DC Cooling Fans: Sizes, Voltage, Wires, Speed Control and How to Choose

Contents List

1. Introduction

Heat is the most common reason an electronic project fails slowly: a regulator that runs too hot, a motor driver that shuts down, a power supply that dies after a summer. A small DC fan is often the cheapest and simplest fix. But fans come in many sizes, voltages and wire counts, and choosing one by looks alone leads to a fan that does not fit, does not start, or is far louder than expected.

This guide explains how a DC fan works, how to read its size and hole spacing, how to turn voltage and current into watts, what 2, 3 and 4 wires do, the difference between airflow and static pressure, bearing types, how to mount the fan so it blows the right way, how to power it and switch it with a MOSFET, how to control its speed, how to use a fan with a heat sink, and how to troubleshoot a fan that does not behave. Every section has worked numbers that you can check against the product page.

2. How a DC Fan Works

Almost every small DC fan is a brushless motor. A ring of permanent magnets sits on the rotating part, coils are fixed in the frame, and a small circuit inside the fan switches the coils in the right order. A sensor (usually a Hall sensor) tells that circuit where the magnets are. Because the electronics are built in, the fan needs only a DC supply: two wires are enough to make it spin.

• No brushes means no sparks and no brush wear, so the life of the fan depends mostly on the bearing.
• The built-in circuit is why swapping the two wires does not reverse the fan. A brushless fan only turns one way, and wrong polarity makes it stay still or, on some fans, damages the circuit.
• DC fans run from low voltage (5, 12, 24 or 48 V). AC fans run directly from mains (for example 110 V or 220 V) and are a different product: see the AC fans. This guide is only about low-voltage DC fans.
• Axial fans (the common square type) move air along the shaft. Blowers push air out sideways at higher pressure.

Browse the full DC fan range and the wider cooling solutions category.

3. Fan Sizes and Hole Spacing

A fan is named by the outside size of its square frame and its thickness: 80x80x25 mm is 80 mm wide, 80 mm high and 25 mm thick. The size of the blades is smaller. When you replace a fan, or fit one onto a heat sink or a case, the number that must match is the distance between the mounting holes.

A larger fan can move the same air at a lower speed, which is quieter. Thickness matters too: a 25 mm fan is the common standard, a 10 to 15 mm fan fits in tight spaces but moves less air, and a 38 mm fan is thicker, stronger and usually draws more current. Some listings call a 92 mm frame “90 mm”: if you must be sure, measure the hole spacing (82.5 mm for 92 mm).

4. Voltage, Current and Power

The voltage printed on a fan is its rated voltage: the voltage it is designed to run at. Small fans are made for 5, 12, 24 and 48 V. Use the rated voltage, and read the current: it tells you what your supply must deliver. Power in watts is voltage times current:

Power (W) = Voltage (V) × Current (A)

• Several fans: add their currents. Three 12 V fans of 0.38 A need about 1.14 A from the supply.
• Margin: choose a supply that can deliver noticeably more than the total, because a fan draws more current while it starts.
• Never run a fan above its rated voltage. A 5 V fan connected to 12 V is damaged. A 12 V fan on 5 V may not start at all, and if it does it moves little air.
• Check with a meter: measure the supply voltage before you connect the fan. See how to use a multimeter.

5. Two, Three and Four Wires

The number of wires tells you what the fan can report and what it can be told:

Reading the tach. Count the pulses per second and convert: RPM = pulses per second × 60 ÷ 2 for a fan that gives 2 pulses per revolution. For example, 100 pulses per second is 100 × 60 ÷ 2 = 3000 RPM. The tach output is usually open collector, so the board needs a pull-up resistor to its logic voltage.

Connectors. Small fans come with a 2-pin connector, and 3-pin or 4-pin headers on a 2.54 mm pitch are common on PC-style fans. Colors are typical but not a standard, and server fans can be wired differently, so check the datasheet. For plugs and housings see the terminal blocks and JST connectors guide and the JST connectors.

6. Airflow and Static Pressure

A datasheet gives two numbers that matter more than the size: airflow and static pressure.

• Airflow is how much air the fan moves in free air, with nothing in front of it. It is quoted in CFM (cubic feet per minute) or m³/h. 1 CFM is about 1.7 m³/h.
• Static pressure is how well the fan pushes against resistance, quoted in mmH₂O or Pa. A heat sink with tight fins, a dust filter or a narrow duct creates resistance.
• The numbers are linked: the more resistance there is, the less air the fan really moves. A fan with great airflow and low pressure can disappoint in front of a dense heat sink.

Fans with a higher speed (RPM) move more air and make more noise. For example, the H.R.Y 8025S24V listing states 4000 RPM, and the H.R.Y 12025S12V listing states 2700 RPM for a larger frame.

7. Bearings and Lifetime

In a brushless fan the bearing is the part that wears. These are the common types:

Datasheets quote life in hours at a stated temperature. Life drops quickly as the fan gets hotter, so a fan that sits in the exhaust of a hot enclosure lasts less than the same fan in cool air. The H.R.Y listings show a sleeve bearing on their label, so keep the orientation in mind when you mount them.

8. Mounting and Airflow Direction

A fan that blows the wrong way is the most common beginner mistake. Look at the frame: two arrows are molded in it, one for the direction of the airflow and one for the rotation of the blades. Usually the side with the motor struts is the side the air leaves from, but always follow the arrows.

• Intake low, exhaust high. Warm air rises, so let the exhaust fan sit high and the intake fan low.
• Let the air reach the hot part. Parts and cables should not block the path between the two fans.
• Do not recirculate. If the intake and the exhaust are very close, the intake pulls back the hot air that was just pushed out.
• Mount firmly, but do not over-tighten. Four screws, or rubber mounts, to keep vibration low. Over-tight screws can distort a plastic frame.
• Keep the cable away from the blades and tie it so it cannot touch them.
• Leave some free space in front of the intake, or the fan will starve and become noisy.

9. Powering a Fan

A fan needs a supply at its rated voltage that can deliver its current. Three practical rules:

• Never power a fan from a microcontroller pin. A pin of a typical board gives about 20 mA, and even a small fan needs 100 mA or more. The pin would be overloaded.
• Use the right supply. A 5 V fan can run from a USB supply or a regulated 5 V rail with enough current. A 12 V fan needs a 12 V supply. If the board has a higher voltage and the fan needs 5 V or 12 V, use a step-down converter: see the buck converter vs LDO guide. A resistor in series to “drop” the voltage wastes power and gives a speed that depends on the load.
• Join the grounds. When a microcontroller controls a fan that has its own supply, the grounds of both must be connected.

Check the polarity before you apply power. Red is normally the positive wire and black the ground, and a reversed fan often does not run and can be damaged. When you extend the wires, use a proper connector or a soldered joint with heat-shrink: see the soldering guide.

10. Switching a Fan with a MOSFET

To turn a fan on and off from a microcontroller, or to change its speed with PWM, use a logic-level N-channel MOSFET as a switch on the ground side of the fan:

1. Connect the fan between + supply and the drain of the MOSFET. The source goes to ground.
2. Connect the microcontroller pin to the gate through a 100 Ω resistor, and add a 10 kΩ resistor from the gate to ground so the fan stays off while the board resets.
3. Put a flyback diode across the fan, with the band (cathode) toward the positive supply. It gives the current of the motor coil a path when the MOSFET turns off.
4. Join the ground of the microcontroller and the ground of the fan supply.

Choosing the MOSFET. It must be logic level: its on-resistance Rds(on) is specified at the gate voltage your board gives (3.3 V or 5 V). Fan currents are small, so the MOSFET stays cool: with a 0.38 A fan and a MOSFET of 0.05 Ω, the loss is 0.38² × 0.05 ≈ 0.007 W. See the full MOSFET as a switch guide for gate drive, and the diodes guide for the flyback diode.

PWM frequency. A typical Arduino Uno gives about 490 or 980 Hz, and a fan can whine at that frequency. A higher frequency, up to about 25 kHz, is quieter. Some fans with built-in electronics do not like a chopped supply, especially at low duty cycles, so test the fan you use. A 4-wire fan avoids the issue because it has its own PWM input.

Browse MOSFET transistors and read the transistors guide for small fans that a BJT can switch.

11. Controlling Fan Speed

A fan at full speed is often louder than needed. Slowing it reduces noise and power a lot, because airflow follows speed, pressure follows speed squared, and power follows speed cubed (see the chart in the wires figure). At half speed a fan moves about half the air with about one eighth of the power.

For small USB-powered fans, a ready-made module with a knob can be enough: the HW-602 USB fan speed controller is listed in the store, and its page gives the supported voltage. Check that the fan and the module use the same voltage.

12. Using a Fan with a Heat Sink

A heat sink spreads heat into more surface, and a fan makes that surface far more effective, because moving air takes heat away much faster than still air. The temperature of the part follows the thermal resistances in series:

Part temperature = Ambient + Power × (Rθ junction-case + Rθ case-sink + Rθ sink-air)

Example. A part dissipates 6 W and the total thermal resistance is 8 °C/W. The rise is 6 × 8 = 48 °C, so in a room at 25 °C the part reaches about 73 °C. The airflow of a fan lowers the sink-to-air resistance, and the heat sink datasheet usually shows by how much for a given airflow. The same 6 W then gives a smaller rise.

• Contact first. The part must press flat against the heat sink, with thermal paste or a pad, and a firm screw. A great fan does not help a loose heat sink.
• Blow along the fins. The air must pass between the fins, not hit them from the side with no way out.
• Match the size. The fan should be about the size of the finned area, so no fins are left in still air.
• Do not forget the rest of the box. A fan on the heat sink moves heat into the enclosure, which still needs an exhaust.

The range includes heat sinks for small parts such as TO-220 and TO-247 packages and for motor driver chips. Browse the heat sinks and read the MOSFET guide for how to estimate the power a part dissipates.

13. Guards, Filters and Safety

A spinning fan can hurt a finger, and a fan in a dusty place slowly fills with dust. A guard and a filter solve both, at a small cost in airflow.

• Finger guards (the metal wire type) protect the blades and the user. Choose one that matches the frame size.
• Filters catch dust. A clogged filter reduces the airflow a lot, so clean it from time to time.
• Wires and loose items must stay away from the blades. A cable caught in a fan can stall it or tear.
• Mains fans are a different job. An AC fan carries mains voltage and should only be wired by someone trained to do it.

14. Server and Industrial Fans

The range also lists many server and industrial fans from makers such as Delta, Sanyo Denki, Nidec, Sunon and Papst, listed as used like new. They are built for high airflow and long life, and they differ from small fans:

• Higher current and power. A 12 V server fan can draw 1 to 4 A, which is 12 to 48 W. Make sure the supply and the wires can carry it.
• Higher speed and noise. They are designed for pressure and airflow, not for quiet.
• 48 V versions exist for telecom and industrial use. Do not mix them with 12 V supplies.
• 4-wire and special connectors. Many have a PWM input and a tach, and some have 6 pins. Find the datasheet by the part number and check the pinout before you connect power.
• Used. They are listed as used, so test one with a supply that can deliver its rated current.

Browse the DC fan category and filter by the voltage and size you need.

15. How to Choose a Fan

Go through these steps in order:

1. Space: measure the room for the fan, and the hole spacing if it screws onto something.
2. Voltage: match the supply you have: 5, 12, 24 or 48 V.
3. Current: check that the supply can deliver the fan current, with margin.
4. Airflow or pressure: open vent means airflow, heat sink or filter means pressure.
5. Noise: prefer the largest fan that fits, run slowly.
6. Wires: 2 wires for on and off, 3 to read the speed, 4 for PWM control.
7. Bearing: ball for long life or any orientation, sleeve for quiet and low cost.
8. Extras: a guard, a filter, a connector and the screws.

16. Troubleshooting

17. Starter Kit for Fan Projects

18. Frequently Asked Questions

What is the difference between a DC fan and an AC fan?

A DC fan runs from low voltage such as 5, 12 or 24 V and contains its own electronics. An AC fan runs from mains. This guide is about DC fans.

Can I run a 12 V fan on 5 V?

It may not start, and if it does it moves little air. For a slower fan, control the speed properly instead, with a speed controller or PWM.

Can I power a fan from an Arduino pin?

No. A pin gives about 20 mA. Use a logic-level MOSFET to switch the fan from its own supply.

What do the 3rd and 4th wires do?

The third wire is the tach, which gives pulses so you can read the speed. The fourth wire is a PWM input that sets the speed.

How do I know which way a fan blows?

Follow the airflow arrow molded in the frame. Usually the air leaves from the side with the motor struts.

Why does a bigger fan sound quieter?

It can move the same air at a lower speed, and the noise grows quickly with speed.

Do I need a diode with a fan?

When you switch a 2-wire fan with a MOSFET, a flyback diode across the fan is cheap protection against the voltage spike of the motor coil.

How long does a fan last?

It depends on the bearing and the temperature. The datasheet gives hours at a stated temperature, and a cooler fan lasts longer.

19. Conclusion

Choose a fan by size and hole spacing, match its rated voltage, and check that your supply can deliver its current. Use 2 wires for simple on and off, 3 to read the speed, and 4 for PWM control. Mount it so it blows the right way, put the exhaust high and the intake low, and use a logic-level MOSFET with a flyback diode when a microcontroller controls it. A bigger fan run slowly is the quiet solution, and a heat sink with good contact and airflow along its fins does the most for the part you want to keep cool.

When you are ready to build, browse the DC fans, cooling fans, heat sinks, MOSFET transistors and all cooling solutions at Makers Electronics.