Backlight LED Tester

I have successfully salvaged LED backlight diffuser assemblies from three different LCD screens. It gave me the confidence to attempt pulling the backlight out of other LCD screens in my pile of less-used and broken electronics, in the expectation that diffuse white light sources will be more useful than low resolution displays. But before I start merrily tear more panels apart, I wanted to address one particular pain point: deciphering mystery LED strings.

Only one of the three examples so far gave me an easy time, where the LED backlight power planes were clearly marked with + and -. The other two had multi-conductor cables that required a little decoding to find a common anode and cathodes for individual strings, and sometimes a few conductors remained mysterious. I had been doing this work by spending a lot of time probing with a multimeter, then soldering wires to test points, and cautiously putting power on those lines with a bench power supply. This has worked so far, but I knew there was room to make this process faster for future panels.

Enter the dedicated LED backlight tester.

I wanted something that could put current-limited power over a set of probes. This would let me probe LED strings directly in a single-step versus my current multi-step workflow of multimeter, then soldered wires, then bench power supply. I considered buying a set of probes that I can connect directly to my bench power supply, but a quick search for dedicated LED testers found them quite affordable and I made the jump to try one.

Looking over several options on Amazon, I decided to try a SID LED KT4H(*) because it advertised a few extra features I thought might be useful enough to worth the extra cost. It has household AC input so I don’t have to worry about a separate power supply or batteries. Separate numerical displays for voltage and amperage allows me to read both metrics simultaneously. Simple dials for current and voltage limits make the user interface simpler than designs that use infuriating combinations of unintuitive button presses. There’s a switch to toggle between two current limits: the one set by the dial, and 1mA for testing purposes. This is much better than turning the current limit knob back and forth. And finally, it can also test the other side of the system: whether the device’s constant current supply is putting out any power at all.

The package also included a convenient carrying case, in the form of a generic First Aid Kit zippered fabric bag. Not the fanciest branding, it gave me a chuckle, but it should be quite sufficient.

The probes had sharp tips more than precise enough to hit the kind of test points I had been probing with my meter.

For a quick familiarization run, I used this tester on a single 5mm through-hole LED and saw it light up dimly in the 1mA test mode and brightly with current limit set at 20mA. Then I moved on to illuminate the recently-liberated backlight from an AU Optronics B101EAN01.5. A nice feature is that the current limit ramps up gradually: there is a slow (2-4 seconds) ramp-up as the device seeks the correct voltage level to deliver 20mA. In comparison to my bench power supply which will snap to a voltage almost instantly. I’m optimistic the slower ramp-up will prove valuable.

Before I could put this tester to work, though, life threw me a curveball and I had a broken clothes dryer to fix. The LED backlights will have to wait a bit.

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

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