LED intensity controlled by voltage level - Page 1
Forum section: electronics / projects

author: okw
author_rank: Frequent Contributor
timestamp: April 14, 2024, 01:55:41 pm
content: Hi! Im' designing a LCD backlight replacement, and want to control the intensity of 20-40 LEDs between two set limits: 6.16V (lowest) and 9.77V (highest intensity). I don't want to mess with the LCD board itself, so these levels are hard spec. How can I control the intensity based on input voltage? I want 6.16V to be lowest, but still readable LCD, and 9.77V to be "almost" too much.I was thinking a cheap MCU with PWM, but is there a simpler way? Space is limited (115x5mm board), I've messed way too much with MCUs over the years, and wanna try a more basic route.

author: langwadt
author_rank: Super Contributor
timestamp: April 14, 2024, 02:01:45 pm
content: >6.16V (lowest) and 9.77V (highest intensity)is that the control input voltage or the voltage to the LEDs ?

author: okw
author_rank: Frequent Contributor
timestamp: April 14, 2024, 02:15:57 pm
content: I just have the GND and the control voltage (6.16V to 9.77V). The voltage is controlled by the LCD boards intensity potentiometer. The original fluorescent backlight has an inverter which gets 6.16-9.77V and steps up the voltage for the fluorescent bulb.I haven't chosen the LEDs yet, but approx 2.8-3.2V (I have to fine-tune the values once I have the board anyway).

author: mariush
author_rank: Super Contributor
timestamp: April 14, 2024, 02:38:41 pm
content: A lot of Diodes Inc led drivers can accept an analogue voltage as brightness signal, but usually it's something like 0.3v to 2.5v for 0% -- 100%For example AL8860 :https://www.diodes.com/assets/Datasheets/AL8860.pdfStep-down driver, Up to 40v input, can do up to 1.5A switch current ... AL8861 and AL8862 also exist, but go up to 1A otherwise similar... oh AL8862 goes up to 55v.Multi-function On/Off and brightness control pin:• Leave floating for normal operation.• Drive to voltage below 0.2V to turn off output current.• Drive with DC voltage (0.3V < CTRL < 2.5V) to adjust output current from 0 to 100% of IOUT_NOM.• Drive with PWM signal from open-collector or open-drain transistor, to adjust output current.Linear adjustment range from 1% to 100% of IOUT_NOM for f < 500HzAL8843 goes to 3A of current :https://www.digikey.com/en/products/detail/diodes-incorporated/AL8843QSP-13/10668306Datasheet :https://www.diodes.com/assets/Datasheets/AL8843Q.pdfMulti-Function On/Off and brightness control pin:• Leave floating for normal operation.• Drive to voltage below 0.3V to turn off output current.• Drive with DC voltage (0.4V < V CTRL < 2.5V) to adjust output current from 10% to 100% of I OUT_NOM.• Drive with an analog voltage > 2.6V output current will be 100% of I OUT_NOM.• A PWM signal (Low level voltage < 0.3V, High level voltage > 2.6V, transition time less than 1μs) allows the output current to be adjusted over a wide range up to 100%Up to you how to reduce your voltage range ... could be something as simple as a bunch of diodes in series ... 1n4007 drops around 0.7v, so just have 10-11 diodes in series to drop 7v... so your 6.16V to 9.77V becomes 0v..2.77v , you'll have plenty of range.Could probably also use an opamp, ...

author: madires
author_rank: Super Contributor
timestamp: April 14, 2024, 04:07:26 pm
content: There's a standard with a 0-10V signal for dimming LED lights. Might be an idea.

author: Zero999
author_rank: Super Contributor
timestamp: April 14, 2024, 05:04:27 pm
content: What forward current?What's the supply voltage?I would use an op-amp to scale the 6.16V to 9.77V range to 0 to 1V across the sense resistor.V2 is the variable input voltage.V3 a fixed 5V reference.The current through the LEDs is 0 to 100mA, as the input voltage is swept from 6.16V to 9.77V.Q1 can be changed for a Darlington pair or MOSFET for higher currents, in which case a frequency compensation capacitor might be required. Another possibility is to duplicate Q1 and R5.Here's the website I used to help me design the circuit.http://earmark.net/gesr/opamp/case8.htm

author: okw
author_rank: Frequent Contributor
timestamp: April 15, 2024, 08:04:47 am
content: Thanks! Looks interesting. But keep in mind, I only have 6.16-9.77V and GND available. I don't have 12V or any other fixed source. Would you recommend I use a 5V LDO to give the opamp a fixed reference and also power the leds from this same regulator (instead of your V1 12V)?And what specs should I look for when selecting the opamp (to make a qualified selection, instead of just experimenting)? I see the LM321 is in SOT23-5 which is a good fit in my 5mm wide PCB/strip.And would you recommend making two identical strips (for left and right side of the screen) with their individual logic? That would distribute the heat generated by the LDO better.9.77V dropped to 3V and 30 LEDs (5-10mA) would generate 1-2W ((9,77V-3V)*(30*0.010A)).Perhaps a PWM LED driver would be better anyway?

author: mariush
author_rank: Super Contributor
timestamp: April 15, 2024, 08:39:15 am
content: You can't tap into a fixed voltage anywhere on the main circuit board? You'd only have an extra wire to route (you already have the ground wire)The existing leds won't be powered by varying voltage, so you must have some power wires or something.If you truly have only 6v or more available, use one of the drivers I suggested and parallel the leds ... the buck regulator in the driver will reduce the voltage to 3...3.2v and provide the string of leds in parallel with the current you set (30 leds x 10mA = 300mA)

author: okw
author_rank: Frequent Contributor
timestamp: April 15, 2024, 08:48:52 am
content: The board only has the two pins out. I could start soldering into pads or pins with ZIF/ribbon cables going to other boards, but I wanted to keep it simple and less invasive.I've already drawn up the AL8860 and think this might be my best bet, both in simplicity and heat management.But designing yourself and experimenting instead of a SoC is always fun.

author: mariush
author_rank: Super Contributor
timestamp: April 15, 2024, 08:59:16 am
content: Can you post some pictures of the boards? Both the one with the leds and the other board (from where wires go to your led board) ?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 15, 2024, 09:04:34 am
content: Quote from: okw on April 15, 2024, 08:04:47 amSo that can provide quite a bit of power ?What output impedance does it have ?You could see if you have enough headroom to drive 2 leds in series ? otherwise you will need a switching current regulator.A TI appnote says thisAs the LED technology has improved, forward voltages have dropped. In many low-power applications, the LED forward voltage is less than 3 V. I

author: okw
author_rank: Frequent Contributor
timestamp: April 15, 2024, 09:12:59 am
content: Quote from: PCB.Wiz on April 15, 2024, 09:04:34 amI have to tap between the original inverter and the PCB and see the current flow and voltage drop. The 6.16-9.77V was without load.

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2024, 10:18:21 am
content: Quote from: okw on April 14, 2024, 02:15:57 pmQuote from: okw on April 15, 2024, 08:04:47 amThen how the heck are you going to power the LEDs?You need an external power supply. That potentiometer will not be able to provide enough current to drive the LEDs. This will not work without another PSU. End of.

author: cosmicray
author_rank: Frequent Contributor
timestamp: April 15, 2024, 10:38:22 am
content: Quote from: okw on April 15, 2024, 09:12:59 amPut a load on thecurrentsupply, and measure the voltages under load. LEDs are current devices, the voltage is in a general ballpark, but the current is the most important figure.

author: okw
author_rank: Frequent Contributor
timestamp: April 15, 2024, 10:48:54 am
content: Quote from: Zero999 on April 15, 2024, 10:18:21 amIt's these same two pins that power the DC-DC inverter for the two fluorescent "sticks". And I would assume this is much less efficient that LEDs, so I assume power won't be a problem.

author: okw
author_rank: Frequent Contributor
timestamp: April 15, 2024, 10:55:28 am
content: Quote from: cosmicray on April 15, 2024, 10:38:22 amI'll rather tap between the motherboard and the DC inverter, to see what current the inverter is drawing at it's maximum (to be safe, since the board is designed to output at least that amount). I don't wanna risk frying the LCD motherboard, by putting too much load on it.

author: Zero999
author_rank: Super Contributor
timestamp: April 15, 2024, 12:22:04 pm
content: Quote from: okw on April 15, 2024, 10:48:54 amJust connect the LEDs in strings of two in series, each with a resistor. It'll give you a very dim light at 6.16V and full power at 9.77V.EDIT: Two pieces of 5V LED strip, cut to the same length i.e. number of LEDs, will give very dim to close to full brightness