MotionBench electronicsHand tools, works as-is

Axial fan

Also called PC fan, muffin fan, case fan.

A framed fan that already solves mounting. PC fans are the standard. The dead ones are dead because of the bearing, which you cannot economically rebuild.

If you are new to this category

These parts spin, push, or pump. A motor is a machine that turns electricity into turning. Some start turning as soon as you connect a battery. Others look identical and will not move until a small circuit tells them which coil to power next. The label, or the number of wires, is how you tell them apart.

Count the thick wires before you apply power. Two wires usually mean a brushed motor that can run from a battery. Three thick wires usually mean a brushless motor. Keep the board that came with a brushless motor.

Computer fan wires

Two wires means power only, usually 12 volts on the label. Three wires means the third wire reports speed, often two pulses per turn. Four wires means the fourth wire is a speed command. Use the voltage on the label. A server fan is louder and draws more current than a desk fan of the same size.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Sizes	40, 60, 80, 92, 120, 140 mm square are the PC standards
Voltage	5, 12, or 24 V on the label. A 12 V fan often starts around 6–7 V.
Wires	2 = power. 3 = tach (open collector, two pulses per rev typical). 4 = PWM on the fourth wire, usually 25 kHz, 5 V.
Current	A quiet 120 mm is about 0.1–0.3 A. A server fan is not quiet and is not 0.1 A.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

Enclosure cooling
Solder-fume fan
The tach wire as a crude speed sensor

Leave this one when

It howls, wobbles, or was packed with dust that has worn the sleeve. Harvest a fan from a machine that died of something else.

Buying a new one is the better idea when

If you need a known CFM and a quiet bearing. Salvage fans are a lottery you can hear.

How to tell if this one still works

Rated voltage. Listen. Feel for vibration. A pulse on the tach wire while it spins means the sensor works.

Tools you will want: Meter, 12 V source.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

PWM: A rapid on-off signal used to control speed or brightness. A four-wire computer fan uses it on the fourth wire.

Where people actually find it

7 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Desktop PC	Prize	1–4	usually	Front, rear, and on the CPU or supply.
Laser printer	Worth it	1–2	usually	Rear exhaust.
Ultrasonic humidifier	Worth it	1	usually	The top.
Space heater	Worth it	1	usually	Behind the element.
Uninterruptible power supply	Worth it	1	usually	Rear.
Desk or box fan	Bonus	1	sometimes	DC-powered modern fans.
Treadmill	Bonus	1	sometimes	Controller cooling.

Prize — take this firstDesktop PC1–4 · usually
In this particular machine
12 V, 80–140 mm. 3-pin or 4-pin.
Where to look
Front, rear, and on the CPU or supply.
How to get it out
Unplug. Do not salvage a screaming one; the bearing is the failure.
The condition to expect
Good if the machine was quiet.
Worth pullingLaser printer1–2 · usually
In this particular machine
12 or 24 V. The label decides.
Where to look
Rear exhaust.
How to get it out
Unplug.
The condition to expect
Good if it is not packed with toner dust.
Worth pullingUltrasonic humidifier1 · usually
In this particular machine
Small DC fan.
Where to look
The top.
How to get it out
Unplug.
The condition to expect
Good.
Worth pullingSpace heater1 · usually
In this particular machine
Often a shaded-pole, sometimes DC.
Where to look
Behind the element.
How to get it out
If it is seized, that is why the fuse opened.
The condition to expect
Seized is common.
Worth pullingUninterruptible power supply1 · usually
In this particular machine
12 V.
Where to look
Rear.
How to get it out
Unplug.
The condition to expect
Good.
Bonus, if you are already in thereDesk or box fan1 · sometimes
In this particular machine
The blade is on a BLDC and a small board.
Where to look
DC-powered modern fans.
How to get it out
The brick is the other half.
The condition to expect
Good.
Bonus, if you are already in thereTreadmill1 · sometimes
In this particular machine
Mains or DC.
Where to look
Controller cooling.
How to get it out
Read it.
The condition to expect
Good.