Thread: modified computer power supply?

http://rc.runryder.com/helicopter/t656700p3/

http://www.helifreak.com/showthread.php?t=355662

http://www.rcrotorz.com.au/showthrea...V-power-supply

These came up with a Google search indicating that I had accessed them before. There should be some information in them.

Dean

Before everybody gets all excited.

One thing must be understood......

Computer power supplies are not designed to be stand alone units.

They are components intended to be mounted in a housing....ie a computer case.

Standing on their own they do not comply with electrical standards.

If you do intend to use computer power supplies for other purposes, I strongly recommend you mount them in some sort of housing.

cheers

Originally Posted by BobL

Computer power supplies need a constant small load applied at start up to stay switched on. If this is not there then they will not turn on.

As I mount my modified power supplies in a suitable (safe) case, I always add a small fan to provide the necessary load as well as cooling. A couple of LEDs will help if the fan is not enough load. If it's a name brand supply, the specification sheet will often mention the minimum load required.

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.

You dont need to put load on the p/s to make it work. from memory you will need to connect 2 wires together

edit: found the link, just short the green to the black http://www.wikihow.com/Convert-a-Com...b-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.

No I am saying exactly what I said.

Computer power supplies are not stand alone appliances and in many ways are not fit for use as such.

There are many reasons for this.

That is before the relationship between mains earth and dc supply common is considered.

The use of a computer power supply can be compared to using a bare unhoused transformer from the point of view of legality, fitness for purpose and safety.

cheers

Originally Posted by jackaroo

You dont need to put load on the p/s to make it work. from memory you will need to connect 2 wires together

edit: found the link, just short the green to the black http://www.wikihow.com/Convert-a-Com...b-Power-Supply

Both are needed. A load is definitely needed - all of the two dozen or so I have dabbled with needed a load to switch on.
Apparently some of the older ones didn't need a load or a short.

Originally Posted by soundman

The use of a computer power supply can be compared to using a bare unhoused transformer from the point of view of legality, fitness for purpose and safety.

Legally maybe, but safety wise I can't see it myself. A bare transformer has direct finger access to HV, PSUs are covered with an earthed case and no direct finger access.
How is it different to any metal bodied electrical device?

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.

Not quite correct. This is covered in the links I posted. It is required that the the "DC neg output" that feeds to the DC pos input of the next supply, is insulated from its case. This is only the "DC" I am talking about. The AC connections are not changed in any way. Both cases are still earthed and safe. I have modified one of my PS's. From memory it involved removing a wire and removing the circle of copper trace on the circuit board for 2 mounting screws.

I have read about disconnecting the earth wire, but as I am sure everyone is aware and I picked immediately, this is a "NO NO".

Originally Posted by jackaroo

You dont need to put load on the p/s to make it work. from memory you will need to connect 2 wires together

edit: found the link, just short the green to the black http://www.wikihow.com/Convert-a-Com...b-Power-Supply

Thanks for this info. It is mentioned somewhere in the links I posted, but there is a lot of reading there. I will try to find where I have "stored" the stuff and make sure this info is written on the PS's so I don't have to look it up when I need it.

Dean

Originally Posted by BobL

Legally maybe, but safety wise I can't see it myself. A bare transformer has direct finger access to HV, PSUs are covered with an earthed case and no direct finger access.
How is it different to any metal bodied electrical device?

A bare transformet has no direct finger access to 240v if the primary is taken thru a terminal block......if the mains cord is joined in a covered terminal block and the primary windings have sleveing over the tails there is not even access to primary insulation

the bare transformer has no issues with ventilation and is guaranteed to have 11kv isolation between the primary and all secondary terminals.

I don't wish to get out AS3100 and estail every way in which a computer power supply does not comply

Computer power supplies are not appliances and should not be left sitting aruound un housed and unproteced.


the easiest way to deal with most of the issues is to mount the computer power supply and whatever else you are building in a computer case.....and put the lids on.

cheers

Originally Posted by BobL

Legally maybe, but safety wise I can't see it myself. A bare transformer has direct finger access to HV, PSUs are covered with an earthed case and no direct finger access.
How is it different to any metal bodied electrical device?

I also would like to know this. The Dell PS's I bought are vastly different to any supply I have seen before. They are roughly 60mm square and about 300mm long. They are much more rigid than a common one.

Dean

[QUOTEA bare transformet has no direct finger access to 240v if the primary is taken thru a terminal block......if the mains cord is joined in a covered terminal block and the primary windings have sleveing over the tails there is not even access to primary insulation
][/QUOTE]

That is a lot of ifs! There is a vast difference between all your "ifs" and the solid metal case of my PS's. I am not saying you are wrong, but I suspect that the main issue is that they were not intended as being stand alone units and so did not get them rated as such. I cannot picture any risk that is different to some of the cheap import appliances I have dismantled and that are rated.

The only risk is electrical. This is not changed as the PS is screwed to the computer case. Structually, my PS's are far better than many applianced I have seen. It just seems hard for me to comprehend how there can be any further risk.

Dean

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.

The metal case of a bench or lab power supply is always connected to its metal housing. Not so the negative/common though, exactly to allow serial connection of several power supplies.

If putting two computer power supplies in series, it would be foolish and dangerous to disconnect the power cord earth lead on one power supply. Instead, it is very easy with most PC power supplies to disconnect internally the connection between the DC common and the metal housing. In Many cases, it suffices to remove the short wire that connects the common (black wirwes) to the chassis or housing. With some other designs, it may be necessary to insulate one or several of the the PCB mounting screws. In either case, I recommend to verify afterwards that the mains earth is still connected to the metal housing - as there is always the possibility of some very cheap PC power supplies being around, that combine DC common and AC earth in one single wire or screw connection.

I fully agree with Soundman on the need for a separate enclosure. Otherwise a conductive object could enter the computer power supply by its cooling air intakes. These cooling intakes are rarely protected by a separate grille or mesh - exactly because these are supposed to be inside the PC case. Alternatively, one should at the very least consider to add a mesh to the open air intakes.

Please be aware that many non-brand name PC power supplies are very very cheaply made. Some housings are so thin walled, that you can squash them by hand like an empty soup can. They are designed to be reinforced by the pc case they are mounted into.

They also may have very large AC ripple voltages overimposed to the DC outputs, making them unsuitable for many tasks. AC ripple of some 400mV (0.4V) in the range from some Hz up to 20Mhz is not that uncommon - that is about 20 times more than you would expect from a real switch-mode lab power supply, and about 200-800 times more than what you would expect from a linear lab power supply. Good brand name PC power supplies will (when new) have much less ripple, more like in the order of 50-100mV. Compare this to the average adjustable linear bench power supply with less than 2mV, often as low as 0.5mV. Or the average adjustable switch mode lab power supply with less than 20mV.

There is also the safety risk of a primary to secondary short when the opto coupler used for sense voltage feedback to the chopper fails - on a $30 power supply, I would not bet my life on this component being chosen to be of a particularly high quality or insulation voltage...

All in all, I would dare to question if conversion of a computer power supply into a lab power supply is a worthwile idea. Consider that nowdays you can buy real (and actually quite good) adjustable linear lab power supplies very cheaply from Chinese and Taiwanese manufacture. One can also find very good professional used (pre-loved) linear bench power supplies for 1-200$. Vastly superior to any modded PC power supply. And much safer too.