Constant current sink driver voltage dropping - Page 1
Forum section: electronics / beginners

author: Rihards Veips
author_rank: Contributor
timestamp: November 07, 2017, 07:46:46 pm
content: HelloI am working on a led thing with a constant current LED sink PWM driver.The LEDs I am using are common anode RGB leds.The max R voltage is 2VThe max G/B voltage is 3VThe LED power supply is 3V but I need the red led to run at 2VMy LED driver max current is set to 20mACan I use a 50 ohm resistor between R and one of the driver outputs to get 2V on the LED instead of 3V?I got the 50ohm value by using this equation:R = (Vs - Vled) / Iled50 = (3 - 2) / 0.02This is what the schematic would look like including the driver output:I don't have much experience with this LED driver stuff so I wanted to ask this here before I make my pcb for the project.Thanks.

author: grifftech
author_rank: Frequent Contributor
timestamp: November 07, 2017, 07:58:26 pm
content: Quote from: Rihards Veips on November 07, 2017, 07:46:46 pmDriver has current limit, no need for resistor.

author: Rihards Veips
author_rank: Contributor
timestamp: November 07, 2017, 08:10:50 pm
content: Wouldn't I have to use the resistor to prevent the LED from burning out because of the 3V supply voltage? The max rating for the red led is 2V

author: kalel
author_rank: Frequent Contributor
timestamp: November 07, 2017, 08:28:44 pm
content: Quote from: Rihards Veips on November 07, 2017, 08:10:50 pmFrom what I understand, if you have a driver that setsconstant current, it should limit the current that the LEDs receive. That means you might need no resistor, if the current limit is set properly and works properly for the purpose. I don't know if there are some limiters that do need an extra resistor.If you had aconstant voltage supply, you would need a current limiting resistor to ensure that the LED doesn't get more than the rated current. This is because if the voltage is even slightly increased after some point, the LED will draw a lot of current, possibly burning itself out (guaranteed at some voltage).

author: Damianos
author_rank: Frequent Contributor
timestamp: November 08, 2017, 10:27:43 pm
content: A circuit of constant-current will regulate the current to the load. The voltage drop on the load will be what corresponds to the value of current. The rest of the voltage, of the power source, will be present at the terminals of the driver. So, a resistance there will prevent the driver to work properly. The problem is with the other two LED lights specified for 3 V, which is equal to the supply voltage; how the driver will adjust the current? Don't forget that the voltage is the force to move charge, current is the result of this movement...

author: Audioguru
author_rank: Super Contributor
timestamp: November 09, 2017, 01:17:05 am
content: Your schematic does not show constant current drivers.Your LEDs driven from Mosfets with NO CURRENT LIMITING except the 50 ohm resistor to the red LED.LEDs set their own voltage. You just need to feed a higher voltage then limit the current.Nobody makes a 2V red LED, they have a range of voltages from maybe 1.7V to maybe 2.2V (look on the detailed datasheet) and you get whatever is available. If your supply is 3.0V and your LED is 1.7V then the 50 ohm resistor limits the current to (3V - 1.7V)/50 ohms= 26mA which is pretty high but the maximum allowed current is probably 30mA. If your supply is 3V and the LED is 2.2V then its current with the 50 ohm resistor is 16mA.A modern bright green and blue LED has a voltage range of from maybe 2.8V to maybe 3.6V so many will not work from only 3V, and they also need a resistor to limit the current.

author: kalel
author_rank: Frequent Contributor
timestamp: November 09, 2017, 02:58:12 am
content: Quote from: Audioguru on November 09, 2017, 01:17:05 amI heard this before, but will they really not work at 3v? Or just not work at optimal brightness.

author: mikerj
author_rank: Super Contributor
timestamp: November 09, 2017, 11:40:24 am
content: Quote from: kalel on November 09, 2017, 02:58:12 amDepends on the forward voltage of the LED. Attached are the IV curves for some typical LEDs. Note that with 3v on the green LED, you will only get about 2mA, and because you only just above the threshold voltage the current will vary significantly with temperature.