Very basic question here that I thought I knew the answer to until last night, but now I am very confused. I picked up some 12v DC squirrel cage fans for a chassis I am building. Here is the Mouser part.


Also here is the supply I am using (RT-125D)


Here is my question. The 12V rail of that supply is rated to 2A. Those fans are listed as 12v at 1.8A each. I got two of them to run in parallel last night off that supply. That should not have worked right? Those fans would have been drawing too much current from that supply right? I'm just try to understand the limitations of the supply. Maybe that was just a poor choice of supply, but it seems silly to me that the cooling fan in a chassis can suck up 90% of the load a supply is capable of (at least on this one 12v rail). Is this a huge conceptual misunderstanding?


  • \$\begingroup\$ They only need that much current to run at 100%. \$\endgroup\$ May 19, 2016 at 20:35
  • \$\begingroup\$ By plugging one directly into power supply I would assume it is running at 100%. I guess by plugging two in parallel to the supply they are running at 50%? Can I scale them back with a pot or fixed resistor? \$\endgroup\$
    – Atomiklan
    May 19, 2016 at 20:38
  • \$\begingroup\$ If they were in free air with no restrictions such as ventilation grilles, etc., they may run at reduced current. Measure the actual current of one running in the open and then add some restriction - a loose weave fabric, for example. \$\endgroup\$
    – Transistor
    May 19, 2016 at 20:41
  • \$\begingroup\$ Hmm I guess not. That would lower the voltage below the threshold. I guess I would have to use PWM to lower the speed of the fan. That goes outside the scope of of this basic question anyways (and my understanding really). \$\endgroup\$
    – Atomiklan
    May 19, 2016 at 20:51
  • \$\begingroup\$ "That would lower the voltage below the threshold." What do you mean by this? What threshold? "I guess I would have to use PWM to lower the speed of the fan." Why are we discussing PWM or speed control? It's not mentioned in your question. It seems that you are quite confused about basic relationship between voltage, current and load. \$\endgroup\$
    – Transistor
    May 19, 2016 at 21:57

1 Answer 1


There is a large latitude in your definition of "run". Were you conducting your test under load? (Where they are sucking and/or blowing against the proposed "load" of the final installation?) Testing in "open-air" is not the same as testing under load. It seems quite possible that two 1.9A fans could appear to operate in a no-load, open-air "test". Was the RPM of the fan the same when you tested it with only one fan? (i.e. was the power supply output "drooping"?)

It is also quite possible that a multi-output switch-mode power-supply (SMPS) could tolerate one of the outputs being over-loaded in cases where the other outputs were not approaching their load capacity.

In any case, it appears that you have a 12V power source that is not adequate to the rating of your two fans. You could use a separate 12V supply for the fans, or you could use mains-powered fans and avoid involving the power supply.

  • \$\begingroup\$ Good point. I'll go take some additional measurements. \$\endgroup\$
    – Atomiklan
    May 19, 2016 at 21:42

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.