how to identify max forward current in an infrared LED with no spec sheet

[t101]

Hello,

I have found myself without a spec sheet but a supply of working 940nm IRs. I have been able to identify the forward voltage (1.25v) and for now have chosen a low value resistor (270 ohms). From this I can determine the current through the LED. I would like to know the LEDs maximum forward amperage though rather than experimenting with the LEDs and destroying a few. The second question is what % of that maximum forward current should be its everyday use e.g. 70%?

Thank you.

 

[Martaine2005]

I would say 20mA max. 10-15mA would obviously prolong the life of the IR LED.


Martin

 

[Alec_t]

Welcome to EP!

I would like to know the LEDs maximum forward amperage though rather than experimenting with the LEDs and destroying a few.

If you can't identify the LED type I think experimenting to see the maximum current before a sacrificial LED fails is the best way. Then run the LEDs at, say, 50% of the maximum. Alternatively, you could assume the power handling ability would be similar to that of a known spec LED of similar physical size/format.
Will the LEDs be used in pulsed mode or continuous mode?

 

[t101]

continuous

 

[bertus]

Hello,

IR leds can have currents upto 100 mA.
The TSAL6200 and TSAL6400 are some examples.

Bertus

 

[CircutScoper]

Hello,

I have found myself without a spec sheet but a supply of working 940nm IRs. I have been able to identify the forward voltage (1.25v) and for now have chosen a low value resistor (270 ohms). From this I can determine the current through the LED. I would like to know the LEDs maximum forward amperage though rather than experimenting with the LEDs and destroying a few. The second question is what % of that maximum forward current should be its everyday use e.g. 70%?

Thank you.

This might work.

1. The voltage drop across an LED, just like most diodes, is junction temperature dependent, decreasing by ~2mV per degree C.
2. Maximum rated LED junction operating temperature is generally around 100C.
3. Junction temperature rise is (approximately) proportional to current.

So...

You could apply a moderate test current (e.g., 20mA) to one of your LEDs and observe the voltage change as it warms up. After giving it a minute or so for the temperature to stabilize, divide the observed voltage change from cold to hot by 0.002V/C to get the approximate temperature rise for the test current. For example, if the voltage dropped from 1.25V (cold) to 1.2V (hot), the calculated temperature rise would be 1.25 - 1.2 = 0.05V and the corresponding temperature rise 0.05V / 0.002V/C = 25C.

Then divide the test current by the calculated temperature rise to get Amps/degree. For the example, that would be 0.02A / 25C = 0.0008A/C

Then choose a maximum junction temperature rise, e.g. 75C for 100C junction temperature in a 25C ambient, to compute max operating current. In this case 0.0008A/C x 75C = 0.06A = 60mA.

 

[hevans1944]

Carefully perform the non-destructive test described by @CircuitScoper in post #6. Repeat the test procedure on several other IR LEDs to verify the correlation between test current and junction temperature. Once you are satisfied your results are typical for several LEDs, decide how close to 100C junction temperature you want to operate, keeping in mind that the results also depend on the ambient temperature. It's up to you, but I would not operate closer than 90C junction temperature, and perhaps much less if you want to extend the life of the IR LED.

 

[t101]

Welcome to EP!

If you can't identify the LED type I think experimenting to see the maximum current before a sacrificial LED fails is the best way. Then run the LEDs at, say, 50% of the maximum. Alternatively, you could assume the power handling ability would be similar to that of a known spec LED of similar physical size/format.
Will the LEDs be used in pulsed mode or continuous mode?

This seems to be the most simple way to achieve. For now I have plenty of LEDs. One maybe 2 won't pause the project. If that fails I can try some of the other answers here.

 

[t101]

Welcome to EP!

If you can't identify the LED type I think experimenting to see the maximum current before a sacrificial LED fails is the best way. Then run the LEDs at, say, 50% of the maximum. Alternatively, you could assume the power handling ability would be similar to that of a known spec LED of similar physical size/format.
Will the LEDs be used in pulsed mode or continuous mode?

So I ended up just measuring the millimamps for the IR diode itself and it equaled 20 ma. Cross checking V=IR,
=.02A x 750 ohms = precisely 15V! Awesome.

My last question what is the target milliamps necessary to use? Would it be 80% of the 20ma, or just the 20ma?

 

[Alec_t]

I'd go with 80%, or even less. It's never a good idea to run components at their maximum rating if you want them to have a long and happy life.