LED TesterCircuit

Posted on Feb 5, 2014

Before you use LEDs, it`s a good idea to check them out. With a LED tester, you can even do it in the dark! LEDs are available nowadays in all shapes and colors. There are types with clear, colorless packages, while others have colored plastic packages. Many modern types of LEDs need less current than older types. Some of them provide quite

LED TesterCircuit
Click here to download the full size of the above Circuit.

a puddle of light if you give them a decent amount of current. When you`re working with used LEDs from the junk box, there`s a good chance that you can`t tell which lead is which any more. (If the leads haven`t been trimmed, the short lead is always the cathode lead and the long lead is the anode lead. ) If you use several LEDs in a display where they all have the same current you naturally want all the LEDs to have the same brightness. But that`s not always the case, even with LEDs of the same type. To save yourself unnecessary soldering work, it`s a good idea to check the LEDs out first. That`s the job of the LED tester described here. This circuit can be used to test up to three LEDs at once, connected in series. You can easily increase that number by using a higher supply voltage. If you do so, you should allow 2. 7 V for each additional LED. The Zener diodes are included in the circuit so it can also be used to test one or two LEDs. Another benefit of the Zeners is that even if one or more of the LEDs are defective or connected with reverse polarity, the remaining ones will light up normally. That makes it easy to spot suspect LEDs. If you extend the tester to handle more LEDs, you must add another Zener diode for each LED position. The test current that ows through the LEDs is held reasonably constant by FET T1, independent of the number of LEDs being tested. The FET is used as a constant-current source to keep the...

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