How to increase the fan speed connected to a usb port

Question

I have 6 PC fans 12V 0.12A connected in parallel. They are connected with a usb cable to a mobile charger. When I was connecting them to the motherboard they were much faster than now though the mobile charger could go up to 9V. As a possible solution for increasing the fan speed came to my mind to use a 1 female 2 male usb splitter and connect 2 males to two supplies. But this wouldnt increase the voltage but maybe the amperage. I know V = R * I. Does this also apply in this case? I mean can I cover up the voltage by increasing the amperage in order of increasing the speed?

Answer 1

I have 6 PC fans 12V 0.12A connected in parallel.

The rating of the fan tells you roughly everything you need to know: That at 12 volts, they will draw 120 mA. The rotation speed (RPM) of the fan will also be maximal at 12 volts*.

When I was connecting them to the motherboard they were much faster than now though the mobile charger could go up to 9V.

A fan header on a motherboard supplies 12 volts. A USB charger supplies (typically) 5 volts.

With six fans connected in parallel, they would draw 720 mA at 12V. At voltages < 12 V, they will rotate at a lower speed and draw less current. If you're not quite sure how current is determined by the load, review this question.

As a possible solution for increasing the fan speed came to my mind to use a 1 female 2 male USB splitter and connect 2 males to two supplies. But this wouldn't increase the voltage but maybe the amperage.

I'm not quite sure what you mean by this. If you want to connect two USB supplies in parallel, you'll in theory be able to deliver more current, but the voltage remains the same. If they were connected in series, in theory the voltage would double, but USB supplies are not meant to be connected in series like that.

I know V = R * I. Does this also apply in this case?

Fans are driven by an electric motor which includes a winding or coil of wire, which is an inductor. Although \$V = R\times I\$ is correct, the resistance component of the motor only applies if measured while not in motion. When the motor is operating, impedance is the property that determines current flow, and is based on the motor's load, construction, temperature, etc. For more information, @Transistor wrote a great answer about resistance in DC motors here.

I mean can I cover up the voltage by increasing the amperage in order of increasing the speed?

Do you mean "Can I make up for the lack of voltage by increasing the amperage?"

If so, no. Increasing the current that a power supply can provide does not automatically increase the current that a load will draw. Review the first question I linked for details.

Unless you are limited to using USB-based power supplies, you will only achieve a maximum speed related to the voltage applied. Be careful not to overload the supply. Ideally you will want to use a 12 volt supply capable of ≥ 720 mA. (You should always plan on ~20% extra headroom for inrush and startup currents. Given the ubiquity and low cost of 12V 1A supplies, that's what I would recommend.) Alternatively you could use a DC-DC converter to boost the 5V (or 9V) up to 12V. It will require more current from the low-voltage side, as well as incur efficiency losses. This question addresses the topic of converting voltages.

*_{Within manufacturer spec.}