripple will still go through so this is not ideal.Voltage stability (little ripple) :Basically all computer power supplies and most wall warts areswitching power supplies. This means that in order to obtain 12v or 5v or whatever voltage it has to output, the power supply switches something on and off thousands of times a second sending pulses of power through a transformer and then on the other side, inductors and capacitors are used to smooth out those bumps of power, those pulses, into a straight DC voltage.It's quite hard to make a power supply have no ripple or minimal ripple ... some wall warts have 300-500mV of ripple, computer power supplies can have 100-200mV ripple.Just think of it like this: if you were to get a multimeter and make 1000 measurements of the output voltage within a second, you won't get the same output voltage more than a few times in a row, the ouput voltage will move around.Here's an example from a cheap 30-40$ power supply and its 3.3v output :The output voltage stays around 3.3v but it moves around between 3.2 and 3.4v which can be bad.This ripple is not a big issue with microcontrollers but it can be if you use the power supply output as a voltage reference to measure something, or some thingsAgain, you can use a linear regulator to generate a lower voltage from the wall wart/power supply and reject some of that ripple, but some will still get through.IsolationComputer power supplies are referenced to ground. Some wall warts also are referenced to ground. That's not good if you want to use an oscilloscope on your circuit, you can blow it up if you test the wrong things.Linear regulators don't help you here, proper lab power supply are isolated (or have an option for that)Current limit :Computer power supplies don't give you an easy way to limit the current going through the wires. If something bad happens in the circuit you make, the power supply can give you 200-500 watts or more. Cables can melt and burn up before a computer power supply will stop by itself in some cases.Wallwarts can't give that much current so your circuit may not start burning, but some of them also don't have circuitry to protect themselves and can burn themselves up or fail sending higher voltage at the output.So ideally you want a way to limit current in case something goes wrong in your circuit.A linear regulator can help you there, as most have internal protections circuits that limit the maximum current they output to a value a bit above what they are designed to output and they will last for some time at that high value. But this is a bad idea in real world, you don't want to rely on it.So conclusion... yes, wall warts are better than computer power supplies and are safer, but they still have quite a lot of drawbacks.You also have to know how that wall wart behaves before actually using (is it regulated, is the ripple low, is it isolated or not?).Search for an adjustable power supply with current limit option, they're a bit more expensive but they're a good investment.Here's some examples:http://www.circuitspecialists.eu/power-supplies/bench-power-supplies

author: CraigHB
author_rank: Regular Contributor
timestamp: April 07, 2013, 02:40:10 am
content: I've seen some really horrible USB wall warts. I've tried ones rated for 1000mAh output, but with that load, voltage falls off to only a couple volts. Even at 500mA, they'll drop out of tolerance for a USB port. USB wall warts are some of the worst offenders. Not just USB ones though. I had one with a barrel connector rated for 2A that I loaded with 2A and the SCR in it exploded.

author: mariush
author_rank: Super Contributor
timestamp: April 07, 2013, 03:05:50 am
content: Indeed.I also bought a small usb charger from an online store, simply because it was less than 1.5$ and needed something worth about $1 to get the total order value over $75 and be able to use a 20% off discount coupon.It was rated for 5v 500mA but it was 5.6v and had about 600mV ripple. At over 200mA, it shat itself with over 1v ripple and at over 350mA it stopped working, it dropped straight to 1v.I made a "review and modding video" here : youtube.com/watch?v=YLjojTnkJq8 (not embedding because I'm not particularly proud of it, it's long, barely edited, was just recovering from a cold and I did not prepare in advance so it's kinda boring)Basically, the design was so bad even using a bridge rectifier and increasing the capacitance it wasn't doing 500mA. The only improvements getting it to do under 2-300mV ripple up to 380mA or so of power consumption.

author: Lui
author_rank: Contributor
timestamp: April 07, 2013, 07:10:27 pm
content: Hi mariush!thanks for the comment. it is very informative.I may use battery for now!

author: CraigHB
author_rank: Regular Contributor
timestamp: April 07, 2013, 08:11:55 pm
content: You can have the same problem with voltage sag using a battery. For example, a AA NiMH cell typically has a couple hundred mOhms internal impedance. Two in series provides around 2.6V which is a useable voltage, but a draw of 1 Amp results in a voltage drop over a half volt. Li-Ion batteries have much lower internal impedance, usually around 100 mOhms for an 18650 round cell and output voltage is quite a bit higher, but voltage sag can still be an issue.There's just no substitue for a proper benchtop power supply when testing your cicrcuits. It's not just the high quality power output. The ammeter and current limiting you get with one are pretty much a necessity. It's probably the very first piece of lab equipment an electronics enthusiast buys.

author: Lui
author_rank: Contributor
timestamp: April 11, 2013, 02:39:00 pm
content: thanks for the reply!I think is better to start looking for a bench power supply then.by asking this question,I learned lots of things along the way.thanks a lot.

author: Excavatoree
author_rank: Frequent Contributor
timestamp: April 11, 2013, 03:06:32 pm
content: I didn't see anyone mention that for lab use, one would want a linear wall wart. I'm so old, that's naturally what I think of when I hear "Wall wart," as I forget the newer switch mode wall warts. Find a HUGE, heavy, clunky one. It'll have a mains freq. transformer and linear regulator, with some capacitors and maybe minimal filtering. Those are good for many projects, and experimenting, if you can't afford a lab quality supply right away.

author: ddavidebor
author_rank: Super Contributor
timestamp: April 11, 2013, 03:40:39 pm
content: Cheap power supply is under 40$

author: smashedProton
author_rank: Frequent Contributor
timestamp: April 11, 2013, 07:17:14 pm
content: I have an atx Based lab power supply. I mainly use it for high power stuff. I am in the process of designing a power supply to be put in my computer

author: Lui
author_rank: Contributor
timestamp: April 16, 2013, 02:11:35 pm
content: @ smashedProtoncan you design a simple bench power supply?!I think building one would make an excellent learning platform.

author: ptricks
author_rank: Frequent Contributor
timestamp: April 16, 2013, 09:02:25 pm
content: Time to go dumpster diving. Chances are you can find an old tv, stereo, something with a transformer, some diodes, a few caps and maybe even a regulator which will work much better than a pc power supply.Cheap current limiting , use a resistor.

author: ttp
author_rank: Regular Contributor
timestamp: April 16, 2013, 11:25:19 pm
content: Quote from: Lui on April 16, 2013, 02:11:35 pmGo to TI.com website and find LM317 datasheet. You'll find simple examples of regulated supplies very easy to build. You'll need to add a transformer, rectifier and filter cap, a fuse would be a good idea too - suitable parts can be found in old/broken equipment for free.

author: johnboxall
author_rank: Unknown
timestamp: April 16, 2013, 11:37:31 pm
content: Quote from: Lui on April 04, 2013, 10:31:40 pmhttps://www.sparkfun.com/products/9774

author: bluey
author_rank: Regular Contributor
timestamp: October 29, 2017, 03:39:41 am
content: Saw a few kits that do the same thing as the Sparkfun one, but not as good.Then you can buy prebuilt ones that look the same from ebay or aliexpress for one third of the price! "24 Pins ATX Benchtop Board Computer PC Power Supply Breakout Adapter Module"After a free old ATX power supply + breakout parts + case + voltage/current controller, seems cheaper to buy a whole switchmode benchtop power supply.

author: ebclr
author_rank: Super Contributor
timestamp: October 29, 2017, 03:55:10 am
content: It's not a good idea, A newbie on electronics, with that amount of current and power, will burn the house, Is a better deal to buy a cheap one like this onehttps://pt.aliexpress.com/item/Yaogong-1502DD-hot-sale-item-15V-2A-ac-to-dc-power-supply-switching-for-mobile-phone/32815341372.html?s=p&ws_ab_test=searchweb0_0,searchweb201602_1_10152_10065_10151_10068_10130_10344_10345_10547_10342_10343_10340_5620011_10341_10548_10545_10541_10307_10060_5700011_10155_5640011_10154_5370011_10056_10055_10539_10538_10537_10536_10059_10534_10533_100031_10103_10102_10142_10107_5760011_10324_10084_10083_10178_10312_10313_10314_5750011_10073_5630011,searchweb201603_2,ppcSwitch_5&btsid=4a1f4fcf-be65-4740-9ae6-e7c0766150d2&algo_expid=1a322dfe-07e5-49db-b2a6-f9d85a59475b-1&algo_pvid=1a322dfe-07e5-49db-b2a6-f9d85a59475b

author: MarkF
author_rank: Super Contributor
timestamp: October 29, 2017, 04:26:28 am
content: I like these little power supplies which are small, linear and quiet (no fan).https://www.eevblog.com/forum/beginners/how-to-power-my-little-projects-kits/msg1327629/#msg1327629If you must use an ATX power supply, here is a cheaper breakout board. There is a case available for it too.https://www.eevblog.com/forum/beginners/how-to-power-my-little-projects-kits/msg1327529/#msg1327529andSeeedStudio

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 29, 2017, 05:11:19 am
content: A great deal depends on WHAT you are trying to power, and HOW you are using it. Neither of which you have mentioned here.You have got suggestions all over the map, from recycled wall-warts up to expensive bench supplies.That is because we each imagine some different application in the absence of any details in your question.If you are powering something with an internal voltage regulator (like a typical Arduino board, etc.) then an unregulated wall-wart is likely sufficient because the load (Arduino board) is providing the final regulation and filtering, etc. But if you are powering some sensitive analog circuit, then you need clean and stable power such as from a bench power supply.Note particularly that recent video that Dave posted about the $20 DIY bench power supply.https://www.eevblog.com/2017/10/11/eevblog-1030-20-diy-bench-power-supply/Highly recommended both as a very sensible solution for power supply question, AND as a valuable project in itself.

author: MarkF
author_rank: Super Contributor
timestamp: October 29, 2017, 05:31:48 am
content: Quote from: Richard Crowley on October 29, 2017, 05:11:19 amIf you look at Dave's follow up video, those PSU Modules are very noisy (timestamp 7:40).https://www.eevblog.com/2017/10/12/eevblog-1031-25-dps3003-psu-module-characterisation/

author: bitseeker
author_rank: Super Contributor
timestamp: October 29, 2017, 05:42:30 am
content: For the sake of clarity, the title, "$20 DIY bench power supply," could be misinterpreted. It's a $20 power supplymodulethat requires a source of DC power to make it go. If you don't already have a suitable source to run it on, you'll need to buy or build that part as well.

author: bd139
author_rank: Super Contributor
timestamp: October 29, 2017, 08:40:25 am
content: My first bench supply was a repurposed car battery charger I found in a skip. I added an LM723 and 2n3055 pass transistor. Heatsink was from an old amplifier I tore to bits which the transformer had smoked. Had settable 100mA, 500mA and 2A current limits. Did 1.2-15v which was enough for me at the time. Had fantastic stability, noise, transient response and regulation, better than some commercial kit!Probably still works today. It’s in my parents’ loft unless they’ve chucked it out.Really you can build your own for virtually nothing.

author: SeanB
author_rank: Super Contributor
timestamp: October 29, 2017, 10:48:09 am
content: Still got the one I made from an old thinnet hub, using some 317 regulators on old CPU heatsinks. Should pull it out and see if it still works.