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Computer PSU - modelling applications

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I have upgraded my computer PSU and I'm wondering if the old one (ATX 750W) might be adapted for model rail use. Any thoughts please?

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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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Thanks. How do you use the various outputs? The most useful appear to be 12V and 3.5V DC

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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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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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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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