Choosing a cheap PWM led drivers (to use with usb charger) - Page 1
Forum section: electronics / beginners

author: Jaunedeau
author_rank: Regular Contributor
timestamp: December 26, 2022, 07:55:07 am
content: Hi,I'd like to play making small lamps with one to ten 3V 150mA led filaments (I would probably dim them). For several reasons, I'd like to power them from USB chargers (free and safe are two of the reasons, reusing is another.)Since I don't want to dissipate up to 3W on something people may touch, I am looking for a cheap PWM current source. I may make a custom PCB (with cheap mcu to handle capacitiv touch and pwm for dimming)So I need a led driver that :- is cheap (probably aliexpress)- can output 3V 50 to 1000mA- can be powered with 5V- has good efficiencyThis onefor exemple covers all the point but not the last (no PWM, that means 0,3W dissipated per led filament, at 300°C/W... not what I'm looking for)If I believe the description,this onemight be close to what I am looking for. I says "6V-24V", it will only provide 5V but since I'll only ask for 3V, that might work ? 20-900mA might work for me. But I don't have the schematics (maybe nor the skills) to say if this will really work :- is this really a current source or is it a voltage ? (some description of some drivers on aliexpress are wrong or misleading)- if it is pwm, what will the ouput look like ? There need to be some filtering because if it just PWM between 5V and 0V, 5V might be too much for the LED event if the average current is the one I need.- how much power would it dissipate ?Do you have any suggestion of something that would "just work" ? Is there a plce where I cna ind the scematics or the chips uses in those drivers ?Thanks,W.

author: py-bb
author_rank: Regular Contributor
timestamp: December 27, 2022, 02:39:25 am
content: You don't have to draw 3 watt. If you connect the LED to the rails it should work (unless it's a USB-C thing, they (can) involve negotiation and it's non-trivial) - provided the LED is rated for 5v. Unless I've misread and you want to use a 3 watt LED (bright)If you do want to go the PWM route you want a 555 timer. They can flash and also are just generally great for PWM. You can make something that turns a voltage level into a PWM signal easily. Great little ICs.I'm reluctant to elaborate further as you should satisfy both yourself and I that going that route is what you really intend further but this shouldn't stop you as 555 timer circuits are abundantly found with any search engine

author: Thunderer
author_rank: Regular Contributor
timestamp: December 27, 2022, 04:37:28 am
content: I know you said 5V, but I suggest using a 19-20V laptop supply instead, to take advantage of the booster efficiency if you really need to up the voltage.I use mainly TI LED drivers and boosters. Depending on what you want, there are plenty of chip manufactured for plenty of combination.In case you go for 19-20V power supply, you can make e multichannel LED fixture with a (similar to a) SCT2016 having 6 in series, and several channels.The SMPS will not dissipate a lot, the main hot spot are the LEDs. If you have 150mA LEDs, do not load them more than 40-50mA. They will stay cooler and have a longer life. Of course, helping the LEDs to cool via the PCB is required anyway.

author: Jaunedeau
author_rank: Regular Contributor
timestamp: December 27, 2022, 07:59:29 am
content: > I know you said 5V, but I suggest using a 19-20V laptop supply insteadThis is totally a viable option, but for now I want to play with reusing usb chargers. That's just part of the funAnd this means learning to solve the constraint of 5V psu with ~3V LED.> You don't have to draw 3 watt.Using 3V LEDs with 5V PSU, without PWM I will have to waste and dissipate 40% of the power, and that's too much.> I use mainly TI LED drivers and boosters.Yes, the final version will probably have a custom PCB with a microcontroller, but I still want to learn about using ready-made modules that could work in my case.> If you do want to go the PWM route you want a 555 timer.If we look atthis one, it indeed uses a 555 to generate a PWM waveform, which is fed to the two channels of a 9926A (schematicshere. There are still three problems for me my case :- it can go up to 100%, which will be too much. There is no easy place to add a serie resistor with the potentiometer. I have not done the math yes but it may be possible to replace D9 with a diode with some higher forward voltage to add some off time to he pwm.- it is a voltage source, not a current source, meaning I'll still need some (lower and less heat dissipating) serie resistor- there is no output inductance, so the LEDs even if they have an ok average current will have a too high pick current (so I still need a pretty high serie resistor)What I'm really trying to learn is to which of those cheap drivers would suit my need. For now my best bet is using a dimmable current source (e.g.this driver) which has an output inductance and something else to PWM it. The PWM would solve the problem of dissipation too much power, and I'd still have to make sure PWM is maxed to about 5/3. If modifying the PWM motor controller is not possible, I can directly use an STM32 or RP2040 board to generate and a potentiometer with some code, but the lamp would look terrible with two boards five wires ^^

author: Thunderer
author_rank: Regular Contributor
timestamp: December 27, 2022, 06:05:57 pm
content: You can buy the 555 PWM generator.But build a constant current source, do not think about voltage control of the LEDs.Like this:http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/index.php?topic=worklog&p=5Some theory about current sources:http://www.pcbheaven.com/userpages/LED_driving_and_controlling_methods/?topic=worklog&p=3