resistor help

buntay

Senior Member
#1
Hello collective,

I need a bit of help, I have a 10mm white led that was prewired for 12v but when I apply 12v at 200mA the resistor gets hot.
Almost enough to melt the solder holding it to the LED.
So, I am pretty sure I need a different resistor to make these work without meting them down.
As for specs on the LED?? I wish I could provide that, however, they were bought bulk from e-bay with no specs.
Does anybody have any ideas other than just hooking up random resistors?

Any help, as always, will be greatly appreciated

Buntay
 

premelec

Senior Member
#2
More information is needed... what do you mean prewired for 12 volts? is there a resistor in series with LED that came with it? Square of the current times the resistance is the formula for power that resistor is dissipating. You need to determine the actual supply voltage, actual LED voltage and what current you want in the LED... with that you can calculate resistor value and wattage size... or you can run from lower supply voltage... ;-0
 
#3
A single white LED element will drop about 3.5v when operating normally. So two LED elements in series will drop around 7 volts. You'll have the do the homework here: put a relatively large resistor in series with the LED (eg 50 to 150 ohms) to reduce the current and measure the voltage across the actual LED's leads. You should see a multiple of around 3.5v, telling you how many LEDs there are within the 10mm package. From that, you can deduce how much voltage the resistor/s have to drop. Then, calculate the power dissipation required from the series resistor.

Resistance (in ohms) = Volt drop / Current (in amps)
Resistor Power (Watts) = Current squared x Resistance

Since you are already working with a component that appears to be badly designed or badly specified, the LED may destroy itself with a current of 200mA.
 

buntay

Senior Member
#7
I am currently doing just that, however, I am using them in conjunction with a LDR as a sensor spanning across 24" and at 5v I only have a b0 variable of 0-3 due to the dimness but with 12v its between 25-30. Since the readADC on an 08m2 command seems to bounces that much its not very reliable. Maybe I should look at another light source :( but I liked the working life of LEDs since they are always on.
 

goom

Senior Member
#8
I suspect that these are mislabeled as being 12V, and are more likely designed for 5V.
They look like "regular" white LED's with a resistor in parallel buried in the heat shrink tubing. The facts that you measured 200mA at 12V, the specified current is 20mA and the resistor gets very hot strongly suggests that they are not designed for 12V operation.
If you measure the value of the built-in resistor, I suspect that it will be around 80 Ohms. It would need to be more like 460 Ohms for 12V.

Putting 200mA through them will certainly cause permanent damage and failure in a short time.
 

AllyCat

Senior Member
#9
Hi,

+1 to the above; there is obviously something "wrong" in the data. You could connect one of the LEDs directly across 12 volts and see how long it continues to shine (but make sure there is nothing flammable nearby). :) Most likely it will be much less than 50,000 hours, probably with the series resistor failing first !

As a solution to your application, you could connect three of the LEDs in series across a 12 volt rail, or maybe 5 across 24 volts and combine their light output. Also, using READADC10 will give a "b0" value 4 times larger and the FVR2048 as the reference for the ADC another 2.5 times. That could get you up to a "b0" well into double figures and maybe over 100 with 5 LEDs.
Code:
#picaxe 08m2
    fvrsetup FVR2048      ; set FVR as 2.048V
    adcconfig %011        ; set FVR as ADC Vref+
    readadc10  c.x , b0    ; select input as required
Cheers, Alan.
 

Flenser

Senior Member
#11
Buntay,

Can you give us the colour bands on the resistor? We can then do the calculations ourselves to confirm what is wrong with your prewired LED,
 

lbenson

Senior Member
#12
Love this from the "Product Details":

"Legal Disclaimer: (for our protection) We will not be held responsible or accountable for any damages or injuries that result from the use of this product. We make no guarantee as to the longevity, safety, or functionality of this product or any other products that we sell. You are buying, using and possessing this item at your own risk."

So much for fitness for purpose.
 

hippy

Technical Support
Staff member
#13
At the other extreme I was recently playing with high power, 75W @ 12V, multi-LED modules which I guess are replacements for bulbs in cars, similar and LCD backlights. No idea how they are wired internally and they do seem to draw draw amps at rated voltage.

What surprised me was, when connected direct to a bench PSU, run at lower voltages they gave out an incredible amount of light while drawing amazingly little current, mere milliamps.
 

lbenson

Senior Member
#14
At the other extreme I was recently playing with high power, 75W @ 12V, multi-LED modules which I guess are replacements for bulbs in cars, similar and LCD backlights. No idea how they are wired internally and they do seem to draw draw amps at rated voltage.

What surprised me was, when connected direct to a bench PSU, run at lower voltages they gave out an incredible amount of light while drawing amazingly little current, mere milliamps.
Can you provide a link?
 

premelec

Senior Member
#15
Seoul Semi used to make some 115 volt LEDs - it's possible - and I plugged in "LED Headlight" in Ebay and there are thousands of 'blind you' auto LED headlight modules for not much money. CREE has made a lot of very bright XML units I have in small brilliant flashlights... These days small down converters which can regulate current efficiently are in supply.
Sorry so OT... cool morning in Colo.
 

hippy

Technical Support
Staff member
#16
Can you provide a link?
Not really. They were observation as I wired things up, turned the PSU voltage up and down. The small ones I have are LED-G4X75W and look similar to this, two COB on four sides, two under lens -

https://images-na.ssl-images-amazon.com/images/I/31VroR5doiL.jpg

8.0V = 1mA
8.5V = 17mA
9.0V = 60mA
9.5V = 115mA
10.0V = 170mA
10.5V = 220mA
11.0V = 315mA
11.5V = 280mA
12.0V = 260mA

Starts illuminating around 7.3V, and current peaks at 11.2V, then starts decreasing. Not sure why.

Physically bigger ones, LED-P13SS54W, with a typical automotive plug socket have 12 COB on four sides, six under lens -

https://i.ebayimg.com/images/g/-yYAAOSwLJ9Z6B4f/s-l300.jpg

8.0V = 2ma
8.5V = 9mA
9.0V = 40mA
9.5V = 90mA
10.0V = 140mA

The really big ones I can't find at present.
 

buntay

Senior Member
#17
Thank you all for the input, it actually gave me an idea. So last night I went to my local Walmart and just bought the brightest LED penlight I could find. Since it takes 3 AAA batteries it works just fine using 5v and is bright as all heck, I am actually hitting 150-155 on my b0 ADC input. I am just using the head so it works out nice.

Thanks for all the help and ideas :)
Onward and upword

Buntay
 
#18
At the other extreme I was recently playing with high power, 75W @ 12V, multi-LED modules which I guess are replacements for bulbs in cars, similar and LCD backlights. No idea how they are wired internally and they do seem to draw draw amps at rated voltage.

What surprised me was, when connected direct to a bench PSU, run at lower voltages they gave out an incredible amount of light while drawing amazingly little current, mere milliamps.
@hippy, I had a problem with some 10W red LEDs that were supplied to me a couple of years ago. The problem was resolved but I had to supply the attached image to explain the difference (One would work at around 6v Ie 3 x 1.8v while the other needed >17v).
Comparison-1.jpg
"12v" white LEDS of any size usually have one or more parallel chains of 3 x 3.5v white elements. BTW, a 75W LED lamp would be equivalent to a 350W halogen!
 

hippy

Technical Support
Staff member
#20
I assumed (after some searching) that a "75W-equivalent" was what was meant.
Could well be. I just happened across that stated rating during a web search trying to discover what I had. Halogen headlamp high-beams seem to be classed around "65W" and that seemed to match with what they appeared to be replacements for.

I guess they are labelled with the wattage of what they would be replacing just so it's easier for people who have no idea of what lumens are all about.
 
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