Constant Current Driver - Explanation Needed - Page 1
Forum section: electronics / beginners

author: Harsh Chandola
author_rank: Contributor
timestamp: March 15, 2017, 07:23:47 am
content: Hi All,So I am designing a system where I'll be controlling LEDs and their brightness. I am given a constant current driver for them. I am unable to understand how constant current drivers/sources can be manipulated. Constant voltage drivers/sources are very easy to use, I can have multiple devices in parallel and they'll have same voltages across but will draw whatever current they require and can also be controlled the way I want using pwm and all but how is the case in constant current drivers/sources? I am going to control LEDs using Atmega chip or ESP8266 wi-fi module for my projects but these devices demand constant voltage. It is not possible to use pwm in case of constant current drivers/sources, also, these microcontrollers need 5 volts to operate, how do I provide 5 volts to them from constant current?Also what happens when we use PWM with constant current driver?How can I manipulate constant current drivers to get my system working like it'd work with constant voltage driver/source?kindly explain me the fundamentals of constant current drivers/sources and how to manipulate them. I am very confused with it any help will be appreciated.Attached is the schematic of one of the project I am working on but I think it might not work with constant current driver/source.Thank You

author: kosine
author_rank: Regular Contributor
timestamp: March 15, 2017, 12:40:10 pm
content: Simplest solution is to switch a basic NPN transistor on and off with your PWM output. Assuming it's a 5V microcontroller, that puts 5V on the base and about 4.4V on the emitter. A 440 ohm emitter resistor then gives you 10mA flowing through the transistor (collector to emitter, the base current will be trivial).Connect however many LEDs you like in series to the collector, and they'll all have 10mA flowing through them. They'll drop whatever voltage is appropriate depending on their colour (about 2V for basic red, yellow, green, about 3V for blue and white), and the rest of the voltage is dropped across the transistor.With a 24V main supply voltage, you should be able to run at least 5 or 6 LEDs this way. If you want a different current, just change the emitter resistor as appropriate. 4400mV/220ohm = 20mA etc.Technically, the LEDs always run at the same brightness, and the PWM just switches them on and off. The more they're on, the brighter they'll look. So you're not actually changing/controlling the current at all. What you're doing is changing the average current over a period of time. The human eye does the rest.Note that your schematic shows the LEDs in parallel, which is not what you want. A parallel configuration will divide the current between them. You want the same current through them, so they need to be in series. (Because current is conserved: What goes in one end, flows out the other and into the next. So you get the same current in every device - LEDs, transistor and resistor all have the same 10mA flowing through them.)

author: phliar
author_rank: Contributor
timestamp: March 17, 2017, 12:45:15 am
content: With a constant-voltage source, you put the loads in parallel (so they all get the same voltage); with a constant-current source, you put the loads in series (so they all get the same current).Any current source will have a "compliance voltage" -- the max voltage they can put out. Which means there will be a limit to how many LEDs you can put in series for your current driver.You can certainly PWM a current source, it all depends on exactly what current source it is.... usually current sources just have a sense resistor in the current path, and feedback makes sure the current stays constant. It shouldn't be hard to control such a beast either on-off (i.e. PWM) or directly (by changing the feedback).What kind of CC source is it?

author: danadak
author_rank: Super Contributor
timestamp: March 17, 2017, 01:14:02 am
content: If you drive a led from a voltage source, led in series with an R,the R effectively makes a crude current source. Problem is one ledto the next may have a significant threshold difference, therefore asignificant brightness sensitivity device to device because of currentvariation. The current source helps to take out one of the errorissues so that all leds are closer to same brightness level.The current source approach also allows for simpler wiring, and lowercomponent count. Of course significant disadvantage is if one led failsopen in string the whole string fails.Regards, Dana.