Power conversionBench electronicsUnplug and unscrew

Switch-mode wall supply

Also called wall wart, USB charger, brick, laptop adapter.

The correct way to get low voltage from the trash: keep the whole sealed brick. Opening one to harvest a controller is rarely worth the loss of a working supply.

If you are new to this category

These parts change wall power into the lower voltage a device uses. A sealed supply with a label is the safer kind. An open board from a lamp can still be connected to the wall on the side that looks like low voltage. Isolated means the output is not electrically connected to the wall. If you are not sure, treat the output as live.

Read the label out loud: volts, amps, and the polarity symbol. Then measure. A number you remember from a similar brick is not a measurement.

A sealed supply versus an open board

Left: a brick. The label is your spec, and the plastic is the safety barrier. Right: a board taken out of a lamp or a charger. Unless you have checked that the output is isolated from the prongs, do not let it touch a project you will handle while it is plugged in.

Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.

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
USB charger	5 V, current printed (1 A to several). USB-PD bricks negotiate; they are not a fixed 20 V until something asks.
Router / disk	5, 9, or 12 V are all common. The current on the label is the number that matters.
Laptop	Often 19 V or 19.5 V at 3–6 A. The plug is model-specific. Center pin ID exists on some Dell-style tips.
Isolation	Most bricks with a transformer inside are isolated. Confirm once. LED-bulb boards often are not, and those are not this part.

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

Bench power for the voltage printed
A permanent supply inside a rebuilt project, left in its case

Leave this one when

It whines, runs hot unloaded, has a melted case, or the no-load voltage is far above the label. Those are failing.

Buying a new one is the better idea when

A new brick with the current you actually need. A free 500 mA supply will not run a 12 V heater.

How to tell if this one still works

Label voltage, unloaded and with a load. Ripple if you have a scope. Isolation resistance from output to the prongs.

Step-by-step on the bench: Check a supply is actually isolated

Tools you will want: Meter.

Words on this page

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

isolated: The low-voltage side is not electrically connected to the wall. A proper brick usually is. Many cheap lamp boards are not.
isolation: The barrier between wall power and the low voltage you can touch. If that barrier is missing, the 'low' side can still shock you.
transformer: Two coils on an iron core. It changes voltage and, in a proper one, isolates the output from the wall. Heavy old adapters are this. Microwave oven transformers are a different, dangerous kind.

Where people actually find it

17 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
Inkjet printer	Prize	1	usually	A metal box inside, or an external brick.
Laser printer	Prize	1	always	Its own metal cage.
Router or modem	Prize	1	always	The external brick.
Set-top box or streamer	Prize	1	always	External brick or internal board.
Wall adapter	Prize	1	always	The whole brick.
LCD monitor	Worth it	1	usually	Internal board or a brick.
LCD television	Worth it	1	always	One side of the rear.
Ultrasonic humidifier	Worth it	1	usually	External brick or a board.
DVD or Blu-ray player	Worth it	1	always	One side.
Powered or Bluetooth speaker	Worth it	1	usually	The brick.
E-scooter or hoverboard	Worth it	1	usually	The charger.
Flatbed scanner	Bonus	1	usually	Inside or a brick.
Desk or box fan	Bonus	1	sometimes	DC fans.
Power strip	Bonus	1	sometimes	Inside USB strips.
Electric toothbrush	Bonus	1	usually	The base.
Seasonal light string	Leave it	1	sometimes	Some plug controllers.
Alarm clock	Leave it	1	sometimes	Capacitive dropper supplies.

Prize — take this firstInkjet printer1 · usually
In this particular machine
Often a 32 V rail and a logic rail. Read the silk. Not always 24 V, not always isolated until you check.
Where to look
A metal box inside, or an external brick.
How to get it out
Unplug the output and label the voltages you measure.
The condition to expect
Often healthy, because the head died first.
Prize — take this firstLaser printer1 · always
In this particular machine
24 V class at real current is common, sometimes higher for the fuser control. Read the label.
Where to look
Its own metal cage.
How to get it out
Four screws and the mains connector. Isolation-check it.
The condition to expect
Often the healthiest part.
Prize — take this firstRouter or modem1 · always
In this particular machine
Commonly 9 or 12 V at 1 A class. Read it.
Where to look
The external brick.
How to get it out
Label voltage, polarity, and measured current capability if you load-test it.
The condition to expect
Usually good.
Prize — take this firstSet-top box or streamer1 · always
In this particular machine
12 V is common.
Where to look
External brick or internal board.
How to get it out
Label it.
The condition to expect
Good.
Prize — take this firstWall adapter1 · always
In this particular machine
Voltage and current on the label. USB-PD is negotiated, not fixed.
Where to look
The whole brick.
How to get it out
Measure unloaded and with a load. Confirm isolation once.
The condition to expect
Cord damage is the laptop-brick failure.
Worth pullingLCD monitor1 · usually
In this particular machine
12 V or 19 V class, plus a backlight enable.
Where to look
Internal board or a brick.
How to get it out
Label the outputs. Isolation-check an internal supply.
The condition to expect
Caps fail here; look for bulges.
Worth pullingLCD television1 · always
In this particular machine
Higher current than a monitor brick. Rails marked on the silk.
Where to look
One side of the rear.
How to get it out
Discharge the primary before you handle it. Label the output plug.
The condition to expect
Caps are a common failure; the rest may be fine.
Worth pullingUltrasonic humidifier1 · usually
In this particular machine
Low voltage, printed.
Where to look
External brick or a board.
How to get it out
Label it.
The condition to expect
Good.
Worth pullingDVD or Blu-ray player1 · always
In this particular machine
5 V and 12 V are common.
Where to look
One side.
How to get it out
Label and isolation-check.
The condition to expect
Good.
Worth pullingPowered or Bluetooth speaker1 · usually
In this particular machine
9–18 V are all common.
Where to look
The brick.
How to get it out
Label it.
The condition to expect
Often lost rather than dead.
Worth pullingE-scooter or hoverboard1 · usually
In this particular machine
42 V class for a 36 V pack. The label is the spec.
Where to look
The charger.
How to get it out
It is not a 12 V brick.
The condition to expect
Good.
Bonus, if you are already in thereFlatbed scanner1 · usually
In this particular machine
Low power.
Where to look
Inside or a brick.
How to get it out
Label it.
The condition to expect
Good.
Bonus, if you are already in thereDesk or box fan1 · sometimes
In this particular machine
An external brick.
Where to look
DC fans.
How to get it out
Label it.
The condition to expect
Good.
Bonus, if you are already in therePower strip1 · sometimes
In this particular machine
5 V.
Where to look
Inside USB strips.
How to get it out
Confirm isolation. Some are nasty.
The condition to expect
Unknown until tested.
Bonus, if you are already in thereElectric toothbrush1 · usually
In this particular machine
Mains to a low voltage into the coil driver.
Where to look
The base.
How to get it out
Leave it in the base if it works.
The condition to expect
Good.
Leave it — present, not yours to reuseSeasonal light string1 · sometimes
In this particular machine
Not a general supply.
Where to look
Some plug controllers.
How to get it out
Leave it.
The condition to expect
Unknown.
Leave it — present, not yours to reuseAlarm clock1 · sometimes
In this particular machine
Not isolated.
Where to look
Capacitive dropper supplies.
How to get it out
Do not reuse as a bench supply.
The condition to expect
Dangerous if misunderstood.