Hi All
I have a small battery powered lamp that I use for illumination at my desk when I need that extra bit of light. The lamp is a simple 4xAA battery poweres 3 rows of 24 white LED's and is quite bright (when it works!!)
It stopped workign a few days ago and today I thought I'd have a quick look to save myself spending £15 on a new one. it seems each row of 24LED's is powered through what I think is a pcb mounted type 100Ω resistor, and one of those has burned out.
The thing is, is that now the entire lamp doesn't turn on!
I am curious as to why this might be happening? Would a resistor burning out cause an entire 24LED x 3 rows to fail?
What should I check?
Make sure it's the 100 ohm resistor and if the components are placed in series or parallel.
trevortjes:
Make sure it's the 100 ohm resistor and if the components are placed in series or parallel.
Hi Trevor
I removed the blown resistor from that row, so i would have thought that what would happen now would be that only two rows work! However, none of the LED's turn on!
I'd say put a new 100 Ohm resistor in and see if the thing works again. Sounds like the rows work in parallel and each row has it's own 100 ohm resistor.
That's what I thought too! Here's a poor iPhone image
They are connected in parallel with each row of 24 LED's going through one of the 100Ω resistors.
I have tried shorting across where the resistor was, but that does nothing too!
That's odd. That means that all the LEDs must be dead or something. Can you get the power supply out and use a diode test to test the LEDs?
Yep, I can remove the power supply, and I have a diode test indicator on my multimeter. Don't I need to remove one of them from the board to test it?
Oh yes you should do that too!
Yep, the LED I removed worked fine! This is weird. I might just give up and appreciate I will have a lifetime supply of bright white LED's 
Thanks for your replies, Trevor, very kind.
That's what I thought, 72 bright white LEDs!
Hi,
Place the LED back in the PCB.
Refit the batteries and use your DMM.
- Measure the voltage out of the battery pack,between 1 and 2, with the switch ON then OFF.
- Measure the voltage between 1 and 3 with the switch ON then OFF.
- Measure the voltage between 1 and 4 with the switch ON then OFF.
- Measure the voltage between 1 and 5 with the switch ON then OFF.
- Measure the voltage between 1 and 6 with the switch ON then OFF.
You have the tools for basic trouble shooting, USE THEM.
Tom...
TomGeorge:
Hi,
Place the LED back in the PCB.
Refit the batteries and use your DMM.
- Measure the voltage out of the battery pack, with the switch ON then OFF.
- Measure the voltage at the PCB where the two blue wires connect, with the switch ON then OFF.
You have the tools for basic trouble shooting, USE THEM.
Tom...
Thanks for those words of wisdom, Tom. Yep indeed you're right, I do have the tools, and a DMM, and you will be pleased to know I have used them, thanks. I had already measured the voltage as you suggested, and at the points you suggested too! Do you really think I'd have posted here with this info if I had not checked for unusual resistance, shorts and voltage readings?
Come on, sir... no need to be so patronising, neither is there a need to use caps to shout.
Enjoy your day.
theMusicMan:
Thanks for those words of wisdom, Tom. Yep indeed you're right, I do have the tools, and a DMM, and you will be pleased to know I have used them, thanks. I had already measured the voltage as you suggested, and at the points you suggested too! Do you really think I'd have posted here with this info if I had not checked for unusual resistance, shorts and voltage readings?
Come on, sir... no need to be so patronising, neither is there a need to use caps to shout.
Enjoy your day.
And what were the results of your measurements.
If you had said what tests you had done in post #1 then we may not be at post #13.
Your question was "What should I check?"
Sorry if I offended, but...
Tom....
1 - haha, nice isn't it?!! 'that' is a direct result of my very poor soldering but is a safe connection nonetheless. I spotted what seemed to be a broken connection to the board on that very pin, and soldered it - badly as you can see. But I tested it and it worked and made a connection.
2 - Yes, spot on. That's where the burned out flat type 100Ω resistor was placed.
I even tried a replacement 100Ω resistor across those pins to see if that fixed it. Sadly not!
Another thing that happened after all I had done in the original thread up to today - was that when I powered up, R1 (top resistor) started smoking, which lead me to believe there was a fault elsewhere, which is when I gave up and decided to open a shop selling bright white LED's for the rest of my days.
PS: sorry if I was a bit arsey in 1st reply to you...
Hi,
Sounds like "one" or more of your LEDs are short circuit.
Just have you the patience/time to unsolder each and test them.
What were your measurements with the 100R out of circuit?
"£15 on a new one" may be worth the effort.
Tom...
TomGeorge:
Hi,
Place the LED back in the PCB.
Refit the batteries and use your DMM.
- Measure the voltage out of the battery pack,between 1 and 2, with the switch ON then OFF.
- Measure the voltage between 1 and 3 with the switch ON then OFF.
- Measure the voltage between 1 and 4 with the switch ON then OFF.
- Measure the voltage between 1 and 5 with the switch ON then OFF.
- Measure the voltage between 1 and 6 with the switch ON then OFF.
You have the tools for basic trouble shooting, USE THEM.
Tom...
1 - 6.2v 6.2v
2 - 6.2v 0v
3 - 6.2v 0v
4 - 6.2v 0v
5 - 0v 0v
Hi
Hmmm, the legs in measurement 4 and 5 appear to indicate that all the diodes OPEN circuit.
6 has a shorted LED or more than one because it smokes a 100R??? resistor.
If they are SMD resistors, 100R is 101, 100 is 10R, that would make sense when you look at the maths.
(6.2 - 1.6)/100 = 4.6/100 = 46mA, not enough for all those parallel LEDs.
(6.2 - 1.6)/10 = 4.6/10 = 460mA, enough for all those parallel LEDs.
They are relying on the fact that ALL the parallel LEDs have the same volt drop, unfortunately that is not the case.
Probably one LED has been consuming more current than designed and popped OPEN and loading up the others that are now over loaded and they go pop in sympathy.
Strange.. :o
Yep, it is a weird one, Tom. Even weirder is the fact that when I removed one of the LED's and tested it on a separate circuit, it worked fine!
So... I'll pop the link to my new online BrightLEDLightsUnlimited dot com later
