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Computer PSU - modelling applications


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  • RMweb Gold

I've used AT power supplies very successfully on both my larger layouts, but the AT only requires a power switch. I know at ATX supply has a different start mechanism but other than that there is no reason why it shouldn't be useful

 

Andi

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  • RMweb Gold

I have the PSU in a separate box under the layout, I use the +12, -12 and +5 supplies (My layouts have significant  TTL type electronics that like a nice stable 5v supply) and the 0volt rail.

The two 12v supplies are used for point control as I find the 24v differential between them is bombproof for twin solenoids. Other items (mostly LEDs) use either the +12 or +5 to 0v supplies dependant on the brightness I want.

 

I have no need to the 3.5v outputs so ignore them.

 

Andi

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It's a while since I looked into this, but some computer PSUs require a minimum load on one of the outputs, used to be the 5V.

 

They can supply a LOT of current. I would add some additional protection between the PSU and your electronics.

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14 hours ago, Crosland said:

It's a while since I looked into this, but some computer PSUs require a minimum load on one of the outputs, used to be the 5V.

 

They can supply a LOT of current. I would add some additional protection between the PSU and your electronics.

Adding to Crosland regarding a LOT of current 

I couldn't find the rated currents for each voltage from an ATX but did find a mention of dangerous output limits of 240VA

So assuming the fault limit for each output is 240VA(240W at unity power factor) then

240W at 12V is 20A & 240W at 5V is 48A

Those sort of currents ARE VERY DANGEROUS ,may be enough to weld with, a metal watch band or a ring will get very hot if accidently placed across terminals, hot enough to cause severe burns

With High current supply's short circuit protection is very important

John

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  • RMweb Gold
7 minutes ago, John ks said:

Adding to Crosland regarding a LOT of current 

I couldn't find the rated currents for each voltage from an ATX but did find a mention of dangerous output limits of 240VA

So assuming the fault limit for each output is 240VA(240W at unity power factor) then

240W at 12V is 20A & 240W at 5V is 48A

Those sort of currents ARE VERY DANGEROUS ,may be enough to weld with, a metal watch band or a ring will get very hot if accidently placed across terminals, hot enough to cause severe burns

With High current supply's short circuit protection is very important

John

The AT supply I use has very fast short-circuit protection built in, it takes a few attempts to get it to start when the layout is first connected while it tries to charge all the capacitors running the point machines.

 

Andi

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20 hours ago, WIMorrison said:

A good set of instructions on how to use the PSU - assuming it is ATX which almost are these days here. It also provides the outputs for the various pins.

 

https://www.electronics-tutorials.ws/blog/convert-atx-psu-to-bench-supply.html

 

very useful supply of a good DC supply :)


Coincidentally, I just had a phone call from a friend asking if he could use a computer PSU to power various aspects of his layout. Your link was very timely, and I have forwarded the link to my mate.

Thanks. :) 

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  • RMweb Gold
7 hours ago, Dagworth said:

The AT supply I use has very fast short-circuit protection built in, it takes a few attempts to get it to start when the layout is first connected while it tries to charge all the capacitors running the point machines.

 

Have you thought about using some form of "Soft Start" ?

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20 hours ago, Dagworth said:

The AT supply I use has very fast short-circuit protection built in, it takes a few attempts to get it to start when the layout is first connected while it tries to charge all the capacitors running the point machines.

 

Andi

The PSU might have short circuit protection built in, but that protection is for lots of Amps and if the wiring of a model layout is to the usual thin wire, it's not going to activate the inbuilt protection. This means it will continue to pump out Amps, until something gives, which won't be the computer PSU.

 

Another form of current limiting is required, DO NOT rely on the inbuilt PSU.

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On 15/01/2020 at 01:51, Dagworth said:

The AT supply I use has very fast short-circuit protection built in,

 

Fine for faults that apply enough load to trigger it.

 

My house has an 80A breaker that operates quite quickly under fault currents. I would not rely on it to protect a lighting circuit but would use a faster, lower current breaker.

 

A 20A supply will happily source 20A forever, possibly to the detriment of whatever is connected downstream.

 

If nothing else, splitting it into multiple supplies, each with a lower current limit will aid fault finding.

 

The 240W quoted above is quite conservative for a PC power supply.

 

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One thing to consider is the type of controller the PSU will be connected to.

 

A computer power supply will only output smooth DC, which may be fine with appropriate protection for many controllers. However, if the controller is a PWM type, or measures back EMF by sampling, it may well not be suitable- most (all?) commercial controllers (and many published d-i-y designs) of that type require an AC input as it (or the unsmoothed full-wave rectified version) is used as a timebase for the PWM frequency and pulse start point - e.g. via a triac circuit, and the zero-crossing as a point to sample the emf. 

 

 

Edited by sharris
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Could be useful for welding white metal kits together?  As for use on the layout it's just too many amps for the sort of wiring we typically use, I have managed to get fish plates to glow red hot on 2 amps let alone 20.   I use all sorts of lap top, game boy and similar power supplies on the layout but no way would I risk anything with an output around 20 amps anywhere near my wiring.    

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  • RMweb Gold

You have to remember that PCs abide by modern power saving methods these days as used by the ACPI interfaces on modern operating systems such as windows....what does this mean.....well it means that ATX class power supplies have to be able react to violent demands....a modern graphics card can draw up to 500w within a few seconds or drawing just 10w now this could be on dual 12v rails or dual 5v rails, either way we are talking about 10s of amps...but what you have to remember is unlike the power supply perhaps in your DCC system or even NON dc system, the ATX power supply will see prolonged high current draws as normal behaviour so I must echo the wise warnings of Kevinlms and crosland.

 

AT power supplies are different they were never designed to supply in reality more than 250w (AT class computers i80286 to super-socket 7) I know some higher level supplies purport more wattage but you will see a stress test will max them out at around 300w at the very most....and the simple reason is graphics cards simply didn't draw that much power and neither did CPUs...AT power supplies therefore do not like repeated power cycling and violent demands, they will either blow the internal fuse or pop the smoothing capacitors. However an AT power supply outside of a computer can be quite dangerous as an AT power supply uses a 240v switch to turn it on....normally in a case it's earthed by the case.....when its not in the case and in your hand its earthed by you!!!!

Edited by pheaton
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For my current build (and if you cross-reference some of my other threads you might suspect that I'm toying with designing my own controllers), I'm considering using laptop power bricks to bring the mains down to something useful.

 

The most common will put out about 3.something amps at between 14 and 19 volts, depending on the type of laptop (generally about 50-60VA) - should be sufficient for my fairly modest requirements  - since I'm using Tortoise motors I don't have current surges on point switching.

 

Other advantages - completely enclosed, certified (assuming it's come from a reputable manufacturer) and, if necessary, easy to PAT test.

 

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12 hours ago, pheaton said:

However an AT power supply outside of a computer can be quite dangerous as an AT power supply uses a 240v switch to turn it on....normally in a case it's earthed by the case.....when its not in the case and in your hand its earthed by you!!!!

 

Can you be a bit more specific?

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Remember that the PC industry transitioned from the AT to ATX form factor in the mid to late '90s so an AT PSU you might find on your favourite auction site is quite likely at least 20 years old, pulled from old equipment and may now be of uncertain electrical condition.

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  • RMweb Gold
7 hours ago, AndyID said:

 

Can you be a bit more specific?

 

An ATX power supply works by connecting via the ACPI interface of a motherboard BIOS, hence the reason in an ATX based computer the power switch is a push to make switch and is connected to the motherboard and runs at 12v.

 

An AT power supply has an external switch on the 240v supply side of the power supply, and its a push and latch switch and you will notice they are quite hefty! this is because the switch and the wire that supplies it handles 240v! or 110v if you are in the US.

 

That is why an ATX power supply will gracefully shut down the computer if the power switch is toggled, and AT power supply will hard power it off because you are breaking the mains supply to it.

 

normally this switch is in your case and as its in your case its earthed by the metal structure which is bolted to supply unit case which is in turn earthed internally......outside of a computer case that switch and is not earthed!

 

 

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2 hours ago, pheaton said:

An AT power supply has an external switch on the 240v supply side of the power supply, and its a push and latch switch and you will notice they are quite hefty! this is because the switch and the wire that supplies it handles 240v! or 110v if you are in the US...

 

normally this switch is in your case and as its in your case its earthed by the metal structure which is bolted to supply unit case which is in turn earthed internally......outside of a computer case that switch and is not earthed!

 

Funny - I have a desk lamp that runs off 240v and the power switch and cable to that are quite light weight. Do you perhaps mean it's quite hefty because it has to pass a lot of current? (somewhere in the region of 3A at 240V or 6A at 110V for a quite generous AT PSU - less if you keep to the 250W you mentioned above).

 

What is most likely to electrocute you with an external  switch hanging on a wire is the mains line voltage exposed on the connections on the rear of the switch -  you should shroud the back of the switch (often you can find an appropriate shoe to fit) to make sure that connections cannot be touched. If a switch really needs a protective earth, I'd rather it had an earth wire connected to it, than relying on the enclosure to provide a good path to earth. 

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  • RMweb Gold

The switch on an AT PSU is on the high voltage input side so you are at the mercy of the 13amp fuse sat in the plug so an earth wire would be by far the safest and most switches were shrink wrapped but you hit the nail on the head in your earlier post where you said generally AT PSUs are now at a minimum 20 years old....but remember the electrical standards they abided to are 20 years old to....

Edited by pheaton
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4 hours ago, pheaton said:

 

 

An AT power supply has an external switch on the 240v supply side of the power supply, and its a push and latch switch and you will notice they are quite hefty! this is because the switch and the wire that supplies it handles 240v! or 110v if you are in the US.

 

 

In Australia we also use 230/240 Volts.

A friend was telling that his son blew up a PC once. Why because he was playing with the case one day and saw a switch, so flicked it to see what it did = BANG!

 

Yep, it was the selection switch between 240V and 110V.  At least he learnt something, without injury!

 

An advantage of ATX PSU's.

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9 minutes ago, kevinlms said:

BANG!

 

Quite some years ago, our company wrote some software for a client who wanted to demonstrate it at conferences, and being new software, we were to be on hand to do the demonstrations. The client plugged in their computers on their trade stand at a European conference where they were to debut the new software, and BANG - guess where they'd been the week before and forgotten to change the voltage selectors back! 

 

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