Power conversionRebuild shelfTest and identify

Film capacitor

Also called run capacitor, motor cap, X capacitor, MKP.

The plastic-film can that starts a motor or sits across a line as an X capacitor. Motor run capacitors are genuinely useful and genuinely mains. Safety-class X and Y parts are not generic capacitors.

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.

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

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
Motor run	Tens of µF, 370 or 440 VAC, oval or round metal can. Value is printed. A fan that used 5 µF will not be happy on 40 µF.
X cap	Across line and neutral, safety marked X1 or X2. Failure mode should be open, not fire.
Y cap	Line to earth. Must be Y-rated. Do not replace with a random blue film cap from a junk box.

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

The same motor
A power-factor or run application that matches the print
Leave X and Y caps in the supply they came from

Leave this one when

The can is bulged, leaking oil, or the value is unreadable.

Buying a new one is the better idea when

A new run capacitor of the same µF and an equal or higher VAC rating. Do not 'go bigger'.

How to tell if this one still works

Capacitance against the print. A run cap more than about 10% off, or with high leakage, is done. Discharge first; motor caps hold a charge.

Step-by-step on the bench: Discharge a capacitor on purpose

Tools you will want: Meter with capacitance, Discharge resistor.

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.

Where people actually find it

3 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
Window air conditioner	Prize	1	always	Side compartment.
Desk or box fan	Bonus	1	sometimes	On larger fans.
Fluorescent fixture	Bonus	1	sometimes	Older ballasts.

Prize — take this firstWindow air conditioner1 · always
In this particular machine
Dual run capacitor, values printed, 370 or 440 VAC.
Where to look
Side compartment.
How to get it out
Discharge, then read the print. Replace with the same µF if you are repairing. Keep it if it measures right.
The condition to expect
Failure is common and obvious: bulged or far off value.
Bonus, if you are already in thereDesk or box fan1 · sometimes
In this particular machine
A small run capacitor.
Where to look
On larger fans.
How to get it out
Read the value.
The condition to expect
Bulged is scrap.
Bonus, if you are already in thereFluorescent fixture1 · sometimes
In this particular machine
A power-factor capacitor.
Where to look
Older ballasts.
How to get it out
Read it. Discharge it.
The condition to expect
Aged.