Using an ATX computer power supply

[BrianB]

I have a few Spare ATX power supplies sitting in the cupboard and as I plan my first layout im wondering for the 12v supply for accessories/lighting/etc that I have these ATX supplies sitting there. Often before ive considered converting one for a bench power supply, there are various "how to" article on the net....

 

but has anyone else done it? or seen it done? For me it saves on more cost, and secondly even a half decent ATZ supply has quite strict power requirements and packs a punch on the 12v line output.

 

thoughts?

[Simon V]

I have a few Spare ATX power supplies sitting in the cupboard and as I plan my first layout im wondering for the 12v supply for accessories/lighting/etc that I have these ATX supplies sitting there. Often before ive considered converting one for a bench power supply, there are various "how to" article on the net....

 

but has anyone else done it? or seen it done? For me it saves on more cost, and secondly even a half decent ATZ supply has quite strict power requirements and packs a punch on the 12v line output.

 

thoughts?

 

Computer SMPS (Switch Mode Power Supplies) often rely on the 5 volt rail being sufficiently loaded to provide regulation of the other supply rails. Or to put it another way, it's the load on the 5 volt rail that regulates the entire supply.

 

Some of the cheaper Chinese imports are really crap designs and it's hardly surprising that motherboards fail when the PSU has a momentary glitch.

 

There is a heck of a lot of Amps available on the 12 volt rail of most computer PSU's. Do you really want to connect 100's possibly 1000's of pounds of model railway equipment to an item that can be purchased for as little as GBP 15?

[tebee]

I've used one of the older AT type supplies in the past, It supplied the power for my first DCC test layout. I seem to remember it needed some resistors as a load to work properly and I think I modded it to supply AC. It did not last very long in service, probably less than a year, but it was probably over 5 years old when I started to use it so it may very well have been near the end of it's life anyway.

 

Replaced it with a 20 GBP Maplin 5 amp PSU.

 

 

Tom

[BrianB]

yes you do have to load up the 3.3v line and also earth one other lines on the PSU... I consider the PSUs I have spare are good...not your 10 quid ATX cheapos.... 120mm fan for cooling... 400w rating etc... its just ive bought those 12v adaptors from places like maplin before and they seemed anything but reliable in true voltage, at low load they always seemed to read high v... but that was years and years ago.... and for the Computer kit I have run in the past the money value is every bit as much as anything im likely to be able to afford in model railways. Oh well, I will try it and see at this stage..... also in Canada we dont have Maplins (I wish) we have "The Source" which sells expensive electronics junk..... worse than useless so I was trying to find a cost effective alternative...

[Dr.Glum]

Simon V obviously knows a lot more about these things than I do, but I've been a very happy user of similar units for a couple of years. I have only used them for lighting circuits (in the scenery), but that has been grand. The maximum rating on the 12v and the 5v is probably printed on the case of the units: do some sums - you should be able to run lots of grain-of-wheat bulbs or hundreds of leds. I always use higher than necessary resistors to derate the leds anyway (to bring their brightness down to a more realistic level for the 1950s period), so if the output were to vary, they are shoould still be safe against over-voltage. If you're thinking of running locos off one, then I don't see the problem as long as the voltage is constrained within what is 'safe' for the motor. I wonder what Simon V was thinking of that I'm missing?

 

I don't understand the price comparison with new PSUs. The service life of ones that I have personal experience of in recent years have not been anything to write home about i.e. short. Whereas your's are free. Go for it!

Cheers, Tony.

[Simon V]

I wonder what Simon V was thinking of that I'm missing?

 

 

 

Very quick lesson on switch mode power supplies.

 

As I said some of the cheaper Chinese units are pretty rubbish design's that have been ripped off from other units. The quality of the parts used leaves an awful lot to be desired and they appear to be built down to a price rather than up to a specification.

 

Good switch modes are great, lots of Amps in very compact units with good protection on the outputs but these cost money.

 

The principle of operation is basically a loop and like a conventional supply there is a primary and secondary side to the supply.

 

In a conventional supply AC mains is fed into a transformer and the AC outputs of the transformer are rectified to give DC which is then regulated, usually with a voltage regulator chip. This is fine for a few Amps but beyond that the transformers get big, heavy and expensive.

 

Enter stage right the switch mode power supply. AC mains is fed in as before but is rectified straight away so we now have something in the region of 340 Volts DC, yes that is three hundred and forty! Which just in case any of you are wondering can be lethal and at best will wake you up. This is fed into a transformer via a transistor. The transformer is no where near the size of it's conventional supply equivalent. The transistor is then switched on and off very quickly - normal AC mains is 50 Hz, it oscillates 50 times per second. This transistor will be switching somewhere in the KHz range, several thousand times per second. This on/off switching induces a voltage in the secondary windings of the transformer. The current is dependant on the rate at which the transistor is switching. In computer supplies the 5 volt rail is usually the one that supplies the most current, 25 Amps is not uncommon and this is enough to weld with! The load on this output is normally monitored and fed back to the primary side via either a small transformer or an optical coupling device. This maintains the isolation between primary and secondary. The whole circuit oscillates within given parameters and we get a nice stable output. The remaining supply rails e.g. -5v +12v -12v are designed to regulate based on what the monitored supply rail is doing.

 

It is not common for the voltages on the primary side to reach the secondary side in failure mode but if the basic mode of operation fails then the secondary voltages can and do, do some strange things. In PC's this normally results in a dead mother board and the whole lot dies very quickly. In a model trains application you will end up with blown bulbs/LED's possibly melted cable and a lot of work and expense to put it right.

 

A WORD OF CAUTION - If any of you are tempted to use a computer supply for your modeling and decide to re-case the supply in a more visually pleasing box beware that the primary side of these PSU's have 340 volts DC running at a high frequency. It is easily enough to kill you and at best will leave you twitching for a while. Don't for one minute think that these things are safe switched off either. There are capacitors in the primary circuit that will sometimes retain their charge for several minutes after switch off, the jolt you will get is as bad as if the thing is switched on.

 

I am not saying don't use them but do understand all of the risks and pitfalls.

[Simon V]

By the way

 

While an older AT style PSU will usually give you a voltage when switched on an ATX supply will not as it's switch on circuit runs through the motherboard to the "soft" on /off switch on the front of the PC.

 

You can persude them to switch on by connecting two of the wires on the multiway connector together but I am not going to tell you which they are as I don't want to be responsible for the possible outcomes of what you are doing.

[Kenton]

Excuse me for bumping this warning.

 

We have had these DIY power supplies posted on RMWeb before - the threads tend to get locked down pretty quick.

 

This sort of thing is no problem for the experienced and knowledgeable electrician but a potential killer for the unwary and experimenter.

The wider membership are generally not electricians and need to read that good advice already given, or better still just steer well clear of the whole side of messing with mains power.

I would hate to see some statement saying "He was just following advice given by RMWeb"

 

A WORD OF CAUTION - If any of you are tempted to use a computer supply for your modeling and decide to re-case the supply in a more visually pleasing box beware that the primary side of these PSU's have 340 volts DC running at a high frequency. It is easily enough to kill you and at best will leave you twitching for a while. Don't for one minute think that these things are safe switched off either. There are capacitors in the primary circuit that will sometimes retain their charge for several minutes after switch off, the jolt you will get is as bad as if the thing is switched on.

[Kenton]

Excuse me for bumping this warning.

 

We have had these DIY power supplies posted on RMWeb before - the threads tend to get locked down pretty quick.

 

This sort of thing is no problem for the experienced and knowledgeable electrician but a potential killer for the unwary and experimenter.

The wider membership are generally not electricians and need to read that good advice already given, or better still just steer well clear of the whole side of messing with mains power.

I would hate to see some statement saying "He was just following advice given by RMWeb"

 

A WORD OF CAUTION - If any of you are tempted to use a computer supply for your modeling and decide to re-case the supply in a more visually pleasing box beware that the primary side of these PSU's have 340 volts DC running at a high frequency. It is easily enough to kill you and at best will leave you twitching for a while. Don't for one minute think that these things are safe switched off either. There are capacitors in the primary circuit that will sometimes retain their charge for several minutes after switch off, the jolt you will get is as bad as if the thing is switched on.

[Dagworth]

Dagworth and Ravensclyffe both run from an old AT type PSU which has been recased. I would second all the warnings above, if you don't know EXACTLY what you are doing then DON'T DO ANYTHING with one of these things.

 

The outputs that I have from mine are +5 volts at 20 amps, +12v at 8 amps, -12v at 0.5 amps. The 5 volts supply runs all my logic and other electronics, the +12 volts supply runs all scenic lighting, and both the + and - 12 volt supplies give me a very useful split rail supply ideal for running tortoises.

I'm on my second one now, the first one had a heat sink come loose and short inside the unit, cue some very big sparks from inside the box.

 

If you do know what you are doing these can make for very reliable and stable power supplies, if you have any doubt at all then stick with a commercial power supply designed for the job you want it to do, or seek the advice and assistance of someone who does know what they are doing.

 

One other comment, the 0volt rail will be connected to mains earth, so if you are soldering on the layout with the PSU on then any of the other outputs are a potential short to the tip of the soldering iron!

 

Andi

[BrianB]

Thanks for the warnings. My other hobby is restoring antique radios, so for me im pretty cautious and know the nasties Capacitors can do... first stage of any antique radio restoriaion is to recap.... but I then test my work behind an isolation transformer and a variac.... and plugged in via a GFCI socket. Also because much of that stuff can make a 70 year old chassis live with ease!

 

I certainly wont be running an ATX PSU out of its original case.... and would advise others dont either

 

ive decided once I get to work on the layout I will use one of my decent ATX supplies to provide 12v supply.... and see how it goes. Its a long way away at this stage, im just planning.... but it will be a budget saver for now.

[Coombe Barton]

I'll just put my 2 pennorth in here. I'm experienced in computers, electrical and electronic stuff, but the potential for future hassle is just too great. If you use commercial power supplies designed for the purpose then, if you exhibit, you're covered. If you hack something around, you ain't. Even if you don't exhibit and some minor unexpected fault in your power supply causes your house to catch fire your insurance may not pay out.

 

Now I have no doubt that I could modify computer power supplies to produce the correct output but I'm not going to. I'm going to buy what I need (after all they're cheap enough) then I'm covered and have saved myself the time. Also computer power suplies have a limited life, after all, as commecial computer writedowns are now two years. Twenty years ago they were five.

 

I'll stick with Maplin, Radspads, or whoever provides the wall warts. I can then fall back on them if anything goes wrong. After all, they should be more skilled than I am. And if I ever exhibit I can carry spares.

[Miss Prism]

There was a very good article on wikihow:

 

http://wiki.ehow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

 

but it seems to have been re-classified somewhere - worth searching for.

[bertiedog]

I have used these switched supplies for MR uses, they do need a basic load though, O/P voltage will be wrong with no load. I would still recommend a proper transformer, and a bridge rectifier plus regulator, if home made.

It's often said , "but what about transformer costs with high ampages," but there is a simple way, use a trickle charger and a lead acid battery, the sealed types. They can be rigged to float charge or just charge before a session of use. Hours of high DC current available.

 

Stephen.

[APOLLO]

Interesting topic. I have a couple of old PSU's in the loft ex old computers, I was going to tinker around with, untill I read this thread. I'll chuck em out tomorrow, at least it'll please the missus, less junk !!.

 

Thanks guys for the warnings. 340v DC, 20amps !!!! Christ, the WCML at the side of my house doesn't use much more !!!!!!!

 

I'll stick with gaugemaster. Expesive, though good stuff, AND you get a LIFETIME guarantee.

 

Brit15