How to know the cut off voltage of IR LED

[sisso]

Hello Everyone,

I have 3 Infra-red LEDs and I want to use them simultaneously. I purchased these from a

local market, so specifications of LED are not available. I find out the resistance of a LED

by multi-meter it came out to be 1.7 K ohm. I connected one with 9V duracell and it got

burnt with a pungent smell Please tell me how could I do that with a 9V battery? Only

thing I know is the forward resistance i.e. 1.7 K Ohm. Would I need more LEDs or resistors

to do that?

Here is one LED : **broken link removed**
Please tell me...........

 

[Diver300]

Well the burnt one probably doesn't work any more.

If you have a multimeter, you need one with a diode voltage range.

Alternatively, supply the LED with a battery in series with a resistor. A 9 V battery and a 1k resistor would be fine. Then measure the voltage across the LED.

You can check that the LED is working with most digital cameras, including video and phone cameras.

 

[alec_t]

If you have a multimeter, you need one with a diode voltage range.

Be aware, the 'diode test' setting on many/most multimeters merely gives a conductivity measure. It doesn't give a forward voltage drop measure.

supply the LED with a battery in series with a resistor. A 9 V battery and a 1k resistor would be fine. Then measure the voltage across the LED.

I agree with that, except that I'd use a 10k resistor to limit reverse breakdown current if the diode gets connected the wrong way round. The measured voltage should be > 2V and < 4V if the diode is ok and correctly polarised.

 

[Diver300]

Be aware, the 'diode test' setting on many/most multimeters merely gives a conductivity measure. It doesn't give a forward voltage drop measure.

I agree with that, except that I'd use a 10k resistor to limit reverse breakdown current if the diode gets connected the wrong way round. The measured voltage should be > 2V and < 4V if the diode is ok and correctly polarised.

With an Infra-red LED the forward voltage is more like 1 V

I would expect 0.8 - 1.6 V with a 9 V battery and a 10 k resistor.

https://en.wikipedia.org/wiki/Led has a chart for the different colour LEDs and Infra-red are the lowest voltage.

I've just tested a red LED at 1.6 V and an infra-red LED at 1.0 V

The infra-red LED showed up clearly on one webcam, and was just detectable on another webcam.

 

[alec_t]

With an Infra-red LED the forward voltage is more like 1 V

My mistake; I was thinking of a normal red LED.

 

[unclejed613]

begin with a rather safe guess that an LED has a maximum forward current of 25mA (this covers most run-of-the-mill LEDs, some are higher, a few older types (mid 1970's vintage) are lower). build a current source that sources 15 to 20mA... better still make it adjustable from 5 to 20 or 25mA. if you build the variable one, you can calibrate the circuit with your multimeter (in current or milliamp mode), so you know where to set the potentiometer for various currents. then change back to reading voltage and insert the LED, and measure the forward voltage at various current settings. if you have a photometer, you can also measure luminosity at various current levels.

shown below is a variable current source for testing LEDs. there are two diode voltage drops applied to the base of Q1. one of those drops is equaled by the forward diode drop across the base-emitter junction of Q1, with the remaining diode drop across the emitter resistor (VR1+R2). the resistors provide negative (or degenerative) feedback when collector current is flowing, maintaining a nearly constant collector current. the collector current is a tiny bit less than 0.7/(VR1+R2). R2 provides a limit of current of about 30mA when VR1 is zero ohms, VR1 provides the ability to adjust the current from about 5mA to the 30mA limit. do not use tiny cermet pots for VR1, use a "normal" size one, typical of a balance control or a motor speed controller. it should also be linear taper.

 

[sisso]

done, thanks all