Power conversionBench electronicsTest and identify

Electrolytic capacitor

Also called bulk cap, filter cap.

The part that dries out and kills supplies. Secondary-side caps that still measure well are worth keeping. Primary-side mains caps belong in the original design or in the bin, not in a new mains supply you are inventing.

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 charged capacitor

Unplugging does not empty a large capacitor. The stored energy sits on the two leads. A resistor of a few thousand ohms, held across the leads with insulated tools, bleeds it down. Then measure. A screwdriver spark is not a discharge method.

Stored high voltage. Capacitors hold a charge with the cord in your other hand. Microwave HV capacitors, CRT anodes, xenon flash caps, and primary bulk caps are the usual ones.
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
Mark	Capacitance, voltage, temperature (85 °C vs 105 °C), sometimes ripple. Polarity is a stripe on the negative lead.
ATX primary	Typically 200 V or 400 V parts, large cans. Stored energy. Discharge before desoldering.
Failure	Bulged vent, crust, high ESR with capacitance that still looks plausible

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

Replacing a known-bad cap of the same values on a low-voltage board
A bulk cap on a DC rail you already understand

Leave this one when

Any bulge, any use as a mains X or Y capacitor (those are film and safety-rated; an electrolytic is not a substitute), any unknown primary cap going into a new design.

Buying a new one is the better idea when

New low-ESR caps of the right voltage for anything you want to last. They are not the expensive part.

How to tell if this one still works

Look, discharge, measure capacitance and ESR if you can.

Step-by-step on the bench: Decide whether an electrolytic is worth keeping

Tools you will want: Meter, ESR meter if you have one.

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.
capacitor: A part that stores a small amount of electrical energy. Large ones in power supplies can stay charged after you unplug. Discharge them before you touch the leads.
electrolytic: A common type of capacitor with a polarity. The stripe marks the negative lead. A bulging top means it has failed.
polarity: Which side is plus and which is minus. Reversed polarity ruins a lot of electronics. The symbol on a power brick is the map.
ESR: How imperfect a capacitor is at passing ripple. A capacitance reading can look fine while the ESR is already high enough to break a power supply.

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
Router or modem	Bonus	2–4	usually	Near the jack.
LED lamp	Bonus	1	usually	In the driver.
Desktop PC	Leave it	many	sometimes	Motherboard and supply.
Microwave oven	Leave it	1	always	Beside the transformer. It is not electrolytic; it is a high-voltage film capacitor around 1 µF at over 2 kV.
Wall adapter	Leave it	1	sometimes	Inside, if you cut it open.
Uninterruptible power supply	Leave it	several	usually	DC bus.
Digital camera	Leave it	1	usually	Flash board.

Bonus, if you are already in thereRouter or modem2–4 · usually
In this particular machine
Input filter caps. If the router rebooted randomly they may be the fault.
Where to look
Near the jack.
How to get it out
Test before keeping.
The condition to expect
Mixed.
Bonus, if you are already in thereLED lamp1 · usually
In this particular machine
This is the part that dried out.
Where to look
In the driver.
How to get it out
Do not reuse it.
The condition to expect
Bad.
Leave it — present, not yours to reuseDesktop PCmany · sometimes
In this particular machine
If the board died of capacitor plague, these are the evidence, not the harvest.
Where to look
Motherboard and supply.
How to get it out
Do not collect a jar of them.
The condition to expect
Suspect.
Leave it — present, not yours to reuseMicrowave oven1 · always
In this particular machine
It holds a charge. The bleeder may be open.
Where to look
Beside the transformer. It is not electrolytic; it is a high-voltage film capacitor around 1 µF at over 2 kV.
How to get it out
Do not take it home 'in case'.
The condition to expect
Charged.
Leave it — present, not yours to reuseWall adapter1 · sometimes
In this particular machine
The output cap.
Where to look
Inside, if you cut it open.
How to get it out
Not worth destroying the brick.
The condition to expect
Leave it.
Leave it — present, not yours to reuseUninterruptible power supplyseveral · usually
In this particular machine
High-voltage cans.
Where to look
DC bus.
How to get it out
Discharge. Do not harvest them for a new mains design.
The condition to expect
Aged.
Leave it — present, not yours to reuseDigital camera1 · usually
In this particular machine
A photoflash capacitor at a few hundred volts.
Where to look
Flash board.
How to get it out
Discharge if you are qualified. Otherwise leave the board alone.
The condition to expect
Charged.