Direct Drive LED and CR123A Current?

I got an LED, rated at:

2.2 Vf
20 mA

This guy here (model # B4304H95):

http://www.linrose.com/leds.html

http://www.linrose.com/images/led19.jpg

I am direct driving it from a 1xCR123A (3volt) light.

So far it is working just fine. Hasn't gone yet.

Question, how do I know if the LED is really drawing 20 mA from the battery? Could it be drawing more or less?

The LED does not have any resistance built-in, as far as I know. So, does the LED determine how much current to draw form the battery or is the battery determining this and giving the LED the max current it can give?

Thanks.

It's probably drawing dangerously high amounts of current, you are seriously overdriving that LED. You might consider getting yourself a cheap ammeter.

I measured the current using a multi-meter placed in-line across the battery and LED.

It's reading 50 mA.

But I also measured the voltage and it's at 1.7 volts through the LED.

So the LED is rated at 2.2Vf 20mA = 44 milliwatts.

I am reading 1.7 volts 50 mA = 85 milliwatts.

More current and less voltage equals the same wattage? Roughly, my crude calculations tell me I am driving the LED at twice the rated wattage.

Does this all sound right?

Could the extra wattage be "inefficiencies" in the circuit? Power is lost along the way?

hello ltiu,

The LED rating is *typical* it may be lower or higher. 1.7v through the LED? Your current measurement method may introduce extra resistance in the circuit. The long leads and shunt resistor in your multimeter can be extra 50-200miliohm, that limits the current flowing through the LED.

In your case, "inefficiencies" lies in the internal resistance of the battery and the LED die and vf. Overdriving a LED may increase the forward voltage and reduce output quickly, it happens to white led but I am not sure about other colors.

your going to need to install a current drop resistor in there with the LED

Resistance = (Vin-Vdrop)/(desired current)
something like a 47 ohm, 1/4 watt resistor oughta do you fine

Illum_the_nation said:

something like a 47 ohm, 1/4 watt resistor oughta do you fine

Click to expand...

Yes, 1/4 watt resistor. That is what the instructions in the packaging says. But I did not install any resistor and so far the LED has not gone yet

its generally recommended to do so, which cuts down the heat outputted from the LED.

on the long run it might burn the LED out prematurely, but hey...there a buck a piece and comes with 2xcr2016 cells

since your running this on cr123A, do whatever you can to keep the contacts apart!

Illum_the_nation said:

since your running this on cr123A, do whatever you can to keep the contacts apart!

Click to expand...

I do not understand, keep the contacts apart? Which contacts, the LED or the battery?

Of cource the contacts are apart until I turn the light on ... did I get you right?

He means to keep the two leads of LED apart, so you don't short out the cr123a battery.

jzmtl said:

He means to keep the two leads of LED apart, so you don't short out the cr123a battery.

Click to expand...

OK, it's a bi-pin socket meant for an incan (the light is originally an incan that I modded to an LED) so the LED pins are always apart.

Thanks.

:thanks: jzmtl for the clarification, by contacts I mean that of the LED, which if shorted to each other and connected to the battery theres going to be a tremendous amount of current rushing through it. the cr123A, assuming the PTC protection is in place, will cause a thermal shutdown to take place, if there is no protection or in the scenario of a PTC failure then your going to be looking at a "lithium thermal runaway reaction" (my fancy way of explaining the 'vent with flame')

Illum_the_nation said:

:thanks: jzmtl for the clarification, by contacts I mean that of the LED, which if shorted to each other and connected to the battery theres going to be a tremendous amount of current rushing through it. the cr123A, assuming the PTC protection is in place, will cause a thermal shutdown to take place, if there is no protection or in the scenario of a PTC failure then your going to be looking at a "lithium thermal runaway reaction" (my fancy way of explaining the 'vent with flame')

Click to expand...

Yes, make sure not to give the CR123A a path of zero resistance.

Hmmm, I'm not really educated when it comes to lights and batteries. The fact that I have a cheap incan light and the original incan in there would glow dim with a used battery. I assumed the light is in direct drive with no fancy chip or resistor in-between.

The fact that the red LED is glowing red just fine at 50 mA and has not burned out. With 4 days of about 4 hours of use total and the CR123A battery has not fried. Does that tell me the light may not be direct drive after all? Or is the LED acting as it's own resistor and limiting the current and protecting itself and the battery.

direct drive damages the LED, but unless your overloading it with too high of a voltage [potential difference greater than 2V] its not likely to see or feel the difference [since the power dissipated from a typical 5mm LED aren't as hot as luxeons] and by the time you see a color change/dramatic increase in heat the LED's probably toast

some manufacturers estimate 5mm LEDs to have the life of 100,000 hours...I highly doubt the accuracy of that statement. I'm more inclined to believe they are around 10,000 hours before you notice any difference. even if you lose 1000 hours because of overdriving, I wouldn't really care as 9000 hours is a very long life by itself and they are what? less than $1/LED?
A LED is a diode, diode are resistors two separate components. there are LEDs that have resistors built in, hence 6V 5mm LEDs, 12V 5mm LEDs can sometimes be found.

Most LEDs die because of high temperatures accumulating in or near the emitter. the long contacts they often come in act as heatsinks, if the LED in operation was to be mounted through-hole/flush with the PCB board, then observe the recommended current input. But if the stems are uncut, the LED can take slightly higher current without damage.
I recommend for input current [for 5mm LEDs] to be around 20-50ma, voltage around +/- 0.5V of the recommended input. anything upwards of that all bets are off as to how long the LED will last as some LEDs are more sensitive than others:nana:

Illum_the_nation said:

direct drive damages the LED, but unless your overloading it with too high of a voltage [potential difference greater than 2V] its not likely to see or feel the difference [since the power dissipated from a typical 5mm LED aren't as hot as luxeons] and by the time you see a color change/dramatic increase in heat the LED's probably toast

Click to expand...

The LED is rated at 2.2 Vf. I am loading it with a 3v CR123A.

some manufacturers estimate 5mm LEDs to have the life of 100,000 hours...I highly doubt the accuracy of that statement. I'm more inclined to believe they are around 10,000 hours before you notice any difference. even if you lose 1000 hours because of overdriving, I wouldn't really care as 9000 hours is a very long life by itself and they are what? less than $1/LED?

Click to expand...

$3.79 per LED.

If the LED lives longer than an incan, then it is worth it. The replacement incan for the light costs something like 6 bucks!

A LED is a diode, diode are resistors two separate components. there are LEDs that have resistors built in, hence 6V 5mm LEDs, 12V 5mm LEDs can sometimes be found.

Most LEDs die because of high temperatures accumulating in or near the emitter. the long contacts they often come in act as heatsinks, if the LED in operation was to be mounted through-hole/flush with the PCB board, then observe the recommended current input. But if the stems are uncut, the LED can take slightly higher current without damage.
I recommend for input current [for 5mm LEDs] to be around 20-50ma, voltage around +/- 0.5V of the recommended input. anything upwards of that all bets are off as to how long the LED will last as some LEDs are more sensitive than others:nana:

Click to expand...

I measured 50 mA. But input is about 0.8V over.

I had to cut the stems to make it fit the socket and have it at the bottom of the reflector. The stems are seated in a bi-pin socket with (I am assuming) lot's of copper all the way to the battery and the aluminum body.

It's still going ...

then it shouldn't be a problem. a new cr123A is 3-3.3V, even at 3.5V theres just a little over 1 V

$4/LED?!
I think I bought my stash of 1000 5mm whites [erm...was 1000 a year ago] for $50 somewhere online, I've built them into LED blocks for shed lighting and so far so good

do a little reading on your own when you have the time, its written by a senior design engineer thats now retired from CPF
Effects of overdrive on 5mm LEDs

Illum_the_nation said:

$4/LED?!
I think I bought my stash of 1000 5mm whites [erm...was 1000 a year ago] for $50 somewhere online, I've built them into LED blocks for shed lighting and so far so good

Click to expand...

What is the mcd and viewing angle rating of those?

ltiu said:

What is the mcd and viewing angle rating of those?

Click to expand...

unknown, a friend ordered them for me off DX [I think :thinking: ]some time ago when I informed him that white 5mms cost $5 each in stores

OK, the reason I asked is I could get 15 LEDs for $3 but these were something like 8 mcd or less. Very dim!

The one I got in my flashlight is 10,000 mcd.

Illum_the_nation said:

your going to need to install a current drop resistor in there with the LED

Resistance = (Vin-Vout)/(desired current)
something like a 47 ohm, 1/4 watt resistor oughta do you fine

Click to expand...

Vin = 3
Vout = ???
current = 20

...and to think I made it through calculus at some point in my life.

-Winston

sorry. double post.