# Power LED Circuit

Discussion in 'The Projects Forum' started by RodneyB, Jan 7, 2015.

1. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
680
15
I came across a constant current LED circuit at

http://electronics-diy.com/power-led-driver-circuit.php

It looks nice and easy to build. However the Calculations for the resistors and the power make no sense to me.

I want to change the N channel MOSFET for a IRF520N and the transistor for a BC547 as I have these items in my stocks.

In the diagram unless I am missing something the Mosfet looks like a P channel yet in the write up refers to an N channel and even the specified part is N channel.

I really want to work out the resistor, for the current limiting.

The LED's I have are have the following ratings

Voltage 3.2 - 3.4 Volts
Current 350mA

I want to run them from a 12 Volt supply in different strings from 1 to 4 LED's

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
The circuit requires an NFET. It will work with at most three of your LEDs if starting from 12V. If running only 1 LED from 12V, the NFET will have to be on a BIG heatsink.

R3 in the posted circuit is 0.65/Iled, so to get 300mA, R3 = 0.65/0.3 = 2.2 Ohms.

Power wasted in the NFET with one LED = E*I= (12-3.3-0.65)*0.3 = 2.5W, so without a suitable heat sink would burn your hand...

Last edited: Jan 7, 2015
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3. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
680
15
Thank you very much for clarifying that for me. Would I work out the Resistor R3 the normal way to limit the current to 320mA

Am not sure how to calculate R1

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
I gave you the approach. Just change the current and push buttons on a calculator...
R1 is non-critical. The 100K given in the original article is fine...

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5. ### RodneyB Thread Starter Well-Known Member

Apr 28, 2012
680
15
thanks very much for your help