Thread: modified computer power supply?

Originally Posted by BaronJ

Hi Guys,
I think what Soundman is trying to convey is that the metal case is connected to both the ground/earth pin on the mains supply socket and the negative/common (Black wires) on the output side. When attempting to use two of these PSU to get 24 volts, one of the PSU cases has to be insulated so that it cannot make contact with the other. This means that the earth lead on one of the PSU has to be disconnected.

The danger now is that failure in either PSU could potentially make not only the case of the PSU's live but the whole output side. Using just a single PSU that is properly earthed does not pose this threat.

This should not be the case.

The earth wire will connect to the case, but the negavtive wires should be floating. The negative output is typically tied to earth via a small capacitor for bypassing electrical noise, but thats about it.

All the secondary side should be isolated from the input. If this is not the case, then that powersupply is illegal and would not conform to the Australian Standards.

Originally Posted by Pearo

This should not be the case.

The earth wire will connect to the case, but the negavtive wires should be floating. The negative output is typically tied to earth via a small capacitor for bypassing electrical noise, but thats about it.

All the secondary side should be isolated from the input. If this is not the case, then that powersupply is illegal and would not conform to the Australian Standards.

It seems you are confusing something. In most personal computer power supplies, the common of each single output voltage (usually a black wire) is actually tied to ground. You can easily verify this with an Ohm-meter. And nowhere does any Australian Standard known to me say, that this should not be the case.

Think about it. Primary and secondary are still perfectly insulated from each other, no matter if the negative terminal of the +3.3V, +5V, +12V outputs and the positive terminal of the -12V output are all connected to ground. The mains input phase and neutral are still insulated from the outputs (that is, somewhere in your main distribution box all the neutral and the ground wires will be bonded together and connected to an earth stake).

Originally Posted by cba_melbourne

It seems you are confusing something. In most personal computer power supplies, the common of each single output voltage (usually a black wire) is actually tied to ground. You can easily verify this with an Ohm-meter. And nowhere does any Australian Standard known to me say, that this should not be the case.

Think about it. Primary and secondary are still perfectly insulated from each other, no matter if the negative terminal of the +3.3V, +5V, +12V outputs and the positive terminal of the -12V output are all connected to ground. The mains input phase and neutral are still insulated from the outputs (that is, somewhere in your main distribution box all the neutral and the ground wires will be bonded together and connected to an earth stake).

And in the case of the power system in use in most Australian Rural Areas such as mine, the neutral and earth are bonded together and connected to an earth stake which then provides the connection to the neutral conductor back to the 3 phase section of the grid. There is only one wire strung on the power poles running down the road. The neutral is carried by the ground, as in dirt. The stuff you stand on. This is called SWER. Single Wire Earth Return.

Dean

Originally Posted by cba_melbourne

It seems you are confusing something. In most personal computer power supplies, the common of each single output voltage (usually a black wire) is actually tied to ground. You can easily verify this with an Ohm-meter. And nowhere does any Australian Standard known to me say, that this should not be the case.

Think about it. Primary and secondary are still perfectly insulated from each other, no matter if the negative terminal of the +3.3V, +5V, +12V outputs and the positive terminal of the -12V output are all connected to ground. The mains input phase and neutral are still insulated from the outputs (that is, somewhere in your main distribution box all the neutral and the ground wires will be bonded together and connected to an earth stake).

I have been repairing switch mode powersupplies for the best part of my working life and I have never seen the negative rail connected to ground. But since you mention it, I have a dead pc downstairs, I shall go pull it to pieces and check.

FWIW, I have seen the negative rail tied to ground via a resistor, also for noise bypassing.

Originally Posted by Oldneweng

And in the case of the power system in use in most Australian Rural Areas such as mine, the neutral and earth are bonded together and connected to an earth stake which then provides the connection to the neutral conductor back to the 3 phase section of the grid. There is only one wire strung on the power poles running down the road. The neutral is carried by the ground, as in dirt. The stuff you stand on. This is called SWER. Single Wire Earth Return.

Dean

All mains power systems in Australia have neutral tied to earth at the switch board. The only exception IIRC is distribution transformers which can use the utility fot the neutral earth connection.

Originally Posted by Pearo

This should not be the case.

The earth wire will connect to the case, but the negative wires should be floating. The negative output is typically tied to earth via a small capacitor for bypassing electrical noise, but thats about it.

I think you will find that this is not the case ! I've yet to see a AT/ATX computer with an isolated negative DC power connection.

All the secondary side should be isolated from the input. If this is not the case, then that power supply is illegal and would not conform to the Australian Standards.

I agree with this comment ! This is why the metal case is earthed and why the output negative is also connected to it. Should a fault occur in the isolating transformer that allows a connection to the incoming mains live then it will trip the ELCB or blow the fuse.

As an aside, I've had computers that would not work in properties that had ELCB protection simply because of leakage current through the suppression capacitors across the mains input. This also includes brand new machines !

Originally Posted by Pearo

I have been repairing switch mode powersupplies for the best part of my working life and I have never seen the negative rail connected to ground. But since you mention it, I have a dead pc downstairs, I shall go pull it to pieces and check.

FWIW, I have seen the negative rail tied to ground via a resistor, also for noise bypassing.

General purpose switch mode power supplies do never have their outputs grounded. It is left to the end user to decide, depending on the application, if analog and/or digital grounds are to be tied to earth or not, and where exactly this tie is to happen. After all, the power supply can be equally well used in a CNC tool machine controller, or in a Medical MRI scanner, or in a TV transmitter.... there are hundreds if not thousands of different uses for such power supplies, all with different requirements.

But with PC power supplies, the end use is always to power a PC. Therefore it makes sense to bond all common grounds together inside the power supply, and to earth them to the PS chassis.

Here some info on ATX supplies:
http://en.wikipedia.org/wiki/ATX

Here some excellent info on switch mode supplies in general, but based on a PC supply:
https://www.onsemi.com/site/pdf/inside_the_power_supply.pdf

Originally Posted by Oldneweng

And in the case of the power system in use in most Australian Rural Areas such as mine, the neutral and earth are bonded together and connected to an earth stake which then provides the connection to the neutral conductor back to the 3 phase section of the grid. There is only one wire strung on the power poles running down the road. The neutral is carried by the ground, as in dirt. The stuff you stand on. This is called SWER. Single Wire Earth Return.

Dean

Hi Dean,

At the risk of this thread going even more off track...

This is not quite correct. There is no neutral with high voltage distribution systems, only phases and in the case of SWER an earth return. To create a SWER system usually two phases of the high voltage are fed to the primary side of an isolating transformer. The secondary side of the isolating transformer is connected to the insulated SWER line and the other side of the isolating transformer is connected to earth. Thus there are two conductors neither of which is a neutral. If you break the high voltage earth wire that runs from a SWER substation to earth, then the full SWER high voltage will appear at the open circuit. This does not happen with a broken neutral although that causes it's own issues. The high voltage SWER earth is usually a separate wire to the low voltage earth on the transformer pole. Note the use of an isolating transformer, without which the grid would see the arrangement as one phase shorted to earth and that section of line would be de-energised.

I notice that quite a few people here are talking about the neutral being connected to the earth wires and stake etc. The connection to the earth stake is not of great importance. I say that because the electrical resistance between the earth stake and the general mass of the earth is usually relatively high. This connection is unlikely to be able to carry much current. There is also no minimum value that the resistance to earth has to be below and it is not something that is tested by your sparky nor the people responsible for connecting the power to your switchboard (that would be people like me). The connection to earth provides a reference for the neutral of approximately zero volts. Your house is likely to function reasonably well without an earth stake (not recommended though). Your house will likely malfunction (quite badly actually, as in "leave the house now") if the main neutral is missing, that is another story however.

The connection between the main neutral, main earth and all the earth wires from the individual circuits is a different animal and quite important. That connection is what trips your circuit breaker in the event of a short to an earthed metal case for instance.

Cheers

The Beryl Bloke

This whole thread has gone off at a tangent and there have been several red herrings thrown in.

In not inclined to get out the standards tonight but a few things that need addressing.

1. in australia we have the M.E.N.( multiple earthed neutral) electrical earthing system where the neutral is bonded the the earth conductor and to the greater mass of earth at certain multiple points in the installed system.....don't get started on this one, the standards take pages to describe how and many very good electricians still don't fully understand it.

2. there is no requirement to make holes in cases so small you cant put small metal objects into them.....the test is the "standard test finger" which is about the size of an average womans or a small mans index finger....the latest version of the test finger is articulated.

3. there is an obligation for stand alone power supplies to have their 0 volt rail and the whole output floating and not connected to the mains earth uness there is a very good reason otherwise.

4. many pieces of electronics both analogue and digital have their 0 volt rail bonded to the mains earth....sometimes this is a solid 0 ohms hard wired bond other times it may be via a resistor, capacitor, inductor or a combination.

This is for both functional shielding, earth referencing or for supression purposes.




My whole point that computer power supplys should not be used stand alone is because of the multiple non-compliances they have as stand alone appliances.

now without direct reference to various standards.

They are not specifiaclly designed to be stand alone devices...they not designed to stand directly on a surface...they have a plie of wires dangling out of them.....they have no properly contrived connections or controlls...if placed unfavorably on a surface their ventilation can be obstructed

AND yes.....their 0 volt rail may be bonded to the mains earth...without a valid reason.....not my major concern though

If we go into detal we can probably look at the strain relieving of the cables and the pull forces those cables can withstand....because these divices are fairly compact inside combined with the poor strain relief of the DC output cables its possible that pulling on said cables may cause internal dammage and reduce internal safety clearances or cause shorts.

Yes there are very specific stength tests prescribed...and none of them involve standing on the product....one in particular involves a point loading impact test...and it may be applied to the weakest point of the device.



A couple of editions ago there was a concept introduced to many australian standards and it bites realy hard and on a wide range of issues in the electrical standards.....that concept is .."fitness for purpose"

AS they stand common or garden pc power supplies are not fit for the purpose of stand alone bench supplies.



I'm not saying it is impossible to modify them so they would be......but this discussion proves that most of you have no idea what is required to make these supplies fit for purpose as a stand alone bench supply.

My recommendation is..if you want to use these power supplies as part of other projects.....mount the PC power supply as it was designed in a computer case and house the project in that case.

Then most of the compliance issues just go away.

cheers

Originally Posted by soundman

Then most of the compliance issues just go away.

Who cares about compliance. We are not talking about on-selling these power supplies, or using them in a commercial situation, are we?

Again, maybe we use a little common sense - there are minimal "true" safety issues with using these.

Originally Posted by Vernonv

Who cares about compliance. We are not talking about on-selling these power supplies, or using them in a commercial situation, are we?

Again, maybe we use a little common sense - there are minimal "true" safety issues with using these.

Yeh its common sence that is definitely lacking here.

They don't write rules for no good reason.

for the most part compliance = safety.

The fact that so many of you can not see the safety issues is what is truly frigthening.



I've just had a second look a some of the power supplies in the stack I have.
They range from no name generis up to mid range antec and thermaltake supplies.

yes I have and will continue to fire these up sitting on my work bench, for testing purposes

but there is no way I would leave these lying arround where other people could be handling them..I certainly would not be carrying one arround like a portable supply.

In spite of having a never ending supply of them..I won't be using one as a general purpose bench supply any time soon...particularly without extensive safety modifications.

cheers

Originally Posted by soundman

The fact that so many of you can not see the safety issues is what is truly frigthening.

The only thing I see is a mountain where a mole hill once was.

I am yet to see in this thread one reasonable safety issue of using a PC PS as a bench power supply in the situation we are discussing.

Originally Posted by Theberylbloke

............

At the risk of this thread going even more off track...

Some good info for Dean on that subject can be found here:

http://en.wikipedia.org/wiki/Single-wire_earth_return
http://ruralpower.org/faq/swer-faqs-3

Originally Posted by Vernonv

The only thing I see is a mountain where a mole hill once was.

I am yet to see in this thread one reasonable safety issue of using a PC PS as a bench power supply in the situation we are discussing.

To charge Heli batteries? No kids in the household that could be hurt? Then I have no safety issue here. Other than you are going to ruin the expensive heli batteries if charging them with a primitive constant voltage power supply. Dedicated chargers have quite a bit of intelligence on board to charge batteries fast and fully without overheating them. Those modern model helis/quadrocopters etc use very light, high power density batteries than can and will explode and catch fire if incorrectly charged.

To drive an automatic feed for the mill? You do your own safety assessment here.
- I would personally not feel comfortable with the risk of metal dust and chips falling into an unprotected ATX supply.
- I would also not feel comfortable in an unheated and sometimes humid workshop with the pressed paper type circuit boards used in most cheap ATX style power supplies. For pressed paper the insulation distances primary/secondary are too small for my taste. I would at least make sure it is an upmarket quality ATX power supply with an epoxy board. That is just me, you may see that differently.
- Also, I would never consider using a "used" PC power supply. I have already seen too many broken ones. For me, it is either a brand new one, or none at all. But that is just me. I have no issue though at using pre-loved professional grade switch mode power supplies for my machine tools - in fact virtually all I use are 2nd hand. But these are far better designed and made than your average home computer power supplies.