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This is a inexpensive knock off of the LUMEX LED Tester Box at 1/4 the cost. It does however use a cheaper header than the screw machined header on the LUMEX tester so the insertion
life will be less. If you need to test hundreds of LEDs you would need a ZIF socket which even the LUMEX tester does not have. The new LUMEX tester now tests 3 terminal devices for the same price but the price is still closer to 40$ than 10$. Good addition to the tool box.
I paid $16 for one of these at a local electronics store for work, and it has been extremely useful for identifying LEDs from the scrap bin. It simply uses a 9 volt battery and resistors (and a push-button switch) to limit the current through LEDs - no fancy current regulation, so the 2-50mA current ratings are a rough indication and not accurate depending on the voltage drop of the LED under test. Accuracy aside this is a great price for a tester that quickly allows you to determine the polarity, color and relative brightness of any LED. Highly recommended!
White LEDs are the "worst case" for testing with their typically high Fv and high current that varies widely. Trying to match color and brightness of a handful of T-3/T-5's is a royal pain. Even using this as a "QC" check for "typical" current hasn't worked the best for me. I'd give it 3 1/2 stars if I could, as it's handy to have. I'd pay triple for a unit I could plug say 10 LEDs into, set the Fv and current I want and test them as a group. Even DIY building something like that would cost $50-75.
There was a defect in mine. The current-limiting resistor for the 30mA branch is 3300 Ohms (the same as the 2mA branch). Other than that, it works fine. I figure it should be around a 230 Ohm resistor instead. It seems like this is an isolated problem, since it works for others.