Infrared LED current ? ( used at surveilance day-night vision camera )

Dear friends,



I´m designing a light source to drive a IR LED set.
In order to take some reference, I measured the spot like bellow :
**broken link removed**
However, I´m facing to some surprises :
1 ) Current regimen is continous, not pulsed ( ok, it seems wright, due it is used to movie, not picture ).
2 ) Current flowing throw LED array is only 4mA ( but, I see datasheets varying from 20mA to 100mA ).

Does it makes sense ?
Are that current value usual ?

+++

The IRED driver may be adjusting for ambient light. Test it in total darkness.

4mA sounds to low ..
I'd say the current should be somewhere between 20 and 30mA, although this may vary with type and manufacturer ..
:wink:
IanP

KJ6EAD said:

The IRED driver may be adjusting for ambient light. Test it in total darkness.

Click to expand...

I done.
I put a black tape upside LDR sensor, and in fact, noticed current increase to maximum allowable.
The circuit, basically consists - in Kirchoff language - to :
12V = I ( 360R + 5*LED ).
Measuring voltage across resistor, gives a continous ~1,5V ( viewed at scopemetter ) --> I = 1,5/360 = 4mA
It is according IR common drop voltage, that is ~1,9V

IanP said:

4mA sounds to low ..
I'd say the current should be somewhere between 20 and 30mA, although this may vary with type and manufacturer ..
:wink:
IanP

Click to expand...

I also believe that is not high.
I found datasheet of a lot of IR LED with If=100mA !
However, due that illuminator maybe is designed not only to indoor temperature, but also consider tropical enviroments ( where I live ), could have been applyed the derating rule :
**broken link removed**
Note that :
>> At 75oC, temperature limit decreases to half of nominal rated.
>> Confined at Dome, accumulated heatting, could not dissipate, quicklly growing.

So, in this case, the 20mA you gave before, would decay to 10mA.
The fact, is that measured value is not what I expected find, but I´m not sure if it was scaled excessivelly careful.


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Be aware also that there are two types of IR LED, the ones used for remote control purposes are rated for high average current although in reality they tend to carry short very high current pulses with long gaps between them. Some IR remotes pulse at several hundred mA.

The other types are the near-IR ones used for surveilence lighting, these are generally shorted wavelength and can be seen as a deep red glow in the dark. They are more like 'super bright' LEDs with an IR phosphor based emitting dye in them which lets them run at much lower current.

I have a camera almost identical to the one in your picture, it works on a 12V supply and has 30 LEDs in it. I will measure the current it draws today (it's a bright morning here) and again after it has gone dark tonight, I'll report the difference in current to you tomorrow. It also has an LDR to shut the LEDs off when it's daytime so the difference in current / 30 should tell the LED currents.

Brian.

betwixt said:

...I have a camera almost identical to the one in your picture, it works on a 12V supply and has 30 LEDs in it. I will measure the current it draws today...

Click to expand...

Hi betwixt,

I thanks a lot in advance, my friend...
Just remarking : I think you can measure anytime the equivalent current on dark, just occluding LDR, that´s not ?


+++

You are correct but the camera is mounted 5 metres up a wall and I'm only 1.75 m tall. :-D

I can only measure at the power feed point which is inside a nearby building. It's still daylight here but I will measure later for you.

Brian.

Ohhh....sorry.
I done in Lab...:smile:

I have taken the measurement. Suprisingly, the current only increasd by 65mA when the LEDs turned on which implies a little over 2mA per LED. This seems too low and I suspect some other circuitry in the camera may be hiding the true LED current. Perhaps the lower 'shutter' speed as the CCD gathers more light also reduces the consumption of the rest of the camera circuits.

Brian.

betwixt,


I also measured a small value, 4mA.
However, I got it directly measuring drop voltage on series resistor.
The conducting regimen was not pulsed, but continous

Maybe there is some trick we don´t know.
It don´t make sense some value so low.

I´m suspecting that is not required too much IR intensity per LED, and pehaps the big amount of components is just to guarantee the geometry of equal distribution of light around camera.


+++

I think you are correct. A single white LED running at 10mA gives a much light as the combined IR array on the camera but over a much smaller angle. They are each running at low current but the combined light makes them bright enough and has a wide enough dispersion angle to illuminate the whole capture area. The speed at which the CCD operates is slower at low light levels so perhaps it uses less current and this offsets some of the LED current measurement.

Brian.

The IRleds use very sharp high current pulses with average energy levels very low .
The 100mA mentioned is a sharp peak current with <microsec width .
Use an oscilloscope to see this .The average current in daytime is thru PWM regulated supply.
During dark ,when IR from the sun is low , the current pulsewidth is higher .As such you can actually see the 65mA current .
This must actually have doubled from the earlier current values ..

vimalkhanna said:

The IRleds use very sharp high current pulses with average energy levels very low ...
Use an oscilloscope to see this...

Click to expand...

I did that, however I found the driving current regimen is continous, not pulsed, at least at the light-source I have in hand.

vimalkhanna said:

The average current in daytime is thru PWM regulated supply...

Click to expand...

The only control is linear performed directly by a LDR controlling a BJT base current. No kind of pulses were detected.
There is not at device some component working on switching behaviour, like an PWM.
Power supply used is an 12V automotive battery.

vimalkhanna said:

The 100mA mentioned is a sharp peak current with <microsec width

Click to expand...

Note at attched chart, that is possible to drive some LED with a current higher than rated to continous conduction.
https://www.edaboard.com/threads/220363/#post937213

+++

Usually IR LEDs are driven at 10%duty cycle so they get enough cooling time in the OFF mode .In case of overload deriven types , the ON time is reduced to 5% .

@vimalkhanna, in this application they are used for illuminating the area in front of a camera, they must be used in continuous mode or it would result in strobing as the pulses clashed with the CCD scan rate.
I still suspect the LEDs are run at low steady current and the small difference in camera current between day and night is not as straight forward as just the LEDs turning on. For example, at night the shutter rate is reduced and the video levels are generally lower, both result in less consumption. These do not seem to be the same kind of IR LEDs as used in remote control units, if they were, I would agree with your low duty cycle information. They seem to be shorter wavelength than remote control types and they can actually be seen glowing deep red when it's dark.

I tried an experiment with a high quality camcorder in night vision mode, I placed it beside the surveilence camera in the dark. The surveilence camera gave a good picture, the camcorder couldn't see a thing!

Brian.