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pier

white led

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You forgot to say what the LEDs must do.
Drive 20 white LEDs at the same time continuously? Blinking all at the same time? In a chaser/marquee mode?

You forgot to say how you will power the LEDs.
6V battery, 12V battery, AC-DC adapter, or powered from a solar panel?

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Audio guru is right . The led draws a max 0f 30mA . I want to power it from a 6 volt AC - DC adapter . The led's are to be in continious on state and should give very long life . I want to arrange it as a small roof lighting . 

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LEDs will draw as much current as the power source will supply until they burn out. You must use a series current-limiting resistor or a constant current source to reduce the current to less than 30mA for each ordinary LED.

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Hi  AG

        Is this the way u told me to conect ?  If so what should be the value of the resistor ? I guess it is 200 ohms . I am planning of using surface mount devices for this can i use resistors of 1206 package to solve the issue ?

Thanks
Pier .

post-22691-142791433387_thumb.jpg

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Hi Pier,
Yes, your schematic is correct.
If the supply is 12V then two LEDs can be connected in series and in series with one current-limiting resistor then half as many resistors could be used.

We cannot determine the value of the resistor until we know details:
1) Is the 6V regulated? Or will it be unregulated and maybe be as high as 10V?
2) What is the forward voltage rating of the LEDs? It will be a range of voltages: minimum, typical and maximum at a current of 20mA.
3) What is the maximum continuous current rating of the LEDs?

We don't know the power rating for the resistors until we know the details above. If the LEDs are 3.5V and you use 25mA for each one then the resistor will be (6V-3.5V)/25mA= 100 ohms. Its power dissipation will be (25mA x 25mA x 100 ohms= 63mW. A 1206 size resistor is rated for 250mW. 

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Hi AG

        As you asked the voltage is regulated . It is a switchig adapter , but i dont know the forward voltage or the continious rating of the led cause there are different brands coming to India and to my surprise some shopkeepers also dosen't know it . They will only be interested to give you the details only if you purchase the devices in a lot ( production ) . All that I know is that it is a white led of 5mm . Is there any website to which I can look into ?

Thanks
Pier .

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Half the white LEDs I bought that were made in India didn't work. Cheap junk. The store (Sayal, owned by people from India) replaced them all with excellent ones but I don't know where they are made. I won't buy anything from India anymore but the store has excellent American surplus products sometimes.

It is easy to connect a current-limiting resistor in series with the battery and the LED then measure the forward voltage of each LED.

I bought an inexpensive factory-sealed bag of 100 LEDs made by Fairchild. Every one was the identical and worked perfectly.

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Hi AG


        I found the adapter to be too expensive so i decided for an alternative one . the LED ckt draws only a max of 625mA so i decided for the ckt shown in here . I used a 2 amps bridge and a voltage divider followed by a regulator so what will be the wattage of the resistor to be used for both in the divider ?


Thanks
Pier

post-22691-14279143343977_thumb.jpg

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Hi Pier,
Your LEDs are very dangerous without a transformer.
The single resistor to pass 500mA and drop 315V is 630 ohms at 158W.
Two resistors in a voltage divider would use more current because the current to ground is wasted.

Do it like this:

post-1706-14279143344063_thumb.png

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Hi AG
        Actually i want to reduce the cost and it is for roof lighting . Is there any other method that i can do because I really want to reduce the cost . the transformer here is very costly and you wont get that correct rating mentioned on it and as you know the indian market . they are stingy and their products are a junk . so i am trying it out to really reduce the cost . please advice me on this .

Thanks
Pier

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20 ordinary LEDs won't make much light. Maybe just enough for a little sign.

When you rectify and filter 230VAC then you have 324VDC. Your LEDs and current-limiting resistors need only 5V and a little more if you have a voltage regulator. The voltage difference is a huge waste of power that makes a lot of heat.

Put 20 LEDs in series then their voltage is 70V.Then use a current limiting resistor that is (324V-70V)/25ma= 10,160 ohms. Use 10k/10W. A regulator won't be needed. The resistor will get very hot. The entire circuit is a shock hazzard.

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Hi AG

        Thats a good idea, but will if one led blows off then the entire ckt wont work naa . I thought of another arrangement  i.e, individual resistors (smd) .          (324 - 3.5 )/25mA = 12.8 k for each led . Please go through this ckt AG and let me know if there is any mistakes . I am new to electronics AG that is why i am asking a lot of doubts . 

post-22691-14279143344197_thumb.jpg

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Sure, why not.
You could use twenty 12k resistors. They will EACH dissipate 8.6W so the total amount of heat from them is 171W! They need to be rated at 10W each. Smd resistors can't dissipate 10W each.

A 6V/500mA transformer will be safe, hardly waste any power and would cost less.

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Hey AG
          I am going to follow your idea of 70 volt i.e, 20 in series . But i need an arrangement to disable mains when the voltage shoots above 240 or 250 . Some one said that there is some special Ic for this purpose . do u have any arrangement for this ?

Thanks
Pier

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You shouldn't operate the LEDs at more than their max current of 30mA each. 500mA/20= 25mA.

I don't know how high your mains voltage surges. I don't know why because mine never does. So I don't know what will prevent it.

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I have been following this thread to see the outcome.  One thing, based on th elast drawing, that should be noted is that you are not working with 20 LED/resistor combinations in series, you are working with 20 LED/resistor combinations in parallel so you might want to rework the numbers a little.

I am also really happy to see the transformer get into play.  Runnig this circuit without a transformer is just asking for someone to get hurt real bad, as AG has pointed out earlier and repeatedly.

As to cost, I can't help but wonder if a 12VDC wall-wort transformer and, maybe,  a 7805 wouldn't be a quick, easy and cheaper solution to building the power supply cicuit.  Even better would be 6VDC wall-wort.

Just some thoughts...

Jim

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Hi AG
        The project is fine now, working and having enough brightness, but for the resistor i put 10k 20watt , it is running cooler now . Thanks for the help . I am making the housing for it now . 


Thanks
Pier .

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