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Registered Member #1334
Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615
Many folk who use ATX PSUs as LAB supplies put a dummy load on the 5V or 12V lines - typically something like 10R (5V) or 25R (12V), i.e. about a 500mA dummy load.
There seems to be a school of thought that such supplies need loading in order to regulate properly or even to avoid damage although I can find no evidence that this is a requirement of any specs... specifically, accoring to the spec at no-load operation is allowed to cause shut-down, but should not cause damage.
Is this just apocrypha? I'm after a definitive answer if possible, not speculation!
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
section 3.5.3
No damage or hazardous condition should occur with all the DC output connectors disconnected from the load. The power supply may latch into the shutdown state.
Some power supplies may play nice, some may shut down. Sounds like something you could try and if it doesnt play nice, add a load as needed.
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
There is no definitive answer as there are zillions of different makes & models. Regardless of any specs, they are designed to make a PC work, which means there will be some load, so considering that they are designed for minimum cost, there is unlikely to be much regard for what they do in non-typical applications. The only answer is to test.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
We're using standard ATX supplies to power a test jig for a single PCI card, and the supplies usually don't start up for the smaller cards (ie go to start and instantly shut down). Plugging in an old disc drive into the supply as well fixes things, and confuses the heck out of anybody that sees it for the first time.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Dr. Slack wrote ...
We're using standard ATX supplies to power a test jig for a single PCI card, and the supplies usually don't start up for the smaller cards (ie go to start and instantly shut down). Plugging in an old disc drive into the supply as well fixes things, and confuses the heck out of anybody that sees it for the first time.
That sounds like a perfect runaround for new guy. "Go find xyz and get the new firmware diskette from them"
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
I have had one early computer PSU go up in flames without a load but has not happened since. Without a proper load the regulation may be completely erratic and impossible to predict. I usually try several ones and select the one that is most stable at low loads. There are some that regulates very well with a very small load and some that needs a significant load on both 5 V and 12 V to behave perfectly.
A trick is to connect a potentiometer between 5 V and -5 V and the wiper to common (pin2) on a 7805 regulator and you got a 0-10 V PSU with just two added parts (as long as the 7805 model is stable without extra capacitors).
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ATX PSU apocrypha...
