white led

[audioguru2]

5 LEDs in series need about 17.5V and the LM317 current regulator needs a minimum of 3.75V so a voltage more than 21.25V would be fine. Use an 18VAC transformer then the unregulated voltage will be 23.8VDC.

 

[pier1]

Hi AG
        What will be the input capacitor value after the bridge rectifier ?. For one LED the value i put was 1000uF when the transformer o/p voltage was 9 volts .

 

[audioguru2]

I have this graph that shows the amplitude of peak-to-peak ripple voltage (at double the mains frequency) but it is for my 60Hz mains and is wrong for your 50Hz mains frequency.
The ripple reduces the average DC voltage by half the amount of the p-p that is on the graph.

For your 350mA then a 1000uF capacitor will have about a 2Vp-p ripple which is fine. Just add 1V to the minimum input voltage of the LM317 current regulator.

View attachment 40460

 

[pier1]

Hi AG
        So i can use 1000uF capacitor only !!!?

 

[pier1]

Hi AG
        Actually I made the circuit on bread board and by mistake i connected the capacitor(1000/25)in reverse but nothing happened to it . Was it supposed to blast ? But it was almost hot by the time i came to know my mistake and switched it off .

 

[audioguru2]

A current of a few Amps or more will blow up an electrolytic capacitor that is connected backwards.

 

[pier1]

Hi AG
        I have seen in the projects Q/A form, your topic on the mood lamp where in the last a heat sink has been suggested for the LED. Should we use a heat transfer paste (white colour) in b/w the LED and the heatsink ?

 

[audioguru2]

Heatsink compound (the white paste) should always be used between a semiconductor's metal surface and the heatsink. It fills microscopic valleys and scratches with the compound that conducts heat very well. The film of paste must be very thin and the items should be tightly bolted together.
The LED datasheet has spec's for a heatsink.

 

[pier1]

Hello AG
            What is lumens and CCT with respect to LED ?

 

[audioguru2]

Hi Pier,
Ordinary small LEDs have their brightness measured in candela or milli-candela units.
Incandescent, fluorescent and very bright large LEDs have their brightness measured in lumens.

One type of measurement takes into account the different sensitivity of our vision to different colors.

 

[pier1]

hello AG,
            I bought a new one (luxion 1 star) The Voltage across the led is exactly 3 volts with the regulator circuit . I have mounted it on to a heat sink(aluminium) of size 57mmx57mm square with 3mm thickness( i cut it myself ) and that is now having heat sinking . I want to screw the heatsink on a hylem sheet of thickness 3mm and length 100x100mm to screw it up again on the roof. Will the hylem sheet get damaged because of heat ? the tempetature on the heatsink just below and opposite to the led is 50 degree centigrade .

 

[audioguru2]

Hi Pier,
If you used thermal grease between the LED and your little heatsink then it is fine. Without thermal grease then the LED is probably too hot inside.

I don't know what is "hylem".

 

[pier1]

Hi AG
        It is used to fix switches and sockets on them. Brown colour on the back and white in the front made by Anchor . I will try to send a pic .

 

[pier1]

I spelled it wrong . It was hylam instead of hylem . Here is a pic of switches and a socket fixed on a hylam board .

View attachment 40471

 

[audioguru2]

Bakelite Hylam Ltd. has made bakelite in India since 1947.
Very old phones and tube radios were made of bakelite.

Maybe now they make a newer plastic. I don't think you should have your 50 degrees C heatsink near it.

 

[pier1]

Hello AG,
              Thanks for advice . I'll screw it up with some space in between (using lingthier screws). What wire shall i use for the led(+&-) I have used one now with 7 strands and 0.2mmthickness for each strand (here we call it 7.2 wire) . 

 

[audioguru2]

I use solid 22AWG wire for most low current wiring and stranded 22AWG wire for flexible wires.

 

[pier1]

Hi AG
        Should I use any protection for the transformer from AC mains voltage fluctuations or will that effect the circuit ?

 

[audioguru2]

A low mains voltage won't harm a transformer. A reasonably high voltage is also fine.
A gross overvoltage (lightning, high voltage by mistake or by accident) would fry a transformer.

Most overvoltage protection devices conduct to blow themselves up or to blow a fuse.

 

[pier1]

A reasonably high voltage is also fine.

How much could this voltage be ?