Hi, I have this little circuit involving 3 AAA batteries in series (4.5 V) + 2 white LED + a resistor or capacitor (don't know which one is it) + a switch of course:



I don't have it right now, but the problem is one LED is blinking like crazy and the other one sometimes stays fine and at full bright, other times blinks just a bit. AFAICT, the blinking does not go from OFF to full brightness, but from 70% to 100% of brightness in the one that kinda works and from 20% to 100% in the one that blinks like crazy.

I basically want to repair it (it's a small light circuit for soldering glasses, ZOOM), and I don't know if it's the LEDs or may be the component on the back (it was tiny, black, bipolar and had 1001 or something like that written on top).

Also, can somebody explain why or how is this happening?

The AAA batteries are all fine, no worries there.

 

Bad solder joint is the first thing I would try. Reheat all the connections on the other side of the board.

 

Solder joints are alright, it's not that

 

looking at the circuit both leds are in parallel , so one will take over more , post a picture of the track side.

 

Solder joints are alright, it's not that

How did you determine that? You cannot always tell by looking at them.

 

As mentioned, LEDs are in parallel. Bad design. Only if LEDs have internal current limit resistors would this be an acceptable design.

 

The 1001 black component is a 1K resistor to limit current. The Vf of a white LED is say around 3V, that leaves 1.5V dropped across a 1K resistor and that gives 1.5ma and that sounds low. Maybe it reads 101, which would be 100 ohms and current of 15mA. In any case two LEDs in parallel is bad/cheap/knockoff design practice. Try removing one LED and see how it works with one. Not positive, but running LEDs in parallel like this can damage LED and results in premature failure.

 

I agree, the LED's are parallel to each other. I doubt that a picture of the other side will reveal anything significant, but still, the picture would prove helpful to some degree. As for the mystery of whether the component is a resistor or capacitor, a resistor has metallization on the bottom, end and top on each end whereas a cap has its metallization on the bottom, end, top AND the sides (on each end).

If one LED is behaving different from the other then it's likely a bad LED. Further testing is advised. You may have to remove one LED to test the other. I'd start by removing the super blinkey LED and see how the other behaves. Likely one is pulling down the circuit somehow. If no change is observed then put the first LED back into the circuit and remove the other. Repeat testing. Then tell us what you've found.

Or just replace both LED's with ones with the same forward voltage.

If it were a bad resistor (probably what's in circuit as DC will not pass through a capacitor) (which is why I'd like to see the back side) then both LED's should respond exactly alike.

 

I have seen a similar failure in 48 inch LED tube lights in a restaurant owned by a friend. In that case it is failing LEDs, because each tube has four series strings in parallel, and the failure starts with one string flashing and eventually ceasing to illuminate.
A failure analysis reveals that in each failed series string at least one LED has developed a much higher forward drop and different current to forward drop characteristics. As the LEDs are surface mount and anchored to the PCB strip very well, repair would not be simple, even if suitable replacement parts were available, which they are not.

And for the assembly in the photo, the red wire is rather well frayed.

 

How did you determine that? You cannot always tell by looking at them.

Because you can see the tracks on the PCB. They are wired in parallel. Plus, you can see the internal contacts. The two triangular connections are connected on the PCB. As are the other two connections.

 

the red wire is in really poor shape and not even connected or insulated properly. also it is heavily corroded. in fact it may be touching the PCB track where the black wire is. not sure if the black wire is in same bad shape or has strands reaching over...

 

Because you can see the tracks on the PCB. They are wired in parallel. Plus, you can see the internal contacts. The two triangular connections are connected on the PCB. As are the other two connections.

My bad. I was asking the TS how he determined the soldering is not faulty. Should have quoted to avoid the ambiguity.

 

Like I said: blink and flicker are failure modes of some LEDs. I have seen it repeatedly.

 

As is stated in post #10, the LEDs are visibly connected in parallel before connecting to the current limiting resistor. To quote Don Klipstein,

"Do not put LEDs in parallel with each other. Although this usually works, it is not reliable. LEDs become more conductive as they warm up, which may lead to unstable current distribution through paralleled LEDs. LEDs in parallel need their own individual dropping resistors."

What might work would be to cut the outermost trace going to the LED on the left at a point between the two LEDs. Then solder one lead of a new resistor to the now unconnected lead of the LED on the left and the second lead of the new resistor to wherever the existing resistor is connected (other than where the existing resistor connects to the LED on the right).

EDIT: Actually it probably would be a good idea to make a little circuit diagram so that the modification is done correctly, perhaps differently from what I have described.

 

LED's can have partial wirebond/diebond failure that causes them to become a thermal oscillator.

LED heats up, connection breaks, LED cools, connection restored- repeat...

 

"S" has described quite accurately the symptoms that I have seen in the LED tube lights, except that in those cases the eventual result is total failure.

 

LED's can have partial wirebond/diebond failure that causes them to become a thermal oscillator.

LED heats up, connection breaks, LED cools, connection restored- repeat...

this is the answer I was looking for, thank you! The WHY!!! As I didn't get what on Earth can make an LED flicker/blink.

This circuit is of course made cheap. I would love to make it better! I use these glasses a lot to work on electronics, soldering and checking things... The battery is x3 AAA 1.5V, making that about 4.5V battery. The circuit must be really small, to fit properly. About the wires being worn, don't worry, that's one of the things I cleaned and fixed, of course that was not the problem and not what was causing the flickering.

 

Since the two LEDs are in parallel but only one was flickering, if the connections were indeed good, then the LED is at least slightly defective.

 

you can experiment with it by introducing temperature change (cold spray, heat gun).

 

2 in-series? one possibly shorted and literally made a short, thus the other is being over-driven?