MotionBench electronicsTest and identify
Hard-drive voice coil
Also called VCM, head actuator, linear motor.
A flat coil between two neodymium magnets. It is a linear motor with a very short stroke, and a passable haptic or galvo. It is a bad speaker and a worse place to put continuous DC.
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.
Touch a 1.5 volt cell to the two terminals for a moment only. A good cone clicks and moves. A scrape means the coil is rubbing, which will sound bad even if it still makes noise. The ohms printed on the magnet, often 4, 6, or 8, tell you what amplifier it expects.
Strong neodymium magnets. Hard-drive magnets slam together hard enough to blood-blister skin and shatter into sharp chips. They also wipe media and bother pacemakers.
Sheet metal and glass. Case steel, shattered platters, lamp glass, and magnet chips cut deeper than they look.
| What to look for | Usual figure |
|---|---|
| Leads | Two, usually a flex cable on the arm |
| Resistance | A few ohms to low tens |
| Drive | A few hundred mA moves it. Sustained DC only heats the coil. |
| Stroke | The arc of the arm, a couple of centimetres at the head end |
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
- Haptic thump
- Small scanner or galvo
- Demonstration linear motor
- The magnets are the larger prize if you do not need motion
Leave this one when
The flex is torn and you cannot reach the coil. Take the magnets and move on.
Buying a new one is the better idea when
A speaker, if you wanted a speaker. A proper voice-coil stage, if you needed a calibrated stroke.
How to tell if this one still works
A 1.5 V cell, momentary, through your fingers so you can let go. The arm should kick. No kick and open resistance means a broken flex.
Step-by-step on the bench: Test a speaker or voice coilTools you will want: Meter, Torx bits.
Words on this page
Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.
- neodymium
- The strong rare-earth magnet in hard drives. It pinches skin and shatters if two of them slam together.
Where people actually find it
1 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.
| From | Call | How many | How often | Where it sits |
|---|---|---|---|---|
| Hard disk drive | Worth it | 1 | always | The actuator arm. |
- In this particular machine
- A short-stroke linear motor. Two leads on a flex.
- Where to look
- The actuator arm.
- How to get it out
- Leave it on the arm. Cut the flex only after you know where the coil ends.
- The condition to expect
- Good unless the flex is torn.