LED constant current circuit

[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)

[Liquid-light]

It's 1.87V
But by the way it's in reality just 27mA or so. Probably because of the part tolerances.

[Benta]

Your Rg is way too high. If you read the article you linked to, it suggests around 1 mA through Rg.

The problem is, that your BC546 has nothing to do, meaning almost no collector current. The base current will be so low, that V_BE will be down to 0.4...0.5 V. The 0.7 V postulated only make sense at I_C and I_B currents that are in the normal operating range of the transistor. And even then, 0.65 V is more likely.

That being said, I don't like the circuit. Relying on V_BE for current limiting is probably the most imprecise way around.

[Liquid-light]

Thanks.
I've just tried an Rg with only 1k. Unfortunately still not working. Everything is the same as before.

[StillTrying]

Are you sure RG is connected to Vcc V+ and not to the LED/drain, it would sort of explain the constant 2.5V.

[Liquid-light]

What a stupid beginner mistake.
You are spot on, that was the reason for the problem.

Thank you very much!!!