Using computer power supply for HO--safe?

This subject has been discussed before, but I'm reviving it with hopes of new input and new info.
A computer power supply, especially some compact ones I've
come across seems like an ideal HO 12V DC source to wire one or more transistorized throttles to. Plenty of amps to spare.
My only trepidation is-- someone brought up on a past thread that these supplies work by switching from the labeled voltages to a high voltage which might introduce itself to the rails if the supply goes defective. Would someone who knows elaborate on this? The risk doesn't seem worth it. That would be a shame because of the pure filtered DC a PS offers.
Thanks, Robby
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The PC power supply rectifies the line power, switches it through the 5V regulator circuit to the various voltages needed by the computer. +12V is obtained by a post regulation of the transformer output as is the other voltages. Please note that it is the 5V output that controls the switching regulation and everything else is just run off of that which means that the 5V output of the supply needs to have some load on it in order to keep things running right. In addition, lightly loading the 5V supply will ionsure that the other voltages won't be able to handle their full outputs either. If you're going to be running layout lights on the layout with the 5VDC and there are a bunch of them, go for it. As a result of the above, you can indeed run the trains on a PC power supply but it really isn't all that great to do so. IN addition, pure DC isn't the best thing to run a train on as the low speed performance of the trains suffer on pure DC as there is no pulsation of the supply to assist in getting the motor to turn.
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robby wrote:

The PC power supplies can furnish a couple of amps of +5V less than 1 amp of +12V power. The PC supplies are cheap, and often available for free from scrapped out computers. They come with a good metal box, to keep small fingers out of the 120 VAC primary power. Additional bennies are a detachable line cord, and a good on-off switch. While failure is always possible, these supplies are widely used in consumer products where safety is a prime concern, 'cause the maker will get sued down to his sox if the PC electrocutes a customer. I have not qualms about using such a piece of gear on my railroad. The enclosure, line cord, on-off switch in PC power supplies is so attractive, that I use old PC supply boxes to contain home brew power supplies for lights and switch machines.
David Starr
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"David J. Starr" wrote:

My concern with using a PC power supply for a model railway p/s is that the 5 and 12 volt grounds are earthed. If the 12 volt supply is commoned to the layout then the common rail of the layout is earthed rather than the tracks being isolated from the mains. Any comments from the experts?
Regards, Greg.P.
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Gregory Procter wrote:

I haven't seen a consumer device with a hot chassis ("earthed") since vacuum tube TV sets went away, and that was quite a while ago. All modern equipment has a transformer that steps the 120 VAC line down to 12 or 5 volts and to provide isolation from the power line. I will admit that I haven't had a PC power supply apart in quite a few years, so I might be mis remembering things. To check, take the cover off the PC supply and look for a power transformer. Follow the 120 VAC input from the 'power entry module' (dingus that accepts the detachable line cord). You should see the 120VAC wiring go thru the power switch and to an iron core transformer. For extra suspenders-and-belt security, ohm out the power transformer. it should show an open between primary and secondary. Or, just ohm out between the output wires and the power line input. It should show open or at least an impedance greater than 100 K from input to output. Remember the PC supply was designed to allow computer users to poke around inside the computer with power on with out getting electrocuted.
David J. Starr
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Computer power supplies do not use a transformer. They are a switching power supply.
"David J. Starr" wrote:

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Chuck Kimbrough wrote:

Hence my concern that "ground" is household earth! ( a little knowledge etc etc :-)
Regards, Greg.P.
PS I once bought a new case with p/s (NZ 240 volts) and discovered the hard way that the case was at 110 volts.

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I used a power supply from a large computer (not a PC) on my last layout.
Only problem I had was that it gave a regulated 12v, where must model rr power supplies give 14 or 16 volts so as to provided that extra "omph" and maybe compensata for voltage drop to ends of the leayout.
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Chuck Kimbrough wrote:

Um, er, ever had one apart?
Look again, chum.
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Robby again. Thanks for replying. Comments:
Bob May: "...pure DC isn't the best thing to run a train on as the low speed performance of the trains suffer on pure DC as there is no pulsation of the supply to assist in getting the motor to turn."
But I thought the modern 5 pole skewed can motors run better on pure DC. I read all over that pulse power is detrimental to newer motors.
David Starr: "The PC power supplies can furnish a couple of amps of +5V less than 1 amp of +12V power."
I've seen 250W and 300W supplies that offer something like 20 amps of +5 and 8 amps of +12. 8 amps can run a large empire railroad!
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The only problem that you will experience with model train motors is the ones that are coreless type motors like the Escap type motors. These have low inertia and will have shortened life problems when used on extremely pulsed type throttles like the ones that are just pulses with no underlying DC to provide contuining power to the motor. Pure DC provides no "kick" to overcome stattic friction to the locomototive and as such, low speed operation tends to be a quick start to a much higher speed than any other throttle will provide. Just rectifying an AC source will provide an excellent source of kick to the motor to allow much lower speed operation. I've operated on pure DC and it was bad for decent switching operations as the locos in question would immediately leap to about 10smph which just doesn't look nice to scale operations. I'll note that the locos in question were of the higher quality drives available. I'll note to another's post that PC supplies do indeed have a transformer in them otherwise the supplies wouldn't be usable on computers as a failure of the switching mechanism could supply 190V to the 5V line. In addition, the other voltages needed for the operation of a computer would be more difficult to do as seperate switching circuits would be needed for those supplies.
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Got into this one late... sorry.
robby wrote:

Typical 250W PS -> 3.3V @ 10A 5V @ 20A 12V @ 9A
Between the 3.3 and 5V only 125W total 108W for 12V

120V is half-wave recitified and filtered then presented to a active semi-conductor. The semi is in series with an Inductor which converts the line voltage to about 380VDC by pulse-width modulation. Feedback from the secondary of the inductor either lengthens or shortens the driving pulses to the semi. Output voltage is picked off from the secondaries and fed into bridge rectifiers and filtered. Having this type of setup lets you use just about any kind of input voltage, could be from 50V to 250V and the thing will stay in regulation.

Just like anything else, catastrophic failures can result in the secondary side of things receiving voltage spikes while the PS is dying. Fast acting crow-bars are sometimes placed on the output supplies that help protect secondary components, but don't always work.

Not pure filtered, but close. Typical noise is about 2% of line and varies according to load. Greater the load, the more noise you get.

Oh, and as a side note the ATX supplies sometimes don't have a power switch so you have to connect one to turn it on -> small momentary push button connected to the voltage sense on the output connector. Don't remember the terminals just now, but shouldn't be hard to find. I do like having the input of any power supply switched as you know it is off. Make sure that you don't switch earth ground, just line and neutral. With computers, they finally got around to putting leds on the motherboards to let you know that off sometimes isn't.

have fun!
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