12v fans, simple question

[brain_police87]

Hello everyone, first off, Im new to the forum. Ive been trolling the past few weeks on different threads and learned quite a bit. Im a hobbyist for the most part, I have quite a love for all things electronics. Ive been trying to search the forum (still getting adjusted to the layout) to find some basic info but still haven't answered my question.

I am trying to wire two 12v DC 40cfm fans on to a older FAT model PS3 with overheating issues. I have my 12v source but I am wondering that if I use two 12v fans, will I actually be pulling 24v? I understand this is a extremely "dumb" question, but as I get more and more into creating my own circuits I need this basic info. How do you calculate draw on power source?

I subscribe to several other forums on different subjects. I typically dont like to be "that guy" but I've searched google and other resources and cant seem to find the information I need. Perhaps I am not phrasing my question correctly? Any help would be great!

 

[KrisBlueNZ]

Hi and welcome to Electronics Point

Voltage is measured between two points, like a distance. You could imagine that a 12V power supply "pulls" its outputs (they're normally called +12V and 0V) apart to a distance of 12 inches. The power supply includes regulation, so its outputs are always 12V apart, unless you overload it (draw too much current from it).

A 12V DC fan requires its wires to be "pulled apart" to about 12 inches to make it run, so if you connect it across the output of a 12V power supply, it will have 12V across it, and it will run.

A fan draws a certain amount of current. The current drawn by the fan may be marked on the fan as a number of amps (A) or milliamps (mA). You can think of current as tension or force. The fan tries to pull its inputs together, but the power supply forces them apart. This force is the current. As long as the power supply can supply at least as much current as the fan needs, the voltage remains at 12V and the fan runs normally.

A second 12V fan also needs 12V across its wires, so you need to connect it across the power supply as well. The two fans will then be connected "in parallel". Both fans will see the same voltage (12V) and each will draw its own current. These currents add together. If the fans are the same, then their combined current drain will be twice the current drain of a single fan.

Provided the power supply can provide the total current required by both fans, it will keep its outputs 12V apart, both fans will run properly, and everything will be OK.

The power supply is also providing current to the rest of the PS3. Generally, power supplies are somewhat over-specified, so you can draw a bit of extra current from them without overloading them.

Try to find out how much current each of the fans will draw, and see whether the power supply has any markings that tell you how much current can be drawn from the +12V output. Provided that the total fan current isn't more than about 10~15% of the power supply's rating on that output, you should be OK. If they will draw more than that, you might want to find some smaller or more efficient fans, or power them from a separate power supply.

If you try to draw too much current from the power supply, it will probably shut down. It may start "hiccuping" - making a ticking sound. This does not damage it. It's also possible that it will continue to run, but get hotter than usual. Therefore you should make sure that at least some of the air from the fan(s) goes through the power supply.

 

[brain_police87]

Thank you so very much for the reply. That is exactly what I was looking for. Lets say for example the two fans together draw more current than rated for the power supply. Am I correct in assuming I could add a resistor to limit the current and thus keep the power supply from being over worked?

 

[davenn]

no, you need a better PSU, else the fans wont work properly

PC fans are very low current usually less than ~ 120mA each
just run them off the 12V from the computer PSU, no need for a separate supply

Dave

 

[KrisBlueNZ]

Thank you so very much for the reply. That is exactly what I was looking for. Lets say for example the two fans together draw more current than rated for the power supply. Am I correct in assuming I could add a resistor to limit the current and thus keep the power supply from being over worked?

In the crude analogy I used in post #2, a fan is like a tension spring. When you hook it between two points that are 12V apart, it tries to pull them together. The power supply is regulated, so it keeps them 12V apart, and this creates tension (current flow) in the spring. If the spring is too strong, it will over-power the power supply's output and force the power supply's output voltage to drop.

A resistor, in this analogy, is also like a tension spring. If you disconnect one side of the fan and hook a tension spring between it and the power supply, the total tension (current) will be reduced, but the distance (voltage) across the fan will also be reduced. Assuming the resistor spring has similar characteristics to the fan spring, the 12V distance will be equally divided between the fan spring (which will have 6V across it) and the resistor spring (which will have the remaining 6V across it). The tension (current) will be halved, but the fan will run slow, or perhaps not at all.

So no, adding a resistor won't help much.

Do you know the output current specification for the 12V output of a Playstation 3 power supply?

 

[duke37]

To limit the current you could use one fan only or put the two fans in series so that each fan would see 6V (and run slower).

 

[brain_police87]

Thanks for all the replies. I ended up using a single fan wired into a 12v dc source. Working out great!