DC power supply for LED array.

[gdlengr]

Hi guys
This is my first posting in this forum. I am trying to make a four feet LED tube light. The power supply will be 200~235V AC. Here is my plan to create the circuit:

220V AC-------->>step down to 12V using transformer --------->> rectifier using 1N4007 diode (bridge)------>>filtering using smoothing capacitor [25V, 100uF]------>>regulate voltage using LM7805------>> dropping 2 volts using 5 W resistor------>>To LED array.

I have a plan to use 200 LEDs. But the problem is, I am confused choosing the proper regulator model? You know, LM7805 has low current rating like 500mA. But my 200 LEDs will draw around 3~4 A current.

Also please tell me would it be wise to use capacitor to drop voltage instead of transformer?

Any advice will be very helpful. I am sorry for my bad English. Thanks in advance.

 

[4pyros]

I would not drop it all the way down to 5 volts!

LM7805 has low current rating like 500mA.

Its good for 1 amp.

Also please tell me would it be wise to use capacitor to drop voltage instead of transformer?

No too much current.
Redo your math for 12 volts and see how much current you need.
Try 24 volts or 36 volts even.

 

[cowboybob]

Just curious. 200 LEDs in one 4 foot light tube?

 

[audioguru]

LEDs are NEVER connected directly in parallel unless they are all measured then matched to have exactly the same voltage. A "3V" LED could be 2.7V or 3.3V or any voltage in between.

In parallel, an LED with the lowest forward voltage will hog most of the current and will quickly burn out. Then the remaining LEDs will also burn out the same way.

Your idea is poor to use a 12VAC transformer to make 15VDC then throw away 10V in a 5V regulator. If the regulator has an output current of 4A then it heats with 10V x 4A= 40W which needs a huge heatsink and a fan for cooling.

Instead, use the 12VAC transformer to make a rectified and filtered 15VDC and don't use a voltage regulator. Connect four 3V LEDs in series and in series with a curent-limiting resistor to make a 15V string. Then connect as many of these strings to the 15VDC that you want.