# How can I use USB output (5V DC) to power a fan 12V DC load?

I would like to power a 5V DC/0.23A fan (beside another device such as 12V LED lamp, and 12V DC water pump). The only possible power source is using USB output, either from computer, mobile phone's charger, and most possible from power bank, which their output are 5V. I was trying to think to use BJT/MOSFET/IGBT, but seems are not possible, as the maximum voltage coming to the drain (in the case of N-channel MOSFET including to cascade the MOSFET, and IGBT) is 5V only, and BJT is not amplify the voltage.

So, how can I do it? What component should I use? Is anyway to do that? Appreciate any helps with design.

• Add up the power requirements of the various loads and compare with the power that can be provided by the 5 volts. It’s a 12 volt fan btw. Jan 4, 2020 at 10:55

Before attempting the project, lets evaluate the power budget available to ensure the USB port responsible to supply power won't get damaged.

As you may know the power that a typical usb port can provide is $$\P = V \times I\$$ where V is 5V and I varies depending on how powerful the source is. It could go up to 3 amperes for very powerful socket USB charger. For PCs it wouldn't really be wise to extract any more then 1 ampere without protection to avoid a very costly motherboard repair for your PC.

To get 12V DC from 5V DC you need to use a power converter. In this case, a boost converter. But we all know the law of conservation of energy, the amount of power that goes into the converter must be equal to the amount of power that goes out.

So in an equation $$\V_{in} \times I_{in} \times Efficiency_{convertor} = V_{out}*I_{out}\$$

Typical boost converters on Amazon would given you an efficiency between .80 to .95 depending on design and load you apply. These boost converters feature a variable potentiometer that allows you to adjust the output voltage and get 12V DC.

So now you know you can get 12V DC from 5V DC but as you increased the voltage, the output current must reduce to respect conservation principle. So in reality you can extract very limited amount of power from your converter. Also remember boost converters aren't really happy when you apply zero load to their output, and this of course depends on how well they have been designed, the output voltage may increase to a large value.

If you have interest you may want to learn about designing a boost converter but that is a discussion for another day and has its own challenges. But do study boost converters online and try making one in a simulation software.

• Thanks you for your nice explanation. I got here good demonstration of boost converter. Seems that it is part of SMPS, also applied in current phone charging. Jan 4, 2020 at 13:34
• As any potential to damage the motherboard, so using power bank and phone charger are the best option. The output current still enough, 1A. Jan 4, 2020 at 13:38
• But, what is the reason of boost converters aren't really happy when you apply zero load to their output? Zero load is not open circuit, right? Jan 4, 2020 at 13:41
• @AirCraftLover What does "zero load" mean? Does it mean no load: no current, infinite impedance? Does it mean zero resistance, a short circuit? If you mean "no load" then yes, that is the same as an open circuit. Jan 4, 2020 at 14:32
• Thank you @ElliotAlderson. Once I heard that open circuit = infinity load, while zero load = short circuit. So if open circuit is quite problem for boost converter, that will be really problem as there will be many chance that one can just take out the load that will make the circuit open, or infinity load, no current, which all the current is bounced back. Jan 5, 2020 at 5:57

One of the other answers has explained the math, so I'll apply it to your task.

You have a 12 volt fan that will draw 0.23 amperes.

If you use a boost converter to get 12V from 5V, then the converter will have to boost the voltage by a factor of 2.4. The current drawn from the 5V supply will go up by that same factor of 2.4.

12V at 0.23A means you will have 0.552A at 5V. That's ignoring the losses in the converter. In reality, you will need even more current from the 5V supply.

Standard USB2 ports on PCs and laptops are rated for 0.5A. Some USB ports will deliver more current, but you can't depend on it. Some will work, some won't - and some USB ports may be damaged.

USB3 can deliver more current, but you'd have to look up the specifications and see if it will do so without having to communicate with the USB controller behind the port.

• Thank you for your explanation, especially the power rating. Seems USB 2.0 is 1A current, so will be enough, so the output of power bank. Avoid damage to mother board, probably using power bank or phone charger are the best option. Jan 4, 2020 at 13:37