Electronics > Beginners
Dimming multiple LEDs - pre built dimmer?
d_brennen:
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
mvs:
--- Quote from: d_brennen on November 25, 2018, 01:25:34 am ---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?
--- End quote ---
NE555 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
Kjelt:
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 planet ;)
Put a fuse in series for safety and if really extra a heatfuse for 80C or something.
Kjelt:
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.
d_brennen:
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 zing ;D I 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
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