# How to lower wattage of circuit

I want to power a computer fan from a 12v motorcycle battery.

Disregarding the motorcycle battery being 12.6 volts (not charging), lets say that it's a regulated constant voltage source of 12 volts.

The fan needs to be 12v driven where 0.58 Amps will be drawn. It is a very old fan, and I can't find a datasheet. But it is exactly this one: https://www.ebay.com/c/1472102822

Ohms law says that a 24 ohm resistor is good for 0.5 Amps or current. However, Ohms law also says that the power being drawn will be 6 Watts. Most resistors do not have this high of a power rating.

How do I power my computer fan the right way without getting a high wattage? I am not looking forward to using bulky, high wattage resistors.

• Part number and link to datasheet of fan required. Please edit the information into your question and not buried in the comments. "Ohm's law says that a 24 ohm resistor is good for 0.5 amps or current." No, it says that a 24 ohm resistor connected across a 12 V supply will cause a current of 0.5 A but you won't be doing that as you're proposing to have the fan in series with it. – Transistor Feb 17 at 21:46
• Why won't I be doing that when I want to put a fan in series with it? What am I not understanding? – Cremus Feb 17 at 21:52
• Because if you drop 12 V across the resistor there would be 0 V left for the fan. – Transistor Feb 17 at 21:54
• 1) 10 W resistors do exist and using a resistor is the simplest solution but not the most elegant as indeed a lot of energy is wasted (turned into heat by the resistor). 2) A more elegant solution will be to use a motor controller which will switch the power to the motor on/off very fast, if the on and off time are equal, the motor will run at roughly 6 V and 1/4 of its power. I suggest looking for a ready made motor control module and make sure it can handle enough current. Building your own using a microcontroller (for example an Arduino board) and a motor driver is also an option. – Bimpelrekkie Feb 17 at 21:54
• @Bimpelrekkie Yeah I know these types of resistors work, and my problem with them is exactly what you say. I am currently building a PCB with a microcontroller, so I could integrate a motor controller on that board. Since that seems like the best solution to me so far. – Cremus Feb 17 at 21:57

You have linked to an eBay ad instead of a datasheet. The fan has four wires so it probably has a PWM speed control input on it and electronics to efficiently control the speed without the high heat loss of a resistor.

Internally the fan is not a simple brushed motor but is a miniature brushless DC motor with a chip generating a three-phase supply to the coils. You will find plenty of reading material on the web explaining their operation.

"No datasheet? No sale!"

• Oh wow! I didn't even see those PWM wires on the eBay ad. I already have the fan, but those PWM wires are missing... I'll start experimenting with that! – Cremus Feb 17 at 22:03
• It's not exactly that one then! – Transistor Feb 17 at 22:06
• Well, it certainly is, but the wires are soldered off. When I look at the solder pads where the PWM wires go, there used to be some wires soldered on. But they are missing. – Cremus Feb 17 at 22:08

The fan you show is a 4-wire PC case fan. Very common, nothing special. Being 4-wire, it includes the ability to do PWM control. If you feed it a PWM signal the power will be reduced in proportion to the duty cycle.

Here's a link to how to make a PWM control using the venerable 555 IC: https://www.electronics-tutorials.ws/blog/pulse-width-modulation.html

Another option is to use a 12V to 5V DC-DC step-down, and use a 5V fan instead. These tend run at lower current than 12V ones. This is a bit like your series resistor idea, but without the losses.

• Thanks for the link to the 55 timer PWM control. I'll have a look at that! However, if it is too much of a hassle, I think I'll just buy a new 5V fan which does have a datasheet. Since I only need to turn it on and off. – Cremus Feb 17 at 22:07
• You could run it with a car USB charger and USB cable. – hacktastical Feb 17 at 23:12

I just realised that ohm's law doesn't apply to a fan since the fan isn't a resistor. The fan takes what it needs in power. When I connect the fan to the 12V battery (which I just tested), the fans spins and draws the amount of current it needs.

If I want to turn the fan on or off using a microcontroller, I just need to switch the voltage supply to the fan. Nothing else, nothing more.

I hope I help someone else who stumbled upon this "problem"