Power conversionRebuild shelfTest and identify

Iron transformer

Also called linear transformer, 50/60 Hz transformer, wall wart transformer.

Heavy, inefficient, and kind. An old linear wall wart or a stereo transformer gives you isolated low-voltage AC and can survive abuse that kills a switcher. Weight is the tell against a switch-mode brick.

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
Frequency	50/60 Hz only. Do not feed it high-frequency PWM and expect it to behave.
VA	Roughly estimated from core size, but the nameplate is better. A small plug-in may be 5–20 VA.
Secondaries	Measure AC unloaded. Loaded voltage sags. Rectified DC is about 1.4× the AC minus diode drops, unloaded.

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

Isolated bench AC
Linear supplies
The core, if you are rewinding and you already know how

Leave this one when

It buzzes angrily, smells of burnt varnish, or the primary is open. Microwave oven transformers are a different, lethal part — see that donor, and do not file them here.

Buying a new one is the better idea when

A boxed isolated supply. Rewinding cores is a hobby of its own, not a shortcut.

How to tell if this one still works

Primary resistance low but not shorted. Secondary AC voltage at the label. Isolation primary-to-secondary should be open.

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.

nameplate: The printed label on the part or the machine. If the label and this index disagree, use the label.
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.
PWM: A rapid on-off signal used to control speed or brightness. A four-wire computer fan uses it on the fourth wire.
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

6 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
Clock radio or stereo	Prize	1	sometimes	Older radios.
Uninterruptible power supply	Prize	1	usually	The heavy part.
Alarm clock	Prize	1	sometimes	Inside older clocks.
Wall adapter	Worth it	1	sometimes	Heavy old adapters.
Microwave oven	Leave it	1	always	High-voltage cage.
CRT television or monitor	Leave it	1	always	The flyback.

Prize — take this firstClock radio or stereo1 · sometimes
In this particular machine
Iron, isolated, the weight in the box.
Where to look
Older radios.
How to get it out
Measure the secondary AC.
The condition to expect
Good if it does not smell burnt.
Prize — take this firstUninterruptible power supply1 · usually
In this particular machine
Inverter transformer, not a friendly low-voltage adapter. Useful if you already work with inverters.
Where to look
The heavy part.
How to get it out
Unbolt after the unit is discharged.
The condition to expect
Good.
Prize — take this firstAlarm clock1 · sometimes
In this particular machine
Small iron transformer.
Where to look
Inside older clocks.
How to get it out
Measure it.
The condition to expect
Good.
Worth pullingWall adapter1 · sometimes
In this particular machine
Iron, 50/60 Hz, isolated.
Where to look
Heavy old adapters.
How to get it out
The weight is the tell.
The condition to expect
Good.
Leave it — present, not yours to reuseMicrowave oven1 · always
In this particular machine
The MOT is a ~2 kV transformer. It is not the doorbell transformer and it is not a bench supply. Unloaded, it saturates and the current is lethal.
Where to look
High-voltage cage.
How to get it out
Leave it in the cage.
The condition to expect
Dangerous.
Leave it — present, not yours to reuseCRT television or monitor1 · always
In this particular machine
High voltage.
Where to look
The flyback.
How to get it out
Leave it.
The condition to expect
Leave it.