Current regulator for low power LEDs - Page 1
Forum section: electronics / beginners

author: T_guttata
author_rank: Regular Contributor
timestamp: June 12, 2025, 09:05:08 pm
content: HiI have a few low power LEDs in 0603 form factor, which I want to use with variable voltage (roughly 4-12V). To get a constant intensity, I don't want to use resistors.I tried using a BCR421U without external resistor. According to the data sheet, the current should be 10mA.Unfortunately when I test it, the current is not constant (no need to measure, since it is obvous due to changing intensity of the light).I'm wondering: is the device not suitable for that task since the current is rather low? Are there any alternatives? I did the wiring according Figure 4-2 of the data sheet (but with only 1 LED).I have used that IC in the past to drive high power LEDs, which worked as intented. But I never used the BCR421U for low power LEDs.Thanks!

author: AlexRG
author_rank: Regular Contributor
timestamp: June 12, 2025, 09:28:15 pm
content: Quote from: T_guttata on June 12, 2025, 09:05:08 pmAccording to the table, the maximum resistance of the additional resistor is 100 Ohm, while the minimum stable current is about 20 mA. If your LEDs consume less, try simply connecting 2-3 of them in parallel to increase the total current.

author: T_guttata
author_rank: Regular Contributor
timestamp: June 12, 2025, 09:51:52 pm
content: 10mA should be possible, but 20mA ist also fine.I think I now understand the difference: BCR420 needs 24V to eneable, BCR421 only 3.3V.Problem is: when I want to use it to drive LEDs with voltages of 4-12V, I don't have constant 3.3V for the enable pin.

author: tunk
author_rank: Super Contributor
timestamp: June 12, 2025, 10:01:11 pm
content: Never used it and wouldn't really know - LM317L in constant current mode?

author: Benta
author_rank: Super Contributor
timestamp: June 12, 2025, 10:11:17 pm
content: Which LED colour?If you're using blue or white types, you can forget the 4 V, the LED voltage drop is simply too large.

author: tunk
author_rank: Super Contributor
timestamp: June 12, 2025, 10:18:51 pm
content: Quote from: Benta on June 12, 2025, 10:11:17 pmYou're right, forgot the "Input-to-output voltage differential" of 2.5V, plus the 1.25V resistor drop.

author: T_guttata
author_rank: Regular Contributor
timestamp: June 12, 2025, 10:22:03 pm
content: Green and Blue. For the blue one: XZCBD67SBut I have found a far better alternative in terms of regulator: AL5809-20P1-7. No enable pin required, just 2 pins. Roughly the size of a 1206 resistor.If it does not work with blue, i might switch to another color.

author: Benta
author_rank: Super Contributor
timestamp: June 12, 2025, 10:29:28 pm
content: Worse alternative. The new device you've found needs at least 2.5 V to operate. The BCR421U only 1.4 V.

author: T_guttata
author_rank: Regular Contributor
timestamp: June 12, 2025, 10:40:21 pm
content: Well ok, but how do you solve the "enable pin problem" if you don't have constant 3.3V?

author: mariush
author_rank: Super Contributor
timestamp: June 12, 2025, 10:44:30 pm
content: AL5809 works as long as the voltage drop across it is minimum 2.5v - so for example with a red led that has a 1.8v forward voltage, you'd need minimum 2.5v + 1.8v = 4.3v to get regulation, and with blue leds with 3v...3.2v you'd need minimum 5v.You need to say what color leds, how many leds, and if you want individual control of each led or not.If you don't need individual control, then probably easiest would be to have all the leds in series and then use a step-up led driver to boost your 4v...12v up to [ n x forward voltage led color 1 + n x forward voltage led color 2 + .... + n x forward voltage color 99 ]For example if you have 6 blue leds in series, you'll have a minimum voltage of 6 x 3v = 18v, so you get a step-up / boost led driver that can boost 4v...12v to around 18.2v - the driver will drop around 0.1v-0.2v on a current sense resistor.If you want individual led control, then probably best option would be to use a step-down regulator to drop your 4v...12v to around 3.3-3.6v and then use linear led drivers with very low headroom to turn on and off individual leds.For example, a 16 channel MBI5035 -https://lcsc.com/product-detail/LED-Drivers_MBI-MBI5035GP-B_C261130.html- can control up to 16 channels with as little as 0.2v drop (typical, maximum 0.25v) per channel.Of course, there's smaller 4 channel or 8 channel such drivers, or led drivers that have individual inputs, not shift register like control.For example AP2502 has 4 channels (not individually controlled) :https://www.digikey.com/en/products/detail/diodes-incorporated/AP2502KTR-G1/4470827RT9300 is similarhttps://www.digikey.com/en/products/detail/richtek-usa-inc/RT9300AGE/2547140and RT9302 has 3 channels :https://www.digikey.com/en/products/detail/richtek-usa-inc/RT9302GJ8/16376590CAT4104 for higher current per channel :https://www.digikey.com/en/products/detail/onsemi/CAT4104V-GT3/2271397

author: Benta
author_rank: Super Contributor
timestamp: June 12, 2025, 10:59:07 pm
content: Quote from: T_guttata on June 12, 2025, 10:40:21 pmNo need for constant 3.3 V. Just connect "enable" to your 4...12 V. Done.But it still won't work with a blue/white LED. Red, green, yellow: sure.

author: T_guttata
author_rank: Regular Contributor
timestamp: June 13, 2025, 05:25:55 am
content: According to the data sheet, the maximum rating for the enable pin is 4.5V (Infineon).I don't have a complicated circuit, just a few status LEDs. But since the voltage is not constant, I was looking for a better alternative to simple resistors.Edit: I checked the data sheet for the BCR421UW6Q (Diodes Inc.); the value is 18V, which makes more sense.

author: Whales
author_rank: Super Contributor
timestamp: June 13, 2025, 06:50:08 am
content: Can you afford an opamp (LM324/LM358 are around $0.05USD), two resistors and a diode? Example with a 6mA setpoint and about 1.5V dropout (which sadly means it can't quite make your minimum 4V operating point):Simple explanation: the opamp is regulating current through the LED. It measures the LED current using the resistor below the LED. It gets a reference voltage from an ordinary diode (left side) of about 0.6V. Iout = 0.6 / ResistorBelowLED.There are a few ways to turn this on or off, depending on what form your input signals are. Best case scenario: 4 leds can be independently controlled with 1x LM324, 5x resistors and 1x diode. Worst case require an extra resistor and diode for every led.

author: Whales
author_rank: Super Contributor
timestamp: June 13, 2025, 07:05:25 am
content: EDIT: ROFL this is the same as the BCR421U's internal equivalent circuit. I swear I didn't look!Whilst I'm here: another option that has a much higher tolerance to voltage range. Should work fine at your minimum of 4V.Explanation: The two diodes provide an approx 1.2V reference. the NPN transistor is operating as a common-collector amplifier (pretend the LED is a short circuit and that VCC is instead lower by about 3V because of the LED). The amplifier is controlled by the 1.2V reference, which means it tries its best to keep 0.6V across the 100-ohm resistor at all times, which creates the 6mA setpoint.Costing if you are hand-assembling: lots more than a single part that does all the current regulation for you.Costing if you are getting a big Chinese board house to assemble this (example): 13 SMD solder joints + 6 SMD parts placed + cost of parts ~= $0.05USD per LED (+ setup costs). Not joking, the PCB surface area will probably cost more than parts and assembly.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2025, 07:15:49 am
content: Quote from: tunk on June 12, 2025, 10:01:11 pmThere isn't enough voltage headroom for that to work well.A 3.3V LDO regulator (voltage mode) with suitable current limiting resistors for the LEDs might be good enough.

author: Perrold
author_rank: Contributor
timestamp: June 14, 2025, 11:43:58 am
content: For low voltage applications, I've used a JFET as a constant current source to drive LEDs (among other things). No issue going down to effectively the diode Vf. Standard disclaimer on the wide spread of JFET parameters applies.

author: MarkF
author_rank: Super Contributor
timestamp: June 14, 2025, 01:56:18 pm
content: I have used a LM334 to drive low power LEDs.Their minimum operation voltage is 1V.The ISET= 67.7mV / RSETapprox. up to 10mA.I've found that modern LED's intensity doesn't vary much over their current range. Only dimming at the low current end of the range. So, I stopped using a regulator and just calculated a resistor for the max voltage the LED would see at a current of 80% (or even 50%) of the LEDs max. Which for my model railroad is 4V to 12V at 5mA for 10mA LEDs.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 14, 2025, 09:48:51 pm
content: This is the classical 2 BJT constant current source / sink. Any beginner with electronics should analyze and build this circuit at least once.

author: Benta
author_rank: Super Contributor
timestamp: June 14, 2025, 09:57:54 pm
content: Quote from: Doctorandus_P on June 14, 2025, 09:48:51 pmBit of an overkill using a BD139 for 10 mA, I'd say.The circuit is not without risks as drawn. It's very likely to oscillate in the MHz region, a series base resistor for the output transistor is highly recommended to quench that.

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2025, 11:09:37 am
content: I would suggest the LM385 plus a couple of resistors and a transistor. It has about 1.5V of headroom so is only suitable for red, orange, yellow or old greenish-yellow LED. It's no good for blue, white or pure green.