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# Current limiting resistor power rating question

#### DiodeDave

Dec 7, 2011
46
I have LED with a 100ma rating, 4.5v voltage drop and 12v power source.

The formulae I have been using call for a resistor of 82 ohms with 1w rating.

I have a 100 ohm 1w resistor in series with this LED and it gets too hot to hold.

What am I doing wrong?

#### BobK

Jan 5, 2010
7,682
No much. A 1W tesistor dissipating 1W will get that hot.

Bob

#### DiodeDave

Dec 7, 2011
46
Thanks Bob.

It's been working for an hour and everything is ok.

For kicks, I put paper towel on it. It's ok too.

#### Audioguru

Sep 24, 2016
3,639
Nobody makes an LED with a voltage of 4.5V. An LED has a range of voltages and yours might be 3.5V and the current is much higher than if it has a voltage of 4.5V or 5.5V.

12V supply, 4.5V LED and 100 ohms produces a current of (12V - 4.5V)/100 ohms= 75mA. The power in the resistor is 75mA squared x 100 ohms= 0.56W which will make a 1W resistor hot. If the LED voltage is actually 3.5V and the 100 ohm resistor is actually 95 ohms then the current is 89.5mA and the heating in the resistor is 0.76W and it will be very hot. Maybe the 12v is actually a car battery charging at 14.4V? You work it out.
Hot resistors must not be enclosed, they must be in free flowing air.

The rating of your LED might be its absolute maximum allowed current and if you do not cool it properly then it will fail soon.

#### DiodeDave

Dec 7, 2011
46
Thanks

The LED has been on for an hour and is still at room temp, or slightly higher.

I put my ammeter in series with the leads on the LED to get my 100ma rating.

The 12v is what I get with my meter across the leads of the 12v battery from my UPS.

The LED is from a flash light with 3 AAA batteries. I measured the voltage across the LED when connected to the 3 AAAs.

The flashlight was advertised as 15000 lumen on Ebay. Not sure if it is 15000, but it is very bright and leaves spots on my eyes when I look at it.

Thanks for all of the input.

#### BobK

Jan 5, 2010
7,682
The best LEDs out there get about 200 lm / Watt. If we believe your 4.5V at 100ma (which we do not, it you actually read the battery voltage when operating the LED it will be less than 4.5V,) then it is using 0.45W. At 15000lm that would be 33,333 lm / Watt!

Also 15000 lm would be as bright as 10 100W light bulbs.

I think you have the units wrong. 15000 mcd maybe?

Bob

#### DiodeDave

Dec 7, 2011
46
Thanks Bob

The flashlight was from Ebay, perhaps there was a translation problem. At the advertised 13000 lumen rate, it is the brightest flashlight I have seen, whatever the actual rating. If you look into the light, you have spots for quite a while!

At any rate, I left the LED connected to the 12v ups battery and the 1w 100 ohm resistor in series for 3 hours. For extra measure, I put a strip of paper towel on the resistor.

After 3 hours, no scorching on the paper towel. and the LED was cool and very bright.

I suspect the LED may have had a voltage rating of 3v or so. At 100ohms and 1w, I think I'm ok.

Thanks for all of your insight.

Dave

#### Audioguru

Sep 24, 2016
3,639
When the light is focussed into a narrow beam then on-axis it is much brighter. An ordinary 100W household bulb is not focussed into a narrow beam so it is not as bright.

You can look at a datasheet for a resistor to see its temperature at its power rating. Paper ignites at 232 degrees C and might scorch at 200 degrees C which will destroy an LED. Your skin is damaged at 60 degrees C.

#### FuZZ1L0G1C

Mar 25, 2014
366
The Vd=4.5 may possibly have it's own built-in current-limiting resistor, although after paging through my M****n catalogue, I didn't find any LED's with a 4V5 rating.
5V, 8V, and 12V are available in catalogue, all with internal Rs.
So the 3-cell flashlight may over-rate the LED to a safe 5V, or as BobK stated, working-load voltage may fall enough to drive lower-rated LED.
A typical U/B white LED is rated If=0.03 and Vd=3 (3volts 30mA).

#### Audioguru

Sep 24, 2016
3,639
My cheap Chinese flashlight has 3 AAA "Super Heavy Duty" carbon-zinc cells overloaded with 24 white LEDs in parallel. No current-limiting resistor was used because the cheap battery is awful. I replaced the cells with modern Western-made alkaline cells and WOW, the brightness is MUCH MORE. I might add a low-dropout voltage regulator.

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