Converting ATX PSU to bench PSU, need to tweak -12v line

A

=ACID RAIN=

Limp Gawd
Joined
May 4, 2008
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146
Is there any way I can ramp up available current on the -12v? It's 0.8 amps and even shuts down if I hook a delta fan to it. I'd like to raise that current limit if possible. Any ideas?

BTW I have soldering skills, but a bit shaky on electronics in terms of diagrams and EE jargon..
 
No way to do it without very extensive mods to the PSU. But I had a PC at work that needed -12V @ 2 amps for some PCI boards that were installed.

I bought a 12 volt 30 watt DC-DC converter with isolated (important) output. I wired the inputs to the PSU +12 volt output, and the + output to PSU ground. The DC-DC converter - output is therefore -12 volts with respect to PSU ground. You need a couple of small capacitors on the DC-DC converter to keep it from oscillating.
 
What are you trying to do? What needs that much negative current?

Perchance you could use your available resources in a different manner? Maybe just hook something up backwards ;)
 
Depending on your PSU, it might actually be possible to hook two PSUs together to create a more capable -12V power supply. Isolated outputs are critical to this. If the PSU has its external (case) ground connected to the DC ground, then it's not possible.

Other than that, though, there's nothing you can do to an existing PSU to increase the capacity, short of the extensive (read: not worth the time) modifications the Frank4d mentioned.
 
Wow Frank, I didn't know you could do that without blowing the thing. Sweet. I guess that's why isolated ground is so important. If the terminology is correct, does that mean ground is floating, so it's just a reference point and how you define ground is based on the setup? If so, I understand it somewhat.

No in my PSU ground is case ground as with most computer PSUs. I would be tempted to make the mod you mentioned, if it will fit inside the case with the other PSU. I have alot of room and I'm good at fitting things like this (example here) so I would enjoy the challenge is it's not a huge brick.

agent, I'm making this to fry the shit out of things. Nah I'll use if for other stuff too, but I'll fry the shit out of things while I use it for testing stuff LOL.

Thanks guy for the help :)
 
I just mention that because I do a lot of prototyping, and it seems fairly rare you need any substantial current on a negative supply rail, unless you're working with amplifiers or something.
 
Ah ok, cool :)

Well I guess on the other hand I could just get a small 24v unit. That's what I was using it for, +12v to -12v for 24v total. Haha.
 
Not enough available current for the stuff I want to do. At this point I'm thinking a small 24v AC/DC or DC/DC boost converter would do.
 
It does matter unless it's your secret weapon for the chimp challenge :p

 
ACID RAIN said:
Does it matter?
Click to expand...

Well, I guess that sums things up.

Good luck with your electronics career. I have a feeling you're gonna need it.

I will also say that a good understanding of how to request help by providing as much information as possible regarding the issue is a useful tool for just about everything in life.
 
LOL Xilikon.

It's hardly a career, agent. I'm a network/PC tech and I just so happen to like playing with electronics when I have some free time. This is purely for kicks and a little bit of functionality. So it may "matter" in some sense as to the design, but in reality it matters very little. I'm just looking for 24v with a little higher current rating than 0.8 amps. Nothing fancy or to exact specs. So when Paul is grilling me for the purpose, then if I must answer, it's for fun.

Maybe I should have explained better in the beginning. I have extra el cheapo rosewill PSU that blew its caps, so I replaced the caps with some higher grade parts, took all the wiring out and replaced it with two 12v lines, one 5v line, one 3.3v line, one -12v line and 4 grounds, shorted green to ground with a solder bridge underneath and now it's working fine, but it has a low -12v current threshold that I want to raise one way or another. That's all. A secondary circuit taken from another 12v/24v DC up converter would be fine if it can fit inside the current case, because I want the unit internalized completely with just mounted clamps on the outside. I'd take a picture for entertainment value, but I can't find my camera at the moment.
 
Agent and Paul, bear in mind I'm new here (not new, but haven't been here for years, and only registered recently) so if there are slight differences in how questions are expected to be asked (happens from site to site) then bear with me. On some sites you would simply say "hey how do I do this" and get an answer. On other sites you would say "hey I want to do this because such and such" and then get an answer. So if I came off as rude, it wasn't my intention.
 
It just helps if we have a better idea of why you're trying to do something--quite often, there's a better way to do something than the OP originally thinks of, and knowing the 'why' behind a question can give rise to simpler, better solutions.
 
I hear ya. Well then for my purposes, we can redneck it. LOL

So would it be better to ask a different question?
How could I achieve 24v from a ATX PSU? Maybe that has an easier answer, maybe not.
 
=ACID RAIN= said:
Maybe I should have explained better in the beginning. I have extra el cheapo rosewill PSU that blew its caps, so I replaced the caps with some higher grade parts, took all the wiring out and replaced it with two 12v lines, one 5v line, one 3.3v line, one -12v line and 4 grounds, shorted green to ground with a solder bridge underneath and now it's working fine, but it has a low -12v current threshold that I want to raise one way or another. That's all. A secondary circuit taken from another 12v/24v DC up converter would be fine if it can fit inside the current case, because I want the unit internalized completely with just mounted clamps on the outside. I'd take a picture for entertainment value, but I can't find my camera at the moment.
Click to expand...

This literally makes no sense. You took and replaced the wiring harness in such a way that you have just 2 12v leads/wire/lines? And one 5v lead/wire/line? and one 3.3v lead/wire/line? Or you "rewired" it with 2 12v rails (from what originally, if this is the case ,with what new OCP circuit you programmed), 1 5v (which it already had), and 1 3.3v (which it already had)?

You are not going to be able to change the -12v current without changing parts beyond the "wiring". And this is why I was asking questions.
 
I needed -12 volts @ 2.1 amps for three synchro-to-digital and digital-to-RVDT cards at work (avionics electronics lab stuff you don't see with most other off the shelf PCI cards) so I came up with the DC-DC converter solution.

The isolated DC-DC converter allows powering the input with any voltage within spec, and referencing the output to another voltage within spec, not nessearily common ground. There are other options; but the thing I liked about it is that it turns on and off with the PC power supply, instead of with a separate power switch.
 
Paul_Johnson said:
This literally makes no sense. You took and replaced the wiring harness in such a way that you have just 2 12v leads/wire/lines? And one 5v lead/wire/line? and one 3.3v lead/wire/line? Or you "rewired" it with 2 12v rails (from what originally, if this is the case ,with what new OCP circuit you programmed), 1 5v (which it already had), and 1 3.3v (which it already had)?

You are not going to be able to change the -12v current without changing parts beyond the "wiring". And this is why I was asking questions.
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Here, I found the camera just for you :D

First, what I making:
http://web2.murraystate.edu/andy.batts/ps/POWERSUPPLY.HTM

Similar, anyways.

Now, pics:
Old caps are yellow, new ones black. I replaced a few that didn't really need replacing, just to be safe.
benchtop_1.jpg

benchtop_2.jpg

benchtop_3.jpg


Now imagine a series of connectors on the back for 12v, 5v, 3.3v, -12v and grounds, and you see where I'm going. The tail will go away and become internal for the connectors. Just an external power source.

As you can see I have tons of room inside the unit, so adding whatever I can squeeze in there can turn this into a very useful box. I could even add a USB header and adapt an internal laptop drive to storage, or have a SATA or IDE interface to plug into. Or I could squeeze in a separate 12v/24v upconverter and give myself more amps on an isolated 24v line. The point is just to have multiple voltage options for any given purpose or thing I might want to plug into it.

edit: let me add that I have given up on ramping up amperage on the -12v line. It's clear from what y'all have said that it requires more knowledge and work than this box is worth.
 
I am going to have to say I would NOT recommend anyone do this in this manner. You have cut the wires that normally spread the current out among multiple outputs and left it such that ALL the current is going to be traveling down ONE wire which is a BAD idea because that is not what that wire is rated for when this unit was designed. The only good news is from looking at that unit you are looking at an old, group regulated, low power, poor quality power supply so it is unlikely it is actually capable of putting out an real amounts of power.

If you are going to continue with this idea I would STRONGLY recommend changing ALL the caps (those Fuhjyyu's could easily be bad, what does the oscilliscope output look like?, and if not are likely to go bad not to mention you have already had bad caps which is something to look into on its own), and ALL the wiring to use thicker gauge wire since you are going to be using just one wire. However, this is a BAD idea as this unit was not designed to function like this.
 
Ooooh-kay. Wow. Wasn't expecting the apocalypse this morning....

I'm gonna go have some breakfast. No offense Paul, I know you're a smart guy, but I think you should lighten up on the caffiene intake (or just lighten up) :). It's a POS PSU and we all know it, and the goal of this thread has been attained.
 
It might be a POS PSU but presumably the gear you're planning to attach it to you might want to not have mains current pumped thru when something goes wrong.
 
I don't think that is likely to happen on any psu, even if it cost 10 cents to make, provided it followed design specs.
 
Frank4d said:
I needed -12 volts @ 2.1 amps for three synchro-to-digital and digital-to-RVDT cards at work (avionics electronics lab stuff you don't see with most other off the shelf PCI cards) so I came up with the DC-DC converter solution.

The isolated DC-DC converter allows powering the input with any voltage within spec, and referencing the output to another voltage within spec, not nessearily common ground. There are other options; but the thing I liked about it is that it turns on and off with the PC power supply, instead of with a separate power switch.
Click to expand...

Replace the "power switch" with a relay controled by the PSU that way it open and close when you power on the system and you don't have to do it manually??
 
=ACID RAIN= said:
So would it be better to ask a different question?
How could I achieve 24v from a ATX PSU? Maybe that has an easier answer, maybe not.
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That really would have been a much better request. It is what you're after, isn't it? ;)

And application makes all the difference in the world. There are usually several ways of arriving at a desired result, but not all of them are going to be economical or efficient. Many times when pondering a solution, we don't think of all the possibilities... that's why stating your goal is important when asking for help. IE you don't have enough current... well, how much is enough current? And for what? Even if it's just for general purpose 'rednecking', that kind of implies some parameters.

All that said, I have to say that short of rewiring some coils and other major mods, it won't be easy to do what you want. It would porbably be much cheaper, quicker and safer to look for an alternative... Places like ALL Electronics and BG Micro have surplus power supplies for very cheap all the time.

Regarding you project as a generic bench supply, I'm guessing you may have seen this?
 
Cyant said:
Replace the "power switch" with a relay controled by the PSU that way it open and close when you power on the system and you don't have to do it manually??
Click to expand...

The way I wired it, described in post #2, doesn't have a power switch. It turns on with the PSU.
 
Thanks agent. I don't know exactly how much current I would want, I just know that what I have with my -12v isn't enough to run a simple high output fan. So while to you I was probably oversimplifying, to me the topic was being over-complicated. I can see the miscommunication now, and I'll try to be more direct later on :)

I have seen some bench PSU hacks, but not the one you linked specifically. I'll check it out though, because I like to grab ideas from different design styles.

I'm looking around to see if I can find a 12v/24v booster that I can incorporate into the unit, or a 120vAC/24vDC converter that I can tie into the incoming wall power inside the unit. For wattage, I'll say 30 watts or similar will be plenty, same as the post above that mentioned a separate unit. The DC/DC booster will probably be easier though, since the PCB will probably be smaller and I can squeeze it all into the box as one unit. In the end I want everything running off of the back switch too, as it does now.
 
Just got the terminals on :)

benchtop_terminals.jpg


Black is ground of course, the rest are identified by wire color.

I just remembered another reason why I wanted to juice up the -12v, because I wanted good 15.3v, 17v and already mentioned 24v.
 
=ACID RAIN= said:
I don't think that is likely to happen on any psu, even if it cost 10 cents to make, provided it followed design specs.
Click to expand...

You haven't seen many Allied (aka Deer) or PowMax reviews have you? All it takes is a transformer melting (due to innefficient cooling or overcurrent from a 200wat supply labled as a 500) and you've got mains voltages on all the rails it supplies.
 
=ACID RAIN= said:
Is there any way I can ramp up available current on the -12v? It's 0.8 amps and even shuts down if I hook a delta fan to it. I'd like to raise that current limit if possible. Any ideas?

BTW I have soldering skills, but a bit shaky on electronics in terms of diagrams and EE jargon..
Click to expand...

[H] gets addictive doesnt it Acid, dont get sucked in!! turn away now! go back to OCF! :)


=ACID RAIN= said:
Agent and Paul, bear in mind I'm new here (not new, but haven't been here for years, and only registered recently) so if there are slight differences in how questions are expected to be asked (happens from site to site) then bear with me. On some sites you would simply say "hey how do I do this" and get an answer. On other sites you would say "hey I want to do this because such and such" and then get an answer. So if I came off as rude, it wasn't my intention.
Click to expand...


it is just part of getting used to new people and such, same happened to me m8 when i came over from OCF and posted more here, you learn how people are, sometimes it is a good way, sometimes a bad way, you always hear tales of the 12 year olds over @ [H] ;) but ilove it none the less, hence the dam post count :(
 
Haha MrGuv, I hear ya. OCF is a bit of a shelter when it comes to real web activity and personalities. I used to visit this site some back in 2000-01, but took a long break. Not because of the site, but because I just forgot about it. You do see alot of glass-half-empty posting though. I mean 3 people swearing I'm gonna blow up and die going "outside of design specs". Man...tough crowd.

Guys, who cares. I'm never going to even push 100 watts though this thing, the heatsinks aren't even warm to the touch (nothing is) and it ran a fileserver for years. It's cheap, yes, but it is not going to have a thermonuclear meltdown anytime soon. And if it does, again, who cares? Bench PSU, not mission critical, not important, not even worth 20 bucks. Nothing attached to it will be worth 20 bucks. It's a toy, and it could be taken out in a field and shot for just as much enjoyment. Just because I did a decent job (my opinion) on the wiring and setup doesn't mean it matters as a piece of equipment. I just don't like doing anything halfass.

Ryan, I've been in this hobby since the 90's. I've seen so many Deer and Powmax reviews that I could write a book on the crappiness. I even owned a couple along the way, they came with a case or two ;). Nuklear, it's not like I ran a wire from the rectifier input to the filter caps on the 5v line. It's not really out of design specs. In fact it's better within design specs in some sense because the caps are lower ESR. Other than that, it is hardly changed. Wiring is not really out of design specs either, since I'm not pulling any more current through them than what a molex connector would see in a PC (actually far less).
 
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