white led

[pier1]

Hi AG

        There is no voltage across 27 ohms resistor when i measured it ? what could be the reason ?

Thanks
Pier

 

[audioguru2]

There is no voltage across 27 ohms resistor when i measured it ? what could be the reason ?

Maybe the pins on the ST13005 transistor are also connected backwards.
This is what the voltages should be:View attachment 40398

 

[pier1]

Hi AG
        I am such a fool, I forgot to connect the 3904 in reverse when i corrected the 13003 properly . Now its fine . The voltage across the 27ohms is 0.56 volts. I dont understand AG how are u able to find faults without seeing the circuit I connected . Thanks a lot AG . as i said is 0.56volts across 27e ok  ?

Thanks
Pier

 

[audioguru2]

I think 0.56V across the 27 ohms resistor is fine.Then the LED current is 20.7mA.

You could replace the 27 ohms resistor with 22 ohms then the current would be 25.5mA and the LEDs would be a little brighter.

 

[pier1]

Hi AG
        I connected 22 ohms . Now the voltage across is 0.58 . How about reducing the resistance little more ?

Thanks
Pier.

 

[audioguru2]

I don't know how good or how poor is the current regulation. When the rectified voltage is as high as 366VDC then you don't want the current to be higher than the max allowed for the LEDs which is 30mA.

 

[pier1]

Hi AG
        Its right as u said . The voltage across the 22ohms resistor is 0.6volts when the mains voltage reaches 345Vdc . The 13005 is getting very hot and i 've put a bigger heatsink now . I will put it under testing and let u know the result after 1 or 2 days . Thanks a lot for ur co-opeation in helping my project AG .

Thanks
Pier

 

[audioguru2]

The current regulation is pretty good. From 300V to 345V is an increase in voltage of 15% but the current increased from 26.4ma to only 27.3mA which is an increase of only 3.4%.

EDIT:
The current regultion is even better than that because the actual circuit voltage is nearly doubled.

 

[pier1]

Hi AG
        The circuit was on test for 3 days continiously and working fine . People are asking me how much power it consumes from the mains . Bulbs consume 60Watts as written on it when we purchase it here . Do we have to connect an Ampere meter in series with the live mains i.e.230/250 volts AC to measure its current and calculate its power ?

Thanks
Pier.

 

[audioguru2]

Hi Pier,
Since you have a full-wave bridge rectifier and 60uF of smoothing capacitors then the DC is without much ripple and is pure DC. You measured the current (calculated with Ohm's Law by the voltage across the 22 ohms resistor) at two voltages so just multiply the voltage times the current to calculate the total power:
300VDC x 26.4mA= 7.9W.
345VDC x 27.3mA= 9.4W.

You could measure the voltage across the LEDs and use it to multiply the current to calculate how much power is used only by the LEDs.

 

[pier1]

Hi AG
        I made a board of 72 X 24 mm in size and the same size for the heatsink with a thickness of 3mm . Would this be a sufficient heatsink for a continious operation . I want to isolate the transistor from the heat sink and screw it up (Heat sink)on the roof of my room .

Thanks
Pier .

 

[audioguru2]

We purchase extruded aluminum heatsinks for power transistors. The heatsinks have many fins for a high surface area. They are rated in their amount of degrees C temperature rise per Watt of power dissipated in the transistor.

 

[pier1]

Hi AG
        Now suppose i have only 21 volts dc at the o/p of the power supply and i have to connect led's in series . Assuming the forward voltage is 3.5volts, can i connect 6 in series so that the total forward voltage reaches 21 volts dc ? I dont know the exactly how to calculate the number of led's to connect in the circuit . Can u please help me in this ?

Regards
Pier.

 

[audioguru2]

Hi Pier,
you cannot be certain the forward voltage of each LED is 3.5V since each one is different. They might be 3.9V then 6 need 23.4V.
also, the transistors in the current regulator need at least 1V to work. So use 24V or 25V.

You don't need a high voltage darlington NPN power transistor and you don't need two resistors in series to increase their voltage rating with a low supply voltage. This circuit should work:

View attachment 40407

 

[pier1]

Hi AG
        Thanks for the circuit. Actually I am not building the circuit for 6 led's but i wanted to know the calculations for deciding the number of led's used for certain voltage available because i read somewhere in the web that u can use leds in series provided the voltage across the led string should not exceed 0.9 X Vcc volts.So As i asked in the topic before as per my assumation i took 3.5volts per led and voltage available is only 21 volts , 6 led's take the entire voltage(21 volts), so i was confused if it was right to do so ????The other circuit u gave me for 49 leds is still working excellent on my roof top . 

Regards
Pier.

 

[pier1]

Hi AG
        I removed the led light from the roof top to put a better heat sink and when i touched the track side i got a good shock, The capacitors were still charged . Should i put and resistor across the capacitor bank so that they get discharged as soon as it is put off ? If so which would u suggest the value to be and its wattage ?

thanks & regards.

 

[audioguru2]

The capacitors were still charged . Should i put and resistor across the capacitor bank so that they get discharged as soon as it is put off ? If so which would u suggest the value to be and its wattage ?

5 pages ago, your circuit had 30uF for its supply filter. The max voltage is 370VDC so two 1/2W resistors in series should not arc. If their value each is 100k then they won't get too hot and they will be nearly completely discharged in (30uF x 100k) x 5= 15 seconds. Actually, the LEDs would discharge the capacitors quickly down to their voltage, then the resistors would continue the discharge. So depending on the total voltage of the LEDs, the discharge time is quicker than 15 seconds.

 

[pier1]

Hello AG
            I bought 2 new 1watt white led (lumileds), Its specifications are

3.5volts DC
350 mA

            Since it is a costly one , As you suggested before i would like to use a isolation transformer(9 volts, 500mA), Bridge rectifier(w10m) a good filter(1000uF/16v) . For the 3.5 volts I dont know what to do . My idea was to use a 7805 with good heatsink and a voltage divider for 3.5volts DC and put the 2nos of 1watt LED's in series . Is this correct ?

Or I have found a link http://www.national.com/webench/ledrefdesigns.do

Should i use something from this Ag . Please Suggest me .

Thanks
Pier .

 

[audioguru2]

How can you power two 3.5V LEDs in series (7V) with only 5V from a 7805?

I didn't check the datasheet, but a 3.5V LED could be 3.0V, 3,5V or 4.0V. Some of them are 3.5V.
Its absolute max current is probably 350mA. Do you want to operate it at its limit?
You need two current regulators set to about 335mA each. Then you add their voltage plus the max LED voltage to determine the voltage that is needed.
An LM317 makes a nice current regulator with just one resistor.

 

[pier1]

Hi AG,
        Sorry for the 7805, it was supposed to be 7808 . On the projects Q/A forum, the lm317 thread you have put up a circuit using 78xx series for current regulation . Shall i follow that ?

One doubt : Assume when two leds(3.5v each) are connected in series the current(each led = 350mA) through both is same. So, is the total current drawn from 78xx series(to220) is the same as the flowing current(i.e, 350mA) ?... ???