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LED & Resistor help..

I was wondering if someone could help solve my problem as follows. I shall make as much detail as possible:

I am running 12 (5mm led) in parallel of 3 in series. (So four rows of three leds)
The Manufactors spec is
Forward max Voltage = 4v
Forward max current = 100mA
Power Dissipation = 100mW

This is all been run off 12V DC supply.
So by my calculations (which may be wrong) it would require 1ohm, 1/4W resistors?
However with all the above been used the last LED in each series of three has blown.
Could anyone explain why or give advice please.

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frollard7 years ago
Can you confirm the 12v supply is actually 12 volts?

wall warts are notorious for putting out upwards of 50% extra voltage.
...that said I've never seen 100mA forward current on a single 5mm led - they always max out at 20-30ma each...

Either way; the specs are wrong, or your power supply is wrong.
goafer (author)  frollard7 years ago
Ok to add some more relevant info... All seperate series in parrallal are all individually resisted with for-mentioned 1ohm resistor. The power supply is running on average 11V (old PC PSU used for build & test purposes.) The LEDs are high power blue - available in UK from maplins, product N22FN. http://www.maplin.co.uk/module.aspx?moduleno=220752
I hope this may help diagnose further???
frollard goafer7 years ago
That spec sheet is VERY obfusicated - light output at 20ma...that would be the typical current; 100 is the absolute max, not the constant max.

The other thing to note, in visual brightness, they will quickly appear 'very bright' and quadrupling the current will only afford a very small gain (after about 20-25ma) - and the efficiency drops off quickly.
FTR: Max spec for If is usually pulse-mode
I suppose thats the case, still not a good idea to drive them at that level all the time, or it will be a sad sad 'splozion.
that's why they also specify tw max
lemonie7 years ago
"The last one in each" is just weird. This doesn't sound right.
Have you got them mounted on adequate heat-sinks, and can you post a picture / schematic? Frollard does right to question the power supply, but the last on in each is odd. You've tested the other ones and they still work - how?

L
goafer (author)  lemonie7 years ago
I have posted this once but adding at end of thread.....
Ok to add some more relevant info... All seperate series in parrallal are all individually resisted with for-mentioned 1ohm resistor. The power supply is running on average 11V (old PC PSU used for build & test purposes.) The LEDs are high power blue - available in UK from maplins, product N22FN. http://www.maplin.co.uk/module.aspx?moduleno=220752
I hope this may help diagnose further???
It still seems odd to be last one in each series!?!?!
 
lemonie goafer7 years ago
Hmm, you're running to the maximum values there, how do you measure the 11V average? For a PSU it should be regulated to 12V

L
seandogue7 years ago
Use the nominal VF for calculations, not the maximum spec. To be especially safe, use a value slightly lower than Vf nom for limit resistor calculation. Also, use a nominal value for the forward current, rather than the max spec.
For instance, based on what you've said, Vf nom is likely 3.6-3.7V..I'll side on safety and call it 3.6V

If nom for an led with max specs of 100mA is probably around 10-20mA. I'll use 10mA to calcualte the limit resistor

12- (3*3.6) = VR = 1.2V

1.2V @ 10mA results in

R = 1.2/0.01 = 120 ohm


In fact, ALWAYS use nominal specs for calculating normal operation. Max values are meant for another set of calculations, including worst-case analysis and certain specific functions which require their characterization by mfgs.
^^^ this series of suggestions == best answer

Lots of strange phenomenon can only be attributed to too much power; "play it safe" is best advice.
AndyGadget7 years ago
As Follard says, 100mA is awfully high for a 5mm LED.  Also, the forward voltage drop sounds too high.  Are these 'normal' LEDs or something fancier? What colour are they?
You can find the actual voltage drop by feeding a single LED with a resistor of  1Kohm or so (from a 12V supply) and measuring the voltage across the diode.

Just clarify the circuit for me - each string of the three series LEDs has it's own resistor, does it?  You haven't got any LEDs in parallel without a resistor?




josh10017 years ago
 according to what u wrote above the calculations are correct. I would use a 1 watt resistor and also place a 12 volt regulator I.C to regulate seeing as max voltage of the led is 4V. Did u place any leds in backwards,that could have blown the last row? BTW if u r using a rechargeable battery you shouldnt turn the leds on with out the regulator I.C in there because the voltage will be higher around 13.2 V or more.
Here is the 12 Voltage regulator KA7812