LED constant current circuit - Page 1
Forum section: electronics / beginners

author: Liquid-light
author_rank: Newbie
timestamp: June 14, 2018, 07:25:46 am
content: Hej everyoneI 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=3For 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

author: mikerj
author_rank: Super Contributor
timestamp: June 14, 2018, 08:13:51 am
content: Check the Vgs requirements of your MOSFET, it may need more than 2 volts to pass the current you are demanding.

author: Liquid-light
author_rank: Newbie
timestamp: June 14, 2018, 06:11:19 pm
content: Thanks, but of course I've checked this point. And the voltage at the gate is always significantly higher anyways.

author: Benta
author_rank: Super Contributor
timestamp: June 14, 2018, 06:42:28 pm
content: 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?

author: Liquid-light
author_rank: Newbie
timestamp: June 14, 2018, 07:40:53 pm
content: 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 10kRs is 22OhmThe transistor is BC546The mosfet is IRLZ34N (logic level obviously)

author: Liquid-light
author_rank: Newbie
timestamp: June 14, 2018, 08:10:26 pm
content: It's 1.87VBut by the way it's in reality just 27mA or so. Probably because of the part tolerances.

author: Benta
author_rank: Super Contributor
timestamp: June 14, 2018, 08:37:47 pm
content: 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 VBEwill be down to 0.4...0.5 V. The 0.7 V postulated only make sense at ICand IBcurrents 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 VBEfor current limiting is probably the most imprecise way around.

author: Liquid-light
author_rank: Newbie
timestamp: June 14, 2018, 09:28:27 pm
content: Thanks.I've just tried an Rg with only 1k. Unfortunately still not working. Everything is the same as before.

author: StillTrying
author_rank: Super Contributor
timestamp: June 14, 2018, 10:29:39 pm
content: Are you sure RG is connected toVccV+ and not to the LED/drain, it would sort of explain the constant 2.5V.

author: Liquid-light
author_rank: Newbie
timestamp: June 15, 2018, 06:39:23 pm
content: What a stupid beginner mistake.You are spot on, that was the reason for the problem.Thank you very much!!!