Hi.
I have 4 red LEDs conntected in series with 20cm long wires to a voltage regulator (9v, 200mA) so each LED gets about 2.25v.
At first they worked fine, but after a while the LEDs started to flicker with very low brightness.
So I tested each one separatly and they are working fine.
I tested with a 9v batteriy and the same flickering occured.
What could be the problem?
Problem is, never ever connect a led without current limiting. And that's apparently what you did.
And because a red led has a normal forward voltage of around 2V you did over drive them quite a bit.
Remedy, trow those 4 leds out (they are broken) and get 4 new leds which you connect in series WITH a resistor 
so each LED gets about 2.25v.
No. Simply not right. Each LED drops a voltage determined by the current through it.
Where is the current limiting resistor in all this?
but after a while the LEDs started to flicker with very low brightness.
Is there a resistor? What controls the current.
So I tested each one separatly and they are working fine.
It could be you are fooling yourself. Again do you have a resistor when you test the individual LEDs?
I tested with a 9v batteriy and the same flickering occured.
Small 9V batteries don't have too much capacity you might have drained it. What is the voltage on the battery terminals when the LEDs are attached?
Hi.
Thanks for your replies.
septillion:
Problem is, never ever connect a led without current limiting. And that's apparently what you did.And because a red led has a normal forward voltage of around 2V you did over drive them quite a bit.
Remedy, trow those 4 leds out (they are broken) and get 4 new leds which you connect in series WITH a resistor
The LEDs are working fine individually so they are not broken no?
Grumpy_Mike:
No. Simply not right. Each LED drops a voltage determined by the current through it.Where is the current limiting resistor in all this?
Is there a resistor? What controls the current.
It could be you are fooling yourself. Again do you have a resistor when you test the individual LEDs?
Small 9V batteries don't have too much capacity you might have drained it. What is the voltage on the battery terminals when the LEDs are attached?
Well I have little idea about electric circuits, so correct me if i'm wrong.
I thought that the LEDs will just drain power they need to work, so 20mA while connected in series. I used 4 LEDs in series so each one gets about 2.25v. If i connect them in parallel, i need to use a resistor (7v/0.2ma=350ohm?), but then current consumption will be 4*0.2=0.8mA?
When i tested them individually i used a potentiometer (to quickly test different color LEDs).
The 9v battery is working fine, I use it to power 3 white LEDs in series, no resistor again (3*3v) and the are working fine (until now) without flickereing.
I think i understand better now (I read this)
That's indeed a nice story about them.
fjtheknight:
The LEDs are working fine individually so they are not broken no?
They are broken. The little adventure powering them without current limiting broke the bond wire. As long as it stays cold it may indeed still make contact but once it heats it disconnects. Hence the flickering. But once broken they are broken and best to toss them out.
Given that you said:-
Well I have little idea about electric circuits, so correct me if i'm wrong.
Did you really think that two members who have 52,216 posts here between them did not know how to power an LED?
Sure you didn't understand this, but a bit more humility is required when making an assertion and it is kicked back.
While having 50+ years in electronics, even I have to ask things sometimes, normally about a new computer language. But I don't go in all guns blazing when I am told things that I assumed are not the case. I ask for further details.
Grumpy_Mike:
Given that you said:-Did you really think that two members who have 52,216 posts here between them did not know how to power an LED?
Sure you didn't understand this, but a bit more humility is required when making an assertion and it is kicked back.While having 50+ years in electronics, even I have to ask things sometimes, normally about a new computer language. But I don't go in all guns blazing when I am told things that I assumed are not the case. I ask for further details.
i'm sorry but I didn't mean any bad thing (i'm not a native English speaker so I sometimes writes what i do not mean).
When i said "Well I have little idea about electric circuits, so correct me if i'm wrong." I am just saying that i will probably write something wrong, so i make mistakes i need your help to correct them
I do apologize if i wrote something disrespectful (because if i did, i didn't mean it).
And thank you for your help ^^
septillion:
That's indeed a nice story about them.
They are broken. The little adventure powering them without current limiting broke the bond wire. As long as it stays cold it may indeed still make contact but once it heats it disconnects. Hence the flickering. But once broken they are broken and best to toss them out.
Thank you for your help
That's fine, have another Karma point.