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# Power Led Driver circuit

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#### Ashkar

##### Full Member level 2
I got a problem,I wanted to drive 50 led's in parallel each led is rated 1watt @3.5v dc that is a total of 50watts of power.It should be controlled through Atmega8 mcu to adjust brightness of led's and to make some other controlling purpose to accomplish. So how would I control the leds work:-D

First you need to have additional details about the power supply to be used for. This may lead to reconsider the arrangement of LED’s, also introducing some small series resistor in order to compensate the variation of LED voltage due to temperature. As an LED heats, its voltage drop decreases and this can encourage the current to increase.
The power may be applied periodically or intermittently, you may use PWM to drive any device (mosfet power transistor) so long as the flicker rate is greater than the human, the LED will appear to be continuously lit.

Dan70

### Dan70

Points: 2
ok,The maximum voltage will be 12v dc

Do you mean rather small LEDs (nominal 20mA)?
They are white I guess. If the supply is 12V, each branch can be made with 3 LEDs in series. So for 50 LEDs, the possible matrices are:
3 * 16 branches = 48 LEDs
or
3 * 17 branches = 51 LEDs

To be in the safe side, each branch will need a current limiting resistor Rs (in series).
Rs = (Vcc – 3*V_led) / I_led

To calculate Rs properly, your first test would be to decide on the current I_led (lower the current, longer the LED life) and get a measurement of the LED forward voltage V_led at this current I_led.
Let us assume I_led is about 20 mA, and you have a 12V supply, the series test resistor is:
R_test = (12 - 3.5)/20 = 425 Ohm => 470 Ohm (standard value)

Now your test circuit consists of:
12V supply , 470 Ohm , white LED (cathode to ground).
Measure the voltages V_rs (over the resistor) and V_led.
The actual I_led is now equal
I_led = Vrs / 0.47 (in mA)
If the brightness of the LED is ok then the measured voltage on the LED is V_led.
Let remember that
Rs = (Vcc – 3*V_led) / I_led
It is easy now to found Rs for each branch.

Sorry, you are right I forgot to put my glasses

If you still need me, just say hello

Kerim

Last edited:

I think it can be grouped as serie of 3 LEDs, for a total of 16/17 rows (48 /51 LEDs)\

At 1W/3.5V we obtain a curent of 0.285A. For 17 groups the maximum current needed= 4.85A.
3.5V * 3= 10.5V the series resistor needed 1.5V/0.285A = 5.1 ohms / 1W

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Dan70

### Dan70

Points: 2
no,I wanted to connect 1watt leds

---------- Post added at 09:28 ---------- Previous post was at 09:26 ----------

@mister_rf Thank you for this great design.would a irfz44n mosfet would be good for the purpose.

Last edited:
Dan70

### Dan70

Points: 2
I would like to drive the mosfet with a totem pole drive.

In this case, yes, it’s OK to use the IRFZ44.

Dan70

Points: 2