ATX power supply load

[D_Electrician]

I want to convert an ATX power supply for powering various motors. I looked online and in all the instructions I found say that a load resistor must be added to the 5V rail to ensure stable operation.

Does the load have to be on the 5V rail or can it be on the 12V rail?

I want to replace the load resistor with a cooling fan, but I'm having difficulties finding a suitable fan (I also have minimal knowledge on the behavior of inductive loads)

My power supply is from 2002 and I think it's revision ATX12V 1.2

[jasminder]

Originally Posted by D_Electrician I want to convert an ATX power supply for powering various motors. I looked online and in all the instructions I found say that a load resistor must be added to the 5V rail to ensure stable operation. Does the load have to be on the 5V rail or can it be on the 12V rail? I want to replace the load resistor with a cooling fan, but I'm having difficulties finding a suitable fan (I also have minimal knowledge on the behavior of inductive loads) My power supply is from 2002 and I think it's revision ATX12V 1.2

dear D-electrician,
Most of the atx power supplies deliver 5V at ~25A and 12V at ~17A. Why you need a load resistor? doesn't your stepper drivers have current limiting? Are you planning to use multiple ps units parallel to get 24 or more volts?
i have used a single atx supply to wire up all the logic circuits, cooling fans, relays, lights etc and never got a issue. The best thing about atx is that they have built-in short circuit protection , so you may save your expensive electronics from burning.

jasminder singh

[TDA]

Originally Posted by D_Electrician I want to convert an ATX power supply for powering various motors. I looked online and in all the instructions I found say that a load resistor must be added to the 5V rail to ensure stable operation. Does the load have to be on the 5V rail or can it be on the 12V rail? I want to replace the load resistor with a cooling fan, but I'm having difficulties finding a suitable fan (I also have minimal knowledge on the behavior of inductive loads) My power supply is from 2002 and I think it's revision ATX12V 1.2

The problem is with cross loading the supply. This is mostly only true for older designs but they will go out of regulation if you have nothing on the minor rails. The problem is exacerbated the closer you get to the max current limit of the supply. If you have a newer designed supply where all minor rails are converted off the 12v then you should have little to no issue. If you want to see this in action go find some old reviews on jonnygurus site. It will show you the effects of cross loading on different supplies.

Originally Posted by jasminder dear D-electrician, Most of the atx power supplies deliver 5V at ~25A and 12V at ~17A.

Not sure what you mean by most supplies have "X" amps on "X" rails. They vary by design and wattage. Most newer ones have much larger 12V capacity as modern computers use it vastly more then the other rails.

Originally Posted by jasminder i have used a single atx supply to wire up all the logic circuits, cooling fans, relays, lights etc and never got a issue. The best thing about atx is that they have built-in short circuit protection , so you may save your expensive electronics from burning. jasminder singh

Make sure you have a good quality supply as not all of them will have true short circuit protection. They are all supposed to but it doesn't work that way unfortunately.

[john_100]

HI D_Electrician ,

some power supplies do regulate without an external load
but for the rest ,its the main +5V supply the needs the load

the +5V output is the primary output that needs regulating
as it supplies the logic ICs (including the microprocessor)

powersupplies shutdown when the +5v rail goes too high
this is why you need the load

the +12v output is for the motors in the diskdrives and fans
and is not regulated to the same extent

John

[vger]

The load for the 5 volt leg can generally be as small at 1/2 amp. A 10 ohm resistor will do that but you will have to use at least a 2.5 watt resistor, better a 5 watt job.