Power LED Circuit

Thread Starter

RodneyB

Joined Apr 28, 2012
697
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
 

MikeML

Joined Oct 2, 2009
5,444
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:

Thread Starter

RodneyB

Joined Apr 28, 2012
697
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.
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
 

MikeML

Joined Oct 2, 2009
5,444
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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