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LED Fail on Magnifying Glass

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Kagord:
Hi,

I purchased this magnifying glass, 3 months later, an LED failed (flickered for awhile and went out), then within a week the other 2 LEDs failed in the same fashion.

Product:   https://www.amazon.com/gp/product/B015VGC15Y

Opening it up.  The circuit is a 20 ohm resistor connected to 3 straw hat 5mm white LEDs in parallel.  Vin is 6V (4 AAA batteries in series).  Looking at datasheets for various LEDs, the LEDs seem to be 3V.

Sample Datasheet:  https://www.iled.com/class/INNOVAEditor/assets/YeniDatasheets/2718-2780(hiledmantar).pdf        (part 2729).

I bought these:  https://www.aliexpress.com/item/wholesale-100-Pcs-5mm-White-Straw-Hat-Water-Clear-Super-bright-Wide-Angle-20000MCD-LED-free/1923509715.html  (White, 3.2V minimum)

It works again, but why, if min voltage is 3.2V and max is 3.4V per LED, how can 6V turn 3 LEDs on, isn't it too much voltage?  How do you calculate the voltage after the resistor? 

DDunfield:
Min voltage 3V to max 3.4V  .. lets guess 3.2
Typical current 20ma

Starting with 6v and dropping 3.2v for the LEDS, that leaves us with 2.8v dropped across the resistor.
By Ohms law,  2.8/20 = 0.14 amps, or 140ma

In an ideal situation, the current would be being drawn equally by the three LEDs, which would mean 140/3 or about 47ma per LED.

Seems a little higher than the typical from the datasheet.
** Not an accurate calculation because we don't know the actual voltage drop on the LEDS, and if the LEDs are drawing more than typical, likely voltage is slightly higher than typical, but the calculation with typical values and finding current more than 2x typical is enough to determine that something is "not right" with the design. You could measure with a multimeter and determine more accurately what is going on.

Also note that driving LEDs in parallel without series resistors on each LED is not good, as they will not be exactly balanced, one will draw more than another. Likely the first one to go was drawing the most current.

It is common in cheap stuff to find LEDs overdriven, presumably in the hopes of getting a little more brightness from them.

I bought a little nightlight to use when travelling - It used 3x1.5V (4.5v total) and a single white LED ... seemed OK for the first few trips, then the LED started flickering very noticeably.
Upon taking it apart, I found a 5 (FIVE) Ohm resistor in series with the LED and batteries. This caused it to draw over 50ma which is more current than I deem suitable for a low power LED (I didn't try to find the data sheet).

I replaced it with a 100 Ohm resistor which gave a current of about 15ma, and a hardly noticeable drop in brightness and no more flashing. Batteries last much longer too!

Based on the datasheet and a lower limit of 3V drop on LED, leaving 3V on the resistor, solving for 20ma I would try a 150 Ohm resistor in series with EACH LED and see where it falls.

Dave

Audioguru:
I agree that each LED should have its own series resistor and operate each LED at its 20mA rating. The resistor for each 3.2V white LED will be (6V - 3.2V)/20mA= 140 ohms which is not available, use 120 ohms or 150 ohms.

Kagord:
OK, please bear with me, I'm new at this, if you're supplying 6V to a 3.4V max input voltage component, is that OK? I guess I'm not getting what voltage drop means.

boffin:

--- Quote from: Kagord on May 30, 2019, 04:17:53 pm ---OK, please bear with me, I'm new at this, if you're supplying 6V to a 3.4V max input voltage component, is that OK? I guess I'm not getting what voltage drop means.

--- End quote ---

That's why there's a resistor.

The LED drops 3.4V
The resistor drops the rest 2.6V
By knowing both the value of the resistor 20R and the voltage across it 2.6, you can calculate total current: 2.6/20 = 130mA, a little over 40mA/LED (in a perfect world)

The problem with the solution you have is that not all LEDs  will drop EXACTLY the same voltage, so the electricity will tend to favour the one that drops the least voltage, burning it out; rather than evenly distributing through each LED  Rinse, repeat until no LEDs remain. 

Take the advice here, and have a separate resistor for each LED, and drive the LED at about 25mA max, or about 100 Ohms (2.6V / 0.025mA) for the resistor.



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