InfraRed Led Diodes - how to build IR illuminator, few questions...

[x-man]

Hi,
I want to build IR illuminator for my cctv camera and on ebay I have ordered this diodes:
**broken link removed**

but on that page I have spec for diodes:
Voltage: 1.5V~1.6V
Current: 20-25 mA
Peak(pulse): 700mA
Power: 200mW

If I good know, if I need 1.5v for this diode and if this diode using 25mA than power can`t be 200mW??!!! Somebody can tell me more about this? For me look like here is something bad...also, this is 10mm diode, why using only 25mA and some other IR 5mm diodes using 100mA?

Also, if this diode need 1.5v and if I have 12v power supply, can I use 8 diodes in series without resistor? I don`t need resistor in this case?

Thanks.

 

[crutschow]

If they are rated at 25mA, then the power dissipated is about 1.5 * 25mA or 32.5mW, not 200mW. And the radiated light energy is much less than that.

For the umpteenth time in these forums, yes, you need a resistor (or some other means) to limit the current through the LEDs no matter many are in series. LEDs are current operated devices, not voltage. You can use them in series but you still need a resistor.

 

[x-man]

But if I have 8 diodes in series, every diode need 1.5v and that is 12v and if I have power supply 12v, can you tell me how I can choose resistor? Ohms, watts...?
Thanks.

 

[ericgibbs]

But if I have 8 diodes in series, every diode need 1.5v and that is 12v and if I have power supply 12v, can you tell me how I can choose resistor? Ohms, watts...?
Thanks.

For a simple circuit you cannot drive 8, 1.5/1.6Vdp IR LED's in series from a supply of 12V.

The easy way is to have two sets of 4 LED's, each 4 would have a current limiting resistor.

So , if they are 1.5V at 25mA, thats 6V drop, so you have to drop the remaining 6V by using a resistor. The resistor would be 6V/0.025 =240R

The most efficient way would be to use only 7 LEDs in series, thats 7 * 1.5 = 10.5V and drop 1.5V across a series resistor, the resistor is 1.5V/0.025 = 60R

 

[x-man]

Why I can`t drive 8 led diodes from 12v power supply if every diode need 1.5v?
Also, like you can see in my first post, specification for this diodes is not good, cheap chinese diodes and I can`t know real spec, they said:
Voltage: 1.5V~1.6V
Current: 20-25 mA
Peak(pulse): 700mA
Power: 200mW

But how can be 200mw with voltage 1.5V~1.6V and current 20-25 mA?!
Also, on ebay don`t have many sellers for 10mm IR diodes, I don`t know why...

 

[ericgibbs]

Why I can`t drive 8 led diodes from 12v power supply if every diode need 1.5v?
Also, like you can see in my first post, specification for this diodes is not good, cheap chinese diodes and I can`t know real spec, they said:
Voltage: 1.5V~1.6V
Current: 20-25 mA
Peak(pulse): 700mA
Power: 200mW

But how can be 200mw with voltage 1.5V~1.6V and current 20-25 mA?!
Also, on ebay don`t have many sellers for 10mm IR diodes, I don`t know why...

The power error was explained by 'crutschow'.
I suspect they eBay seller has quoted the pulse peak power, NOT the continuous power.

If you think you can drive 8 LED's in series from a 12V supply , why don't you try and see.???

 

[x-man]

Yes, I already do that and that working but I want to do it at best possible way, I need all possible power (IR) from this diodes and I don`t need burned led diodes...you know what I mean...

 

[ericgibbs]

Yes, I already do that and that working but I want to do it at best possible way, I need all possible power (IR) from this diodes and I don`t need burned led diodes...you know what I mean...

With LED's its important that the current thru the LED be limited, else it will burn out.
The easiest way to do this is by having a series current limiting resistor.

There is a chance in certain conditions that the 12V to 8 LED's without current limiting could cause the LED's to blow.

The maximum constant current for your LED's is only 20 to 25mA

 

[audioguru]

Think the forward voltage of LEDs decreases as they heat up. If you connect eight 1.5V LEDs in series and without a current-limiting resistor to 12V then their current will keep increasing which heats them more which reduces their forward voltage which increases the current which increases their heating .... then they all burn out!

 

[MrAl]

Why I can`t drive 8 led diodes from 12v power supply if every diode need 1.5v?
Also, like you can see in my first post, specification for this diodes is not good, cheap chinese diodes and I can`t know real spec, they said:
Voltage: 1.5V~1.6V
Current: 20-25 mA
Peak(pulse): 700mA
Power: 200mW

But how can be 200mw with voltage 1.5V~1.6V and current 20-25 mA?!
Also, on ebay don`t have many sellers for 10mm IR diodes, I don`t know why...

Hi there,

I thought i might help clear up the voltage paradox problem a little here.

As others have already mentioned, the voltage of the 'diode' can change without you knowing it, and while they are running. You set them up to run and measure their voltage and it might be 1.500 volts, but once they run for a while and after a few months the voltage measured might be 1.400v or something like that.

This is a common wonder when people first encounter LEDs. They read that the LED voltage is like 1.5v, and it seems that they will always run at 1.5v. I think this is because we are previously so conditioned to think in terms of light bulbs which have filaments, and those devices usually require a fixed voltage. LEDs are not light bulbs though, and they need attention to current more than voltage. So you see we can not really say truthfully that an LED 'needs' 1.5v, we have to instead say that the LED 'needs' 20ma. Even though the data sheet will mention voltage as well as current, it's the current that we have to pay most attention to very unlike filament bulbs. Yes, the LED needs *at least* 1.5 volts to operate, but that's just a very rough estimate of what the voltage will be when it is first run up and after several hours of running and after several weeks or months of running when it may change to something else. Thus, our circuit must make sure that the current level, not the voltage level, stays constant for the life of the LED, even with the voltage changes i talked about above.

The basic rule for a filament light bulb is: "Let the current change to whatever it wants to, but keep the voltage as constant as possible".

The basic rule for an LED is: "Let the voltage change to whatever it wants to change to, but keep the current as constant as possible".

The reason we usually use a resistor in series with the LED is because that is the simplest way to limit current to a safe level during the entire life of the LED. There are better ways, but a resistor is the simplest and cheapest. The idea is to make sure you have enough voltage for the LEDs plus a little more, and use a series resistor to drop that little more. The result is an LED that can run for years.

 

[x-man]

Okay, I understand it but can you tell me how resistor control current to diodes, if I using resistor for 12v and 8 diodes (1.5v per diode) and if for some reason diodes need only 1.4v, with 12v constant power supply that will generate more current per diode...if I good understand all resistor will use this extra 0.8v and on that way current will be normal per diode?

Also, now I using 10mm large IR led diodes and they need only 25mA but now I see some 5mm led diodes that need 60mA per diode, this small diodes need more current than this large, is this mean that this small diodes will generate more IR light than this large?

 

[audioguru]

A good and powerful IR LED is rated for 60mA to 100mA.
But the very cheap ones you bought are rated for only 20mA to 25mA so they will be dim and might have a very narrow beam angle.

If you use six LEDs in series and they are all 1.4V then a 150 ohm current-limiting resistor limits the current to 24mV.
If the six LEDs are all 1.5V then the 150 ohm resistor limits the current to 20mA.

Then do the simple calculations with more LEDs and less resistance. If the LEDs are all 1.4V the current will be high and if they are 1.5V the current will be so low they are useless.

 

[x-man]

Yes, now I see it, now I searching for high power IR leds, maybe somebody know where I can buy high power IR leds online? Will be good sizes 8-10mm...thanks.

 

[OlPhart]

I think there's a "sweet spot" for current limit value: less than 50 ohms starts making voltage fluctuations show as higher current variation, greater than 100 you start giving away power without benefit.

This is more opinion than fact, but a colleague used to design to 20 ohms, sometimes less. He periodically groused about burning a string, while my guideline hasn't caused me grief. <<<)))

 

[MrAl]

Hi,

Yes, that optimum point is about half way between low extreme and high extreme. So for a target design of 20ma we would accept 15ma as a low and 25ma as a high, approximately.

 

[Jaguarjoe]

Here is the V-I curve for a diode (like and LED is a diode):

https://upload.wikimedia.org/wikipedia/commons/a/a5/Diode-IV-Curve.svg

Look what happens as the voltage is increased along the "X" axis in the positive (right) direction. At the start, no current flows ( up on the "Y" axis) and it doesn't flow for a long time until you approach the forward voltage of the diode (1.5v for your LED). Now look at the current and how rapidly it rises with just a tiny increase in voltage. Without something to limit the current to the diode while running at a constant supply voltage, it will burn up as soon as the forward voltage of the LED drops even a little bit.