Sound, light, heatBench electronicsTest and identify

LED array or COB

Also called COB LED, backlight strip, MCPCB, LED bulb board.

Many LEDs in series on aluminum board, from a bulb, a flashlight, or a TV backlight. You do not power these from 12 V because 'LEDs are 12 V sometimes'. Count the series junctions or measure the string voltage from the original driver.

If you are new to this category

Speakers, lamps, heaters, and displays. To test a speaker, touch a 1.5 V cell to the terminals for a moment. The cone should click once. A heater can start a fire if you remove the part that was supposed to shut it off.

Heat and bright light usually mean household voltage. Unplug first. If the original safety part is open, find out why before you replace it.

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
Bulb COB	Often a series string run at tens to a couple of hundred volts by a mains driver. Treat the board as mains until you know the topology.
TV backlight	Series-parallel strips. A whole strip may want 30–100 V at a few tens of mA. The original driver is the power supply.
Flashlight star	One or several LEDs on a star, often a single white LED around 3 V, or a small series.

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

Lighting, with a constant-current driver
The aluminum board as a small heatsink

Leave this one when

Any LED in the string is black. Series strings go dark from one open die, and paralleled strings then hog current.

Buying a new one is the better idea when

A 12 V strip with the voltage printed, if you do not want to design a current source.

How to tell if this one still works

Diode test may show the whole string if the voltage is within the meter. Otherwise use the original driver, current-limited.

Step-by-step on the bench: Check a supply is actually isolated

Tools you will want: Meter, The original driver.

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.
series: Connected in a chain. Cells in series add their voltages. LEDs in series all go dark if one of them opens.
parallel: Connected side by side. Cells in parallel share the current and stay at the same voltage. Only match cells that are in similar condition.

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
Flashlight	Prize	1	usually	The head.
LED lamp	Worth it	1	usually	Under the diffuser.
Flatbed scanner	Bonus	1	sometimes	The scan bar, on LED-era units.
LCD monitor	Bonus	2–4 strips	usually	Edges of the panel.
LCD television	Bonus	strips	usually	Behind the panel.

Prize — take this firstFlashlight1 · usually
In this particular machine
One white LED on a star, or a small COB.
Where to look
The head.
How to get it out
The star is the heatsink.
The condition to expect
Good.
Worth pullingLED lamp1 · usually
In this particular machine
Series LEDs on aluminum.
Where to look
Under the diffuser.
How to get it out
One black LED opens the string.
The condition to expect
Mixed.
Bonus, if you are already in thereFlatbed scanner1 · sometimes
In this particular machine
A light bar at whatever voltage the board used.
Where to look
The scan bar, on LED-era units.
How to get it out
Keep the little driver if there is one.
The condition to expect
Good.
Bonus, if you are already in thereLCD monitor2–4 strips · usually
In this particular machine
Series LEDs. One open LED darkens a whole string.
Where to look
Edges of the panel.
How to get it out
Do not peel them casually; they crack.
The condition to expect
Mixed.
Bonus, if you are already in thereLCD televisionstrips · usually
In this particular machine
Long series strings.
Where to look
Behind the panel.
How to get it out
Keep the driver with them.
The condition to expect
One dead LED kills a strip.