Power conversionRebuild shelfHand tools, works as-is

Fuse

Also called cartridge fuse, blade fuse, thermal fuse is separate.

A fuse is a specified weak link. The salvage rule is to read the current, the voltage, and whether it is fast or slow. A fuse that is the wrong speed will either nuisance-open or fail to protect.

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
Glass	5×20 mm and 6×30 mm. Current and voltage printed. T or S means slow; F means fast.
Automotive blade	ATO/ATC sizes, current by color and by the number. Voltage is low; do not use a car fuse on mains.
PTC	A resettable polyfuse looks like a yellow disc and is not a one-shot glass fuse.

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

Protecting the next prototype
Replacing the same fuse in the same device after you found the fault

Leave this one when

The printing is gone, or you are about to put a 20 A fuse where a 1 A fuse was, because you have more 20s.

Buying a new one is the better idea when

A small kit of 5×20 mm fuses. Knowing the value is the whole component.

How to tell if this one still works

Continuity. A good fuse reads a short. That does not tell you its rating; the stamp does.

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.

mains: Ordinary household wall power. It can kill. Unplug before you open anything that used it, and do not experiment on live wiring.
continuity: A meter mode that beeps when two points are connected. It is how you map a switch: beep, then click, then see if the beep changes.

Where people actually find it

5 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
Microwave oven	Worth it	1–2	usually	On the line input.
Uninterruptible power supply	Worth it	several	usually	Input and battery.
End-of-life car	Worth it	a row	always	The same box.
Power strip	Bonus	1	sometimes	A breaker or a thermal link.
Fluorescent fixture	Bonus	1	sometimes	Inline.

Worth pullingMicrowave oven1–2 · usually
In this particular machine
Ceramic mains fuse. Replacing it without finding the fault is how the new one opens.
Where to look
On the line input.
How to get it out
Read the rating.
The condition to expect
Often open, which means it already did its job.
Worth pullingUninterruptible power supplyseveral · usually
In this particular machine
Read them.
Where to look
Input and battery.
How to get it out
Do not upsize.
The condition to expect
Good.
Worth pullingEnd-of-life cara row · always
In this particular machine
Blade fuses. The number is the rating.
Where to look
The same box.
How to get it out
Do not replace a blown one with a larger one to 'test'.
The condition to expect
Good.
Bonus, if you are already in therePower strip1 · sometimes
In this particular machine
Some strips have a resettable breaker.
Where to look
A breaker or a thermal link.
How to get it out
Test it.
The condition to expect
Good.
Bonus, if you are already in thereFluorescent fixture1 · sometimes
In this particular machine
Read it.
Where to look
Inline.
How to get it out
Test.
The condition to expect
Good.