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LED constant current circuit
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Liquid-light:
Hej everyone

I have a small strange problem with one of my circuits and I have the impression that I'm getting blind while I try to find the solution. So a new perspective would be very much appreciated.

I try to drive a few leds with a constant current setup like the one on this site :
http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/?topic=worklog&p=3

For testing purposes I started with a simple 5mm led and appr. 30mA. That means a sense resistor of 22 Ohm. Rg is 10k. As transistor I've used a standard Bc546 and the mosfet is a IRLZ34N.
So nothing crazy so far.

But for some reason I need 3,3 to 3,5V more voltage in order to reach 28mA. Following the calculations I should only need 0,7-1V extra. And it doesn't matter if I use 1 or 2 or 3 Leds. It's always 3,5V more needed. So obviously something is wrong. But I don't know what. Any ideas?  :-D
mikerj:
Check the Vgs requirements of your MOSFET,  it may need more than 2 volts to pass the current you are demanding.
Liquid-light:
Thanks, but of course I've checked this point. And the voltage at the gate is always significantly higher anyways.
Benta:
I've no idea what you mean by "3.3 to 3.5 V more." What is your supply voltage? And what is your gate resistor?
Liquid-light:
As I wrote Rg (according to the schematic the resistor at the gate and the transistor collector) is 10k.
And when I say 3.5V extra I mean led voltage (3,3V) + 3.5V extra supply voltage  in order to reach 30mA.

And, as I wrote, it doesn't matter how many leds in series are used. It's always 3.5V extra. So for example with 3 white leds it's 9.9V + 3.5V in order to reach 30mA. But according to the circuit description (please check the link)  it should only be 9.9V + 0.7V.

So what could be the issue here.
I've attached the circuit from the page.
Rg is 10k
Rs is 22Ohm
The transistor is BC546
The mosfet is IRLZ34N (logic level obviously)
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