Power & batteryCommon

Uninterruptible power supply

Also called UPS, battery backup.

A lead-acid battery that is usually already dead, an inverter transformer, relays, and a fan. People scrap the whole unit because the battery died. The battery is the heavy part and often the only dead part.

A donor is the dead machine. The parts below are what is still worth removing from it. “Leave it” is a part that is physically in there and should stay in there: too dangerous, too worn, or only useful as part of a repair you are not doing.

The wires that matter on a computer supply

Yellow is 12 volts, red is 5 volts, orange is 3.3 volts, black is ground. The supply stays off until the green wire is connected to a black one. Purple is a standby 5 volts that is present whenever the cord is plugged in, even if the fan is still. Unplug and discharge before you open the metal box.

Why it is in the trash while the parts are not

It started beeping, which means the battery will not hold. Sometimes the inverter itself failed.

Before you open it

Do this while it is still unplugged, cool, and not leaking. Unplug. The battery is live. The inverter bus is high voltage DC. Discharge before you touch the board.

Where the dead machine usually is

Home office — In the house
Office cleanout — Commercial
Bulk trash day — Curb

Offices replace these on a schedule. The battery is a hazardous-waste item even when you want the transformer.

How it comes apart

Battery out first. Note the voltage. Then the cover. Do not short the battery terminals with the case.

Tools: Phillips, Meter, Discharge resistor, Gloves.

Words on this page

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.

What to take

The table is the short version. The notes under it describe that part inside this machine, not only the general description.

Parts in this donor
Part	Call	How many	How often	Where to look
Sealed lead-acid battery	Worth pulling	1–2	always	The tray.
Iron transformer	Prize	1	usually	The heavy part.
Power relay / contactor	Worth pulling	1–3	usually	Transfer relays.
Axial fan	Worth pulling	1	usually	Rear.
Fuse	Worth pulling	several	usually	Input and battery.
Electrolytic capacitor	Leave it	several	usually	DC bus.
Piezo horn	Bonus	1	usually	The beeper.
Mains cord	Worth pulling	1	always	Input.

Prize

Prize — take this firstIron transformer1 · usually
Inverter transformer, not a friendly low-voltage adapter. Useful if you already work with inverters.
The heavy part.
Unbolt after the unit is discharged.
Good.

Worth pulling

Worth pullingSealed lead-acid battery1–2 · always
12 V, often 7 or 9 Ah. Frequently sulfated.
The tray.
Load-test. Recycle if it sags immediately.
Often the reason the unit was scrapped.
Worth pullingPower relay / contactor1–3 · usually
Mains contacts.
Transfer relays.
Read the rating.
Good.
Worth pullingAxial fan1 · usually
12 V.
Rear.
Unplug.
Good.
Worth pullingFuseseveral · usually
Read them.
Input and battery.
Do not upsize.
Good.
Worth pullingMains cord1 · always
IEC or fixed.
Input.
Inspect.
Good.

Bonus

Bonus, if you are already in therePiezo horn1 · usually
The beep you hated.
The beeper.
Small.
Good.

Leave it

Leave it — present, not yours to reuseElectrolytic capacitorseveral · usually
High-voltage cans.
DC bus.
Discharge. Do not harvest them for a new mains design.
Aged.