Dimming multiple LEDs - pre built dimmer? - Page 1
Forum section: electronics / beginners

author: d_brennen
author_rank: Newbie
timestamp: November 25, 2018, 01:25:34 am
content: I'm making a project that has about 100 3528 SMD LEDs and I'm contemplating how to power them. I am comfortable selecting LEDs and resistors to match the source voltage etc but I want brightness control, as the project is an ambient/desk light. Is there a premade do-da that can do duty cycling on a common rail for multiple sets of parallel LEDs? Can I build one with a pot style nob? I'm familiar with duty cycle dimming on Arduino type micros but not for so many LEDs and I'd rather keep this project as "dumb" as possible and avoid micro controllers/programming if possible.TIA

author: mvs
author_rank: Frequent Contributor
timestamp: November 25, 2018, 09:58:27 am
content: Quote from: d_brennen on November 25, 2018, 01:25:34 amNE555 timer can be used as pulse width modulator.You may find premade modules, that are using it. They are typically advertised as PWM motor controller and LED dimmer.https://www.aliexpress.com/item/Mini-DC-5A-Motor-PWM-Speed-Controller-3V-35V-Speed-Control-Switch-LED-Dimmer/32709044080.html

author: Kjelt
author_rank: Super Contributor
timestamp: November 25, 2018, 10:22:47 am
content: Just use current control.First choose your Serial voltage, take a safe value like 24V.Then deduct a "voltage drop" across the regulator and "led-aging" margin of 10-15% so calculate the number of serial leds per string, for instance 6. You have then 15 or 16 paralel strings of 6 leds which is quite a lot. Each string has to have the same serial drop voltage so leds should be from the same batch a d the strings measured equally. You could add a low resistor per string of 1 ohm or lower but tthis should not be necessary with measured equal batch leds.Electronics is the good old LM317 in current mode (see datasheet) with a potmeter.The LM317 can be leading (24V in current to leds out) or trailing (24V to leds then to lm317 current to ground) it does not matter but do calculate max current and take care of the little heat.If your ledstringsvoltage is much lower than the supply voltage you are heating the lm317 and the planetPut a fuse in series for safety and if really extra a heatfuse for 80C or something.

author: Kjelt
author_rank: Super Contributor
timestamp: November 25, 2018, 10:27:45 am
content: Oh yes: If human DC safety is not an issue due to very good isolated and designed construction, you can use larger strings and higher voltages which leads to lower currents.The lm317 is rated for 36V but since it is floating it will only see the difference between its in and output.So if for instance you have a 60VDC supply and your ledstrings has a Vf of 57V the lm317 has a difference of only 3V between its in and output.

author: d_brennen
author_rank: Newbie
timestamp: November 25, 2018, 12:37:11 pm
content: Thanks for the replies. It's a table/feature lamp for my desk, made of thick enamel copper wire with SMD LEDs wired to very, very thin enamel wire (scientific wired I believe it's called, .2mm). Hadn't considered "human safety" as it's just me and my mitts that might get a zingI was thinking of using 9-12v from an adjustable wall wart DC adaptor and appropriate numbers of LEDs in series.The PWM dimmer linked above looks the ticket, there's versions on Amazon UK.I'll get one ordered and experiment with a section of 3/4LEDs, resistor and different voltages. I don't want to drive these hard at all as it's just ambient light rather than a lamp

author: Nusa
author_rank: Super Contributor
timestamp: November 25, 2018, 01:21:01 pm
content: The other approach, since you have so many LED's in the assembly, is simply to vary how many of them are turned on. Turn on every other one, and even at the same brightness, you've dimmed the result. Turn on every 10th one, and you've dimmed the results a lot! Turn on only one LED....how dim is that?

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2018, 08:58:12 am
content: Quote from: Kjelt on November 25, 2018, 10:22:47 amIt's better to use a single resistor per string of LEDs, rather than attempting to match the LED forward voltages. Resistors are cheap.Quote from: Kjelt on November 25, 2018, 10:27:45 am60VDC is actually the upper limit for exposed connections in dry areas, so it would be OK to just use that. The LM317 needs an extra 1.25V on top of the usual drop-out voltage, when used in constant current mode, as it's the voltage across the sense resistor.

author: Kjelt
author_rank: Super Contributor
timestamp: November 26, 2018, 04:25:45 pm
content: Quote from: Hero999 on November 26, 2018, 08:58:12 amYeah I know, that was what I always thought and did. But since 2011 I have looked at about 20+ ledpanel designs from 4 A brand companies and really nobody does this.Selling point is efficiency and serializing multiple panels.There are pro lighting parkinglights with 15yr guarantee between them, just strings of 11 leds serial 3 parallel strings per panel and then 5 panels per light, no resistor.The PCB layout could be part of this since you could argue that copper traces are low ohms resistors

author: John B
author_rank: Frequent Contributor
timestamp: November 27, 2018, 06:00:53 am
content: Quote from: Kjelt on November 25, 2018, 10:27:45 amWith a caveat. Better to keep the entire series voltage under 36V, as if the LM317 has to drop the entire >36V, for reasons like shorting the LEDs or thermal shutdown, the output on the LM317 will go into breakdown and ultimately runaway failure.

author: Zero999
author_rank: Super Contributor
timestamp: November 27, 2018, 09:12:00 am
content: Quote from: Kjelt on November 26, 2018, 04:25:45 pmI suppose manufactures have better access to matched components.It will only improve the efficiency if a switching regulator is used. If an LM317 is used then using current sharing resistors will make no difference to the efficiency.

author: Kjelt
author_rank: Super Contributor
timestamp: November 27, 2018, 12:48:59 pm
content: Agree, the loss in efficiency is in the lm317 case negligable.