white led

[audioguru2]

Hello AG
             One more doubt before connecting the load, should the 3.9 Ohms resistor also be 2 watt ?

The 3.9 ohms resistor dissipates only (1.25V x 1.25V)/3,9= 400mW. A 1/2W resistor would be extremely hot, a 1W resistor will be fine.

 

[pier1]

Hi AG
  Since i am weak in electronics i am getting confused . In the ckt the 3.9 Ohms is connected from o/p to adjust terminal so when the LED's are drawing 350mA current, then is the current flowing through the 3.9 Ohms resistor ? Or from where is it drawing current ?

 

[audioguru2]

Yes, all the LED current flows through the 3.9 ohm resistor. When the current causes a voltage of 1.25V then the LM317 reduces the current by reducing the voltage to the load.

View attachment 40435

 

[pier1]

Hello AG
            Could these heat sink be sufficient for the regulator ? I first used PI49 and found the regulator over heating .

View attachment 40453

View attachment 40454

 

[audioguru2]

Heatsinks are not selected for their looks. They are selected by calculating how much cooling you need, then selecting one with the correct spec's. The manufacturer will say how many degrees C the semiconductor's case temperature will rise per Watt of dissipation when it is bolted to the heatsink. It is assumed that no insulator is used and that thermal compound grease is used between the semiconductor and the heatsink.

 

[pier1]

Hi AG
        What should be the resistor value instead of 3.9 Ohms if i put only one LED ?

 

[audioguru2]

What should be the resistor value instead of 3.9 Ohms if i put only one LED ?

The 3.9 ohm resistor sets the regulated current to 325ma if there is only one of if there are many LEDs in series. With only one LED then a higher voltage will be across the LM317 causing it to heat more.

 

[pier1]

Hi AG
        So I would have to use a 6 volts stepdown 500mA transformer,bridge and a filter which would give an o/p voltage of 8.64 volts dc across the filter capacitor to reduce the voltage to lm317 ?

 

[audioguru2]

So I would have to use a 6 volts stepdown 500mA transformer,bridge and a filter which would give an o/p voltage of 8.64 volts dc across the filter capacitor to reduce the voltage to lm317 ?

Almost. The rectifier bridge reduces the voltage to 7.1V. Then the max output voltage of the LM317 will be about 4.6V and the 3.9 ohm resistor reduces the max voltage for the LED to only 3.35V.
Doesn't the LED need 3.9V?

 

[pier1]

Hello AG
            The LED forward voltage i measured is 3.48volts . So it wont be sufficient na ?

 

[audioguru2]

There is not enough voltage if the  LM317 is a weak one.
If your mains voltage drops then the LED might dim more.

 

[pier1]

Hi AG
Then I will have to use a 9 volts 500mA step down transformer. Will that give enough voltage for lm317 ?

 

[audioguru2]

A 9V transformer will provide plenty of DC voltage for the LM317 current regulator to drive a single 350mA LED. The LM317 will overheat without a small heatsink.

 

[pier1]

Hi AG
        I connected a 11 Ohms resistor as the load . I noticed that the o/p voltage of the lm317 regulator was 3.6volts dc when the input voltage to the regulator was varied from 7.82volts dc to  17volts dc . The lm317 is cooler then before now . Is this enough AG  ??? as the forward voltage for the led is 3.48 volts dc ? 

 

[audioguru2]

3.6V in 11 ohms is a current of 327ma. If the forward voltage of your LED at 327mA is 3.48V then this current regulator is fine.

 

[pier1]

Hi AG
        I've seen some lens specially made for luxion 1 watt LED's at
        http://ledsupply.com/l2-op-025.php  . What is your suggestion on this ?

 

[audioguru2]

Who makes that lens? Does it work? Will it work well with a Luxeon LED?

Philips probably make a suitable lens for their Luxeon LEDs.

 

[pier1]

Hello AG
            Philips has a certified a manufacturer for this http://www.polymer-optics.co.uk/Supa-Hex%20Range%20(Luxeon).pdf .

 

[audioguru2]

Nice lenses.

 

[pier1]

Hi AG
        Assuming the current drawn by each LED is 350mA . I have connected a 500mA transformer for the whole circuit . Now to make 5 LED's work together, Will it be good to put them in series with a transformer of more o/p voltage? Because the i/p to the regulator is maximum 40 volts i guess !! I am not sure. Will it be safe ?