Power conversionBench electronicsSpecialist reuse

LED constant-current driver

Also called backlight driver, LED PSU, AMC7135.

LEDs want current, not a random voltage. The driver is the part that does that. Backlight drivers produce tens of volts at a regulated current. Flashlight boards based on the AMC7135 are linear 350 mA sinks and can be stacked.

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.
CCFL backlight inverters. Old LCD panels use a cold-cathode lamp driven at about a kilovolt of AC. The inverter is small and looks harmless.

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
Backlight	Often a boost to 20–80 V or more for a series string. The current is in the tens of mA. Open-circuit voltage is high; do not probe carelessly.
AMC7135	350 mA linear, needs the LED and a supply only a little above the LED voltage. Paralleled on many flashlight boards.
Bulb driver	May be non-isolated. Do not mix this up with a safe brick.

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

Drive the LED string it came with
A flashlight rebuild

Leave this one when

You have lost the string it was matched to and there is no current marking.

Buying a new one is the better idea when

A CC module with a labeled milliamp rating.

How to tell if this one still works

Only with the original LED load attached, and only after you know whether the input is mains.

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

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.
isolated: The low-voltage side is not electrically connected to the wall. A proper brick usually is. Many cheap lamp boards are not.
series: Connected in a chain. Cells in series add their voltages. LEDs in series all go dark if one of them opens.

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
LCD monitor	Worth it	1	usually	With the supply or beside it.
Flashlight	Worth it	1	sometimes	Behind the star.
LED lamp	Bonus	1	usually	In the base.

Worth pullingLCD monitor1 · usually
In this particular machine
Constant current for that panel's strips.
Where to look
With the supply or beside it.
How to get it out
Keep it with the panel.
The condition to expect
Often the actual fault.
Worth pullingFlashlight1 · sometimes
In this particular machine
AMC7135 boards show up in hobby lights. Cheap lights use a single resistor or a mystery boost.
Where to look
Behind the star.
How to get it out
Read the chip.
The condition to expect
Good.
Bonus, if you are already in thereLED lamp1 · usually
In this particular machine
May be a capacitive dropper. Isolation-check before the output touches anything else.
Where to look
In the base.
How to get it out
If there is no transformer, it is not isolated.
The condition to expect
The cap is the usual failure.