ATX to PSU??  No negative? - Page 1
Forum section: electronics / testgear

author: gooseEL34
author_rank: Regular Contributor
timestamp: March 09, 2013, 04:59:26 pm
content: Ok folks.I am just starting my interest in EE projects and knowledge acquisition.I see a lot of vids on Youtube about converting a computer ATX power suppy unit to a benchtop voltage supply.Here's my question....With ICs, don't you usually need + AND - voltages?For example, I have a solid state guitar amp that uses 741s and 4558 ICs.Those are operated on +/- 15.Am I missing something? Or are there many instances where one would only need a positive DC voltage supply?ThanksTim

author: nctnico
author_rank: Super Contributor
timestamp: March 09, 2013, 05:12:12 pm
content: An ATX power supply is the worst choice for a bench PSU. It can push out way too much current and is poorly regulated.

author: cyr
author_rank: Frequent Contributor
timestamp: March 09, 2013, 05:53:16 pm
content: Generally you only need a negative supply for audio and some other analog applications, you don't need it for stuff like microcontrollers.But I agree, an ATX supply is a horrible choice for a lab PSU, the only thing it is good for is making things blow up in your face if you make a mistake.

author: Unknown
author_rank: Unknown
timestamp: March 09, 2013, 05:54:55 pm
content: I agree with ntcnico that an ATX power supply makes for a very poor lab supply. But to answer your question, no, modern electronics does not generally need negative rails. For example, many modern (from seventies onward, like the LM324) op-amps are often designed for single supply operation, which means that the op-amp can swing to its negative rail (ground in that case). Common digital ICs and micro-controllers don't need negative voltages either.The µA741 is an ancient late-sixties design, when symmetrical +/- 15 V power supplies were common. You can be sure the amplifier in your smart phone won't have +/- 15 V power supplies, however.

author: nctnico
author_rank: Super Contributor
timestamp: March 09, 2013, 06:15:32 pm
content: Actually many opamps (including the LM324) can't swing to the positive or negative rail. For that you need an opamp which is specifically designed for rail-to-rail output. And there is also something called rail-to-rail input. In other words: read the datasheet carefully regarding input levels and output swing.You can use a single supply voltage and use a resistive divider to keep the inputs halfway the supply voltage.

author: mariush
author_rank: Super Contributor
timestamp: March 09, 2013, 07:29:09 pm
content: If you plan on modding an ATX power supply, don't rely on the -12v and -5v outputs. -5v is not even in the current ATX standards, only very old designs of power supplies still have it. And -12v... well, usually you only have 0.1-0.5 A of power on that, very little.Modern ATX power supplies have little ripple on the outputs, but still much higher compared to linear power supplies. While a linear power supply can go below 5mV ripple, most ATX power supplies have around 40-100mV ripple on the 12v output.Really, a small +/-15v 1A transformer is about 12-15$ shipping included. If you don't need positive AND negative, transformer will be even cheaper.All you need after that is a bridge rectifier (or four diodes), a few capacitors and a couple of adjustable linear regulators and some resistors to set the output voltage. And if you want to limit current, that's also very simple to do, but you're already limited to 1A by the transformer or the linear regulators you're going to use.

author: c4757p
author_rank: Super Contributor
timestamp: March 09, 2013, 07:42:40 pm
content: Quote from: nctnico on March 09, 2013, 06:15:32 pmI'm pretty sure LM324 can swing to within a couple millivolts of the negative rail.

author: SeanB
author_rank: Super Contributor
timestamp: March 09, 2013, 08:10:54 pm
content: 324 can swing to negative rail only with a pulldown resistor, and will not swing to positive rail then. You need a CMOS opamp to do rail to rail, and even then the swing has very limited current source or sink capacity near the rails, while it can be a lot more in the middle. Even then they only approach within 50mV or so to the rail.

author: gooseEL34
author_rank: Regular Contributor
timestamp: March 10, 2013, 03:32:10 am
content: Thanks for all the supplies. Gonna pony up for a decent bench supply.Thanks for the suggestions.Tim

author: toli
author_rank: Frequent Contributor
timestamp: March 10, 2013, 07:05:19 am
content: As people have stated already, a PC PS is a very poor bench PS. That said, it has its uses. I have one of these converted into a basic PS with only 5/12V output. I use it when I need high current for things like motor drivers when I don't need higher voltages (some of my steppers can give me quite a significant speed even at low voltages as 12V - in fact, at the moment my CNC is operating from this PS since I need the higher voltage PS for other things).There are a few issues with these PS's, the main points are:- Its grounded, so no floating rails - it might not be an issue for some needs, but it is a limiting factor- You can only get significant current from the 12/5/3.3V rails so you can't get high voltages without modification- Very poor regulation (can be improved by adding a regulator which may also be used for current limiting - but this will lower the maximum attainable voltageSome of the older supplies (I think the newer models don't have that issue), actually have a limit on the minimum allowed load current. I had to install a couple of high power resistors (on the 5V and 12V rails) to draw some constant current so the PS won't fail even under no external load conditions (some PS's even have this minimum load stated on the sticker usually a few 100's of mA's).As for the 324, it can only swing close to the negative rail with very low current. Anything above 50uA will make the output rise to about 0.6V.

author: potatogun96
author_rank: Contributor
timestamp: March 11, 2013, 03:37:50 am
content: I have a simple ATX bench supply, and it has +12, -12, +5, -5, and 3.3. I can get up to 1 amp on each negative rail. It works ok for simple stuff, but it has horrible ripple and enough current to melt stuff. I only made one because I had the stuff lying around and was broke. I hope to get a real bench supply soon, it's safer.I got this kit from Jameco, it has positive and negative adjustable rails, but low current. It might work for your needs.http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_20626_-1

author: moemoe
author_rank: Regular Contributor
timestamp: March 12, 2013, 11:23:06 am
content: Please also keep in mind that most ATX PSUs require some minimum load to generate stable outputs.So you would end up with adding some dummy load resistors, probably a voltage regulator (if you need just 5v it's fine to hook some L7805 between +5V and +12V, so you just get the 2V required voltage drop for the chip). But it's still main earth referenced.If you have one spare lying around, just using it for the first steps could be okay, but your first goal should be to replace it with some serious power supply.