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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 4th Apr 2022, 3:06 pm   #1
Roger Ramjet
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Post Desktop PSU Alternate Use

Hi, I am trying to diagnose a fault with my sons garden lights which are powered via a 5 Volt 3A 15W plug top PSU which I suspect the is faulty because it gets very hot & buzzes even with no load applied.
So my question is, to prove the PSU fault, can I substitute it with a salvaged Desktop PSU by connecting the LED string to the 5V O/P wires on the assumption that it can deliver 3A for a short while.
The lights are good quality ones from Costo so worth saving.
Thanks, Rog
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Old 4th Apr 2022, 3:17 pm   #2
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Default Re: Desktop PSU Alternate Use

It's possible to do this, though not very convenient. You will need to briefly ground pin 16 of the ATX connector (green wire) to get the PSU to switch on.

Another 5V wall wart would seem to be a better idea. Many forum members will be able to provide one for the price of the P&P, or you could use a Poundland USB charger which offers a 5V supply.

To simply demonstrate that the lights aren't faulty, use 3 AA cells.
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Old 4th Apr 2022, 3:25 pm   #3
Mark1960
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Default Re: Desktop PSU Alternate Use

It might be better to test the voltage output of the psu with a multimeter, both disconnected from the lights and connected. If you don’t have a multimeter then try a bulb from an old torch or bicycle lamp, but look for one with more than 5v rating, maybe in local pound shop.
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Old 4th Apr 2022, 3:52 pm   #4
Roger Ramjet
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Default Re: Desktop PSU Alternate Use

Thanks Mark & Paul for your brill advice on both counts.

Trying to avoid purchasing a PSU in case it was faulty LED string that blew the origional one. Like the idea of 3 x 1.5 V batteries in series though so may try that if I cannot get the ATX PSU to switch on.

I have a DMM & diagnostic knowledge if push comes to shove.

Cheers, Rog
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Old 5th Apr 2022, 10:54 am   #5
Roger Ramjet
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Default Re: Desktop PSU Alternate Use

Hi again Paul & Mark,
Found an old Compaq PSU that had an on / off switch on rear. Gave out a steady 5 Volts (rated at 18 amps) so more than enough for the 3 amp load on the LED strings.
Anyway the lights worked a treat so I will now leave it to my son to obtain a replacement PSU from Costco.
Will set up the Compaq PSU on my workbench noting more & more gadgets now needing a 5 Volt supply.
JD & once again thanks for your earlier advice.

PS - Slightly off thread the faulty Costco PSU was full of water despite IP rated !
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Old 5th Apr 2022, 11:54 am   #6
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Default Re: Desktop PSU Alternate Use

One thing to be aware of is that PC power supplies can supply a great many amps from their 5V output, which they will try to maintain even into a very low resistance load.

If you are powering something which doesn't need anything like that many amps it might be an idea to put a sensibly rated fuse in the +ve output. Otherwise if you try to power, say a project which needs 5V and introduce an accidental short, the power tracks on your project will probably vapourise before you can blink.
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Old 5th Apr 2022, 12:10 pm   #7
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Default Re: Desktop PSU Alternate Use

Another way to solve the problem with a PSU with a supply above the limitations of a 7805, is to use more than one in parallel, with one leg of each paralled through a diODE.
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Old 5th Apr 2022, 12:17 pm   #8
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Default Re: Desktop PSU Alternate Use

Quote:
Originally Posted by paulsherwin View Post
It's possible to do this, though not very convenient. You will need to briefly ground pin 16 of the ATX connector (green wire) to get the PSU to switch on.
I think you need to hold the green wire to ground to keep it on?

I use something like this https://www.ebay.co.uk/itm/224454029...hoCa7cQAvD_BwE

It has fuses to stop big accidents but do note it still needs care as it is easy to leave it loose and let the terminals or back of the PCB touch ground!
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Old 5th Apr 2022, 12:42 pm   #9
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Default Re: Desktop PSU Alternate Use

No, you just need to ground it momentarily to get the PSU to start up. That's why the power switch on a PC doesn't latch. I think they will also work if you ground it permanently though.

They can indeed deliver a very substantial current, so care needs to be taken.
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Old 5th Apr 2022, 2:40 pm   #10
Roger Ramjet
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Default Re: Desktop PSU Alternate Use

Quote:
Originally Posted by SiriusHardware View Post
One thing to be aware of is that PC power supplies can supply a great many amps from their 5V output, which they will try to maintain even into a very low resistance load.

If you are powering something which doesn't need anything like that many amps it might be an idea to put a sensibly rated fuse in the +ve output. Otherwise if you try to power, say a project which needs 5V and introduce an accidental short, the power tracks on your project will probably vapourise before you can blink.
Yes i can understand that but makes me wonder how "safe"' this arrangement would have been on these older machines i.e. the PSU output sockets often plug straight onto the motherboard so what would happen if a component e.g. capacitor failed to short circuit ?
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Old 5th Apr 2022, 3:42 pm   #11
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Default Re: Desktop PSU Alternate Use

Things could get quite lively. Normally in circumstances like that something will get burnt open or blown open circuit.
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Old 5th Apr 2022, 3:46 pm   #12
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Default Re: Desktop PSU Alternate Use

I once fitted a floppy disk power connector incorrectly causing a short. It melted the 5V line wiring - lots of smelly smoke and melted PVC. I got the power off before a fire started.
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Old 5th Apr 2022, 4:05 pm   #13
Roger Ramjet
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Default Re: Desktop PSU Alternate Use

I can relate to that Paul, although my folly was with a motorbike wiring loom !

In my naivety, I would have expected some form of internal over current protection on these PSU's noting that modern switch mode units shut down if excess current detected.

Now I wonder if any desktop PSU's ever went up in smoke ?

A fuse, a fuse, my kingdom for a fuse LOL

Rog
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Old 5th Apr 2022, 4:43 pm   #14
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Default Re: Desktop PSU Alternate Use

I think in theory a dead dead short would be detected and some kind of limiting would kick in, but where you have a fault condition exhibiting a resistance of a few ohms, that may look like a legitimate load to such a high current capable supply.

Last edited by SiriusHardware; 5th Apr 2022 at 4:59 pm.
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Old 5th Apr 2022, 4:57 pm   #15
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Default Re: Desktop PSU Alternate Use

PC PSUs do have over current protection built in; it's just that the current that a given rail can supply is rather high and quite capable of melting wires or PCB tracks if abused. Looking at some typical current ATX desktop PSUs, the 5V rail can supply 20A.
Since the Pentium 4 was introduced around 2000, the most important rail is +12V as this is used to supply the DC to DC converters that power the CPU and memory. A CPU may need a core supply of around 0.9V with peak currents of around 200A.

John
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Old 6th Apr 2022, 7:53 am   #16
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Default Re: Desktop PSU Alternate Use

A point worth noting when re-purposing old computer psus is that they generally require a minimum load on the primary output rail, generally the 5V one, before the other rails regulate properly.

Peter
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Old 6th Apr 2022, 10:47 am   #17
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Default Re: Desktop PSU Alternate Use

This point is often made, but I must say that it hasn't been the case in my experience. All I can think of is that some PSUs need a light 5V load to work and others don't.
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Old 6th Apr 2022, 11:53 am   #18
Phil__G
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Default Re: Desktop PSU Alternate Use

For bench-testing 5v projects I find a single-cell 'power bank' (for charging mobile phones) is very handy. The small ones dont need a lot of current to stay on. Portable projects like my Roberts radio wifi conversion can be permanently powered by a small internal powerbank
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Old 6th Apr 2022, 11:57 am   #19
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Default Re: Desktop PSU Alternate Use

Post No.5- By the way, IP ratings are predictably complex!

https://www.enclosurecompany.com/ip-...-explained.php

Dave
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Old 6th Apr 2022, 12:26 pm   #20
Roger Ramjet
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Default Re: Desktop PSU Alternate Use

Quote:
Originally Posted by The Philpott View Post
Post No.5- By the way, IP ratings are predictably complex!

https://www.enclosurecompany.com/ip-...-explained.php

Dave
Yes agreed Dave, I remember from college when doing 16th Edition IEE.

Plug top PSU spec is IP44 which tallies with full immersion in water up to 1.8 metres. Clearly not up to the job.

Rog
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