help me make led lamp like this

[khankll]

hi

can anyone help me in identifing this circuit ?

http://www.ebay.com/itm/LED-Super-F...4153983?pt=US_Light_Bulbs&hash=item20c599f1ff

it is a led lamp.. it takes 10-14 volts and has 24 leds ..

can anyone tell me how the led and resistor is arranged ? it seems ut has 3 series leds in a parrallel string of 8.. and a single resistor to control overall current.. ewhat is the value of led ? seems like 200 M ohms ..with 1 % accuracy..

i have red that these leds require 2.9 to 3.5 volts to work with full brightness at 3.5 v @ 20 mA..

how has the resistore been chosen in the above case ?

i want to make one like it..

 

[(*steve*)]

It is a really bad design that one should not wish to emulate unless one is keen on killing a heap of LEDs.

It has 8 strings of three LEDs with a single current limiting resistor.

If you want to know the right way to do it, see here.

 

[khankll]

thanks i have read that post before asking here..

its the value of resistor thatmade me comnfused how they using such a humengious resistor.. these leds are normally operated at 2.9v to 3.5 voltas at if max of 20 mA.. how is it achieving that ? that resistor wont even allow passing such current .. and the toal power consumption is given as 1.6 Watt which makes every led consume .066 Watt or 66 mW each..

and at 1.6 watt consumed from say 14 volts the current flowing in ckt is 11mA ..? and such a huge 2 Mohm resistor ..

 

[BobK]

That is a 20 Ohm resistor. Assuming it is operating at 12V and each LED is dropping about 3.3V, the resistor is dropping 2.1V, so there is 105 ma total current. So it in the right range at least. The problem with this arrangement is that the current will not be equally balanced between the 8 strings because the drops of each individual LED varies by about 300mV at the same current.

Bob

 

[diefoce]

tnx for the info but if i use 12v battery from my motorcycle how about the amp???

 

[BobK]

I said that in the last post, I estimarte 105ma.

However, you should not make it that way, you should use a separate resistor for each string of LEDs. The resistor would then be calculated to drop the difference between 12V and the forward votage of the LEDs in the string at the desired current for the string.

Bob

 

[khankll]

]That is a 20 Ohm resistor.[/B] Assuming it is operating at 12V and each LED is dropping about 3.3V, the resistor is dropping 2.1V, so there is 105 ma total current. So it in the right range at least. The problem with this arrangement is that the current will not be equally balanced between the 8 strings because the drops of each individual LED varies by about 300mV at the same current.

Bob

thanks i was reading the fourth band as yellow.. but it seems its gold..

ya its not good to have a single resistor at to limit the current for all.. atminimum every series string neds on resistor...

but how can i compensate for the changine input voltages.. e.g a charged 12volts battery is 13.5 volts and discharged is at 10 volts.. huge differnce .. how can i compensate this ?
so i get optimum brightness.. i.e max within limits of the led..?

 

[KJ6EAD]

Use an LM317LZ in constant current configuration to drive each series string of 2 LEDs. Use a 68Ω resistor as the control for the LM317. The schematic is in the datasheet.

 

[khankll]

Use an LM317LZ in constant current configuration to drive each series string of 2 LEDs. Use a 68Ω resistor as the control for the LM317. The schematic is in the datasheet.

thanks i will try that way..
can u tell me how much the input voltage be ?
e.g if i connect 2 3.3v leds that need 20mA of current what could be the absolute minimum input voltage..


i am always confused by this.. that is how minimum can i go on input voltage ..

also it seems there are a lot of lm317 regulators.. is there aqnyone that can also handle currents upto 350 mA so i can also use the,m to power high power led.. something which doesnt needs heatsink

thanks a lot

 

[CocaCola]

thanks i will try that way..
can u tell me how much the input voltage be ?
e.g if i connect 2 3.3v leds that need 20mA of current what could be the absolute minimum input voltage..

6.6 volts if the LEDs are in series but this doesn't give you much room to install a limiting resistor without the risk of the LED dropping out, a 1Ω resistor will work in this instance as it will have little effect overall and in a perfect world no resistor would be necessary... Most people will pump a little more voltage in and up the value of the resistor a little bit to provide some buffer... Unless you have a constant current circuit already...

If they are parallel you only need 3.3 volts and the same deal applies for the resistor...

also it seems there are a lot of lm317 regulators.. is there aqnyone that can also handle currents upto 350 mA so i can also use the,m to power high power led.. something which doesnt needs heatsink

Yes, several should handle in excess of that current, but you might just find it easier to use a fixed value regulator closer to the supply value, and work from there... Unless you are using it as described above in a constant current configuration...

 

[KJ6EAD]

Can you tell me how much the input voltage must be? e.g. If I connect 2 3.3V LEDs that need 20mA of current, what could be the absolute minimum input voltage?

9.6V, (2 X 3.3V + 3V headroom).