0
\$\begingroup\$

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?

\$\endgroup\$
2
  • \$\begingroup\$ Connecting fans directly to usb port is probably a bad idea. You are risking burning your motherboard \$\endgroup\$
    – Eugene Sh.
    Nov 12, 2021 at 19:33
  • 1
    \$\begingroup\$ In general, motor speed is determined by voltage and torque is determined by current. If you don't increase the voltage you can't increase the speed. I have no idea what you mean to do with a USB splitter - whatever that is. \$\endgroup\$
    – Transistor
    Nov 12, 2021 at 19:55

1 Answer 1

1
\$\begingroup\$

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.

\$\endgroup\$
5
  • 1
    \$\begingroup\$ A motherboard sends 12V to fans, unless the voltage is less to slow them down as motherboards may have control over the voltage to slow them down. An USB port would give only 5V, and even if an USB charger would be able to give 9V out, it will do so only after the device has negotiated with the supply that it is OK to give out more than 5V. Maybe a boost converter will work for you. \$\endgroup\$
    – Justme
    Nov 12, 2021 at 20:48
  • \$\begingroup\$ @Justme Depends on the motherboard I suppose. Some use PWM so may still send 12V but on a <100% duty cycle. (But we'll leave this detail alone for the sake of comparing 12V to 5V supplies!) \$\endgroup\$
    – JYelton
    Nov 12, 2021 at 20:49
  • \$\begingroup\$ @Yelton and newest motherboards might give fixed 12V and PWM control on separate pins. Some do have PWM control, or analog control of the supply voltage. \$\endgroup\$
    – Justme
    Nov 12, 2021 at 20:51
  • \$\begingroup\$ @Justme I'm not disagreeing, I'm just saying that the OP likely was seeing a fixed 12V and obtained the faster speed. \$\endgroup\$
    – JYelton
    Nov 12, 2021 at 20:55
  • \$\begingroup\$ Oh I am not disagreeing either - my comment was supposed to be comment on the question, not your answer. Sorry for the confusion. \$\endgroup\$
    – Justme
    Nov 12, 2021 at 21:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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