Driving/regulating LEDs in parallel - Page 1
Forum section: electronics / projects

author: sebmadgwick
author_rank: Regular Contributor
timestamp: May 24, 2014, 11:54:35 am
content: I will pulse a group of 10IR LEDsperiodically, e.g. for 50 ms every 1 seconds. The LEDs will be powered from lithium ion cell (nominal 3.7 V) and switched using an N-channel FET inthe obvious way.My problem is that I am not sure how best to regulate the LED current. Each LED is capable of running up to 100 mA but I would want to limit the total current of the group to 300-700 mA. There aresingle chip solutionsbut the application examples always indicate the LEDs in series. My battery voltage is too low for this.Is it 'bad practice' to drive identical LEDs in parallel? Presumably because forward voltage variations risk one LED drawing too much current?How do you suggest I drive these LEDs?

author: SeanB
author_rank: Super Contributor
timestamp: May 24, 2014, 12:31:58 pm
content: Single resistor and take a chance on killing a LED. Use a big capacitor across the input side ( really big, 1000uf or more) and you will have a very high pulse capacity and will not be worrying about battery internal resistance as it ages. You probably would want a series resistor or around 1R per LED, and arrange the tracks so each LED has the same length of track. Best is to have 2 long traces and feed the power in the one end and the ground at the other, so that the LED's all see around the same current. Best to have all the LED's come from the same batch, so order a pack of 10 and use them all.Pulse driving IR LED's is fine with a small overload, as they often in a remote only have the on resistance of the power switch, the battery internal resistance and the track resistance as current limit. The 100mA is for a continuous rating, with a shorter pulse than 50mS ( 10mS or so) you can go to a much higher current.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: May 24, 2014, 02:42:32 pm
content: Quote from: SeanB on May 24, 2014, 12:31:58 pmThis is something else I am interested in but did not ask. The battery cannot provide more than 1 A continuous but I would like the IR pulse to be as bright as possible. Thedatasheetindicates a potential 1 A pulse for 100 us. Space is limited but I could maybe fit some flat tantalums to create the bulk.I'm expecting there is a risk that after a pulse, the drained capacitor would cause the rest of the circuit (microcontroller, wifi, etc) to brown out. In which case a circuit similar attached could be the solution.Quote from: SeanB on May 24, 2014, 12:31:58 pmGiven the specified 1 A pulse for 100 us, a single series resistance to the parallel LED array could be very low. Perhaps the value would be chosen to lengthen the [capacitor fuelled] pulse rather than protect the LEDs.Any comments welcome.

author: Richard Crowley
author_rank: Super Contributor
timestamp: May 24, 2014, 02:56:29 pm
content: My understanding is that YES it is bad practice to attempt to drive LEDs in parallel without series equalization resistance. Unless you want to buy a bushel-basket full of LEDs and measure each of them and hand-select a set of perfectly matched units (if that is even possible?).Else the ones with slightly lower equivalent resistance will "hog" the current away from the LEDs with slightly higher impedance. Same principle as when you parallel transistors where you put small resistance in series with each unit to equalize the current through all the parallel devices.

author: mariush
author_rank: Super Contributor
timestamp: May 24, 2014, 04:20:14 pm
content: You could just use a driver like this for each led :http://uk.farnell.com/infineon/bcr421u/ic-led-driver-0-5w-leds-6sc74/dp/1791068RLSet the current limit through external resistor, send a few volts on the enable pin to enable it, so no need for mosfets , just connect a microcontroller pin (or the output of a 555 timer or whatever you want) to it and you're done.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: May 24, 2014, 04:45:28 pm
content: hi, i saw the datasheet of those led and the Vf is only 1.45V for If 100mA, so with 10 led you will need a bit more than 14.5V that's not much at all, you only need a step up circuit to boost the 3.7V to 15 for example. you apply this voltage to the led string with a resistor in series to limit current to 100mA (for 15V you only need 5Ohm resistor). i used once an LMR62014 to supply an infra-red sensor that need 12V from a Li-ion 3.7V no problem at all.the best way though is to use a led driver as it regulate current and usualy have dimming option i think LM3410 can do the job for you

author: StrangeloveMD
author_rank: Contributor
timestamp: May 24, 2014, 04:53:36 pm
content: Paralleled diodes will preferentially degrade due to slight variances in real world performance; or that is my understanding.

author: SeanB
author_rank: Super Contributor
timestamp: May 25, 2014, 07:31:22 am
content: True, that is why I suggest a series resistor for each led to balance them a little. Yes the life will be shorter, but how long does this have to last, and how many pulses will it transmit in the lifetime. The capacitors can easily have a SMD inductor added in the battery side ( with a resistor of 10R or so across the inductor to damp it so it will not ring with the capacitor) and another with a capacitor on the supply to the microcontroller to provide decoupling to it as well. Easy to add, and will work to keep it running.

author: sebmadgwick
author_rank: Regular Contributor
timestamp: May 26, 2014, 03:04:56 pm
content: I built a circuit to test using 4400 uF and 5 LEDs each with a 1.5 R series resistance. I used a scope and Dave's uCurrent to measure the voltage and current with each pulse.See attached schematic and resultant transient voltage and current.The currents are generally lower than I expected: 500 mA peak (when only 1.5 R matter) and 200 mA steady-state (when 10R has effect).