O.K Robert, understood. Whenever it will be comfortable to you, just remember and do me the favor of testing a low brightness led with your tester and let me know what message you get. I am only curious if it sees a typical diode or anything else.

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I'm pretty sure that the LED was a ultrabright since it was used as a flashlight. I replaced it with a ultrabright and it works fine. These ultrabrights have a Vf of about 2.8V. I can't find the old LED right now. I don't know if I misplaced it or threw it away when I was half asleep sometime.

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Hi Robert

It is known that leds behave like zener diodes. They have excellent stabilizing characteristics for that purpose and I have seen them used as zeners many times.
Now, in your case, I understand that your tester tells you "no component detected" if the crystal junction of the diode is open. This is of course natural to happen. But if the instrument tells you "this is a 4V zener diode", this is strange indeed because the led should work normally in this case. Perhaps without full brightness but it should work.
Anyway I don't own that type of tester and cannot give you any logical explanation about its result. I either use my digital multimeter in the diode function or my analog multimeter in resistance function, which has a special range for led diode testing. The difference between the two instruments I use, is that in general digital multimeters cannot test diodes with forward drop voltages greater than two volts, or even less than that depending on the manufacturer, whereas my analog multimeter has a special range for leds, this is the Rx10 range, which lights up any led with Vf up to 3V providing maximum current of 15mA. So I have the information of both the forward voltage drop and the current draw of the led. This is therefore a complete dynamic test of led diodes, which is the most reliable one.
Well,as far as I know about leds, the Vf of the low brightness types is between 1,5 and 1,7V. The high brightness types extend to 3,2V, the nominal current for both being usually 20mA. But I don't know about leds with Vf above 3,2V. At least I have never came across such one.
It would be therefore interesting if you tried to connect that failing led to your bench psu, limiting its current to say 10mA and the voltage around 5V and tell me what happens...
It would also be interesting for me to know what your tester tells you when you connect to it a low brightness led...

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I already know that. What I'm saying is that the LED quit lighting up and when I tested it with my DCA 75 Pro it said that the LED was a 4V zener diode. I've never seen anything like that before and was wondering if you had. Usually when a LED quits working my DCA 75 Pro says, "no component detected", which is what I expected. But for some reason this LED burned out and somehow reads as a zener diode - the first time I have ever seen that happen before.

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Hi Mark

Just keep in mind that there are no power transistors with more than three pins. So don't get confused about that. But, as it almost always happens, there are some minor exceptions to that. These include the RF power transistors which have two emitter terminals (but only one emitter inside their case) and power MOSFETs (usually bricks) with four terminals, the fourth being the so called sense terinal (for measuring the drain current through them). Thus any component having more than three pins is an I.C (although there are exceptions again, such as I.Cs having three pins i.e the TL 431 or even I.Cs having two pins only, like the TAA 55O). This is the marvelous world of electronics!

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Robert,

If this circuit uses a 1,5V zener instead of a resistor as per the normal scenario I used above, keep in mind that the circuit behaviour will be different. The difference will be that the led, although healthy, will not light at all if the battery is discharged below a safety low level. This is because the zener diode will act as a switch not allowing current to pass through it. So the led will light only when the batteries are fully charged. With my scenario, the led will keep on lighting much below that safety voltage threshold, which is not good for the health of the battery. 18650 cells must not be disharged below 3 Volts and respectively not be charged above 4,2 Volts. This is their rule of longevity. Once again, the golden medium is needed!

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Hi Robert

Oh,I have seen many leds failing! Nothing is immortal in our world! The point is that if the circuit design is correct leds last about 50000 working hours. I am a little bit confused with your description. Does the Atlas Peak see the failed led both as a simple diode and as a zener as well? Or the led is fed from its source through a zener diode? Normally there should be a voltage dropping resistor between the source and the led in order to protect it from burn out. The calculation is straitfoward. It's a direct application of Ohm's law. You need to subtract the led's operating voltage from the total source voltage and then divide your result with the nominal working current of the diode. The new result will be the value of the voltage dropping resistor. If the calculation does not coincide with a standard resistor value, which is most likely, choose the next higher value available. Next, check the power this resistor consumes in order to choose a resistor of three times higher of that of your calculation finding. Again, power equals the square of the led's current, multiplied by the resistor's value. Use a resistor of three times this power (approximately again). Finally, if you take a decreased value of the led's working current in your first calculation, you will increase very much the longevity of your led. (Practically the calculation will give you a higher than the normally needed resistor).
I hope I have covered all your possible questions about the topic.

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Hi Paris,
My son has a USB battery pack that uses 2 18650 batteries. It also has a little flashlight that uses one 5mm white LED. The LED stopped working. So I took it apart and removed the LED. My Peak Atlas DCA 75 reads the LED as a 4.2V/or so zener diode with a Vf of about 1.5V. It's the first time I've seen a LED burn out and read as a zener diode. Have you ever seen that? I didn't write an article about it since it was such a simple repair. A new LED has it working just fine.

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