dimmer for LED strip. - Page 1
Forum section: electronics / beginners

author: robotic
author_rank: Contributor
timestamp: July 11, 2017, 02:27:51 am
content: So I made a light table and want the brightness adjustable. It runs on 12V all white leds (I have ~400 leds inside maybe more or less). It runs at around 2A-3A depending on input voltage.I've controlled some 12V RGB leds using an arduino before using some STP16NF06 (16A 60V) Power MOSFETS for each colour channel.I'm wondering if I need to PWM or something. I found this old dimmer switch (like the ones you use in houses), it might act like a potentiometer and just have a resistance inside. Is this safe to use? Or should I go arduino and pwm it. I would like to use a knob. I have both potentionmeters as well as rotatary encoders that would work if going the arduino method.

author: welpester
author_rank: Contributor
timestamp: July 11, 2017, 03:55:54 am
content: PWM is way to goArduino -> OpAmp -> MosfetDrive PWM to OpAmp thru filter-welpester

author: VoidsWarranties
author_rank: Contributor
timestamp: July 11, 2017, 04:22:19 am
content: Chances are the dimmer switch doesn't act just like a potentiometer, but there might be one inside of it that you could use with the arduino instead of a normal potentiometer. If you use pwm with an arduino be sure to use the pins with the highest pwm frequency(pins 5 and 6 run at 980hz one the uno) and I have gotten head aches from pwm lights so now I add a smoothing capacitor but if you don't get headaches then don't worry about it. I wouldn't bother with rotary encoders unless you need very fine control over the brightness. You could also use a cheap buck converter and use the trim pot to set max voltage and add a separate pot for brightness and they don't use pwm.http://m.ebay.com/itm/DC-DC-CC-CV-Buck-Converter-Step-down-Power-Module-7-32V-to-0-8-28V-12A-300W-USA-/182602817113?epid=1981076948&hash=item2a83f9de59%3Ag%3AbuUAAOSw5cRZMQV-&_trkparms=pageci%253Abac8cb5d-65ef-11e7-ae0e-74dbd1802e6d%257Cparentrq%253A2fdcf5d015d0ab14f7b551eafff9d4b0%257Ciid%253A11This one comes with constant voltage and constant current(they do have inflated ratings so don't use them to full power but I have used them at 150w no problem)If you have any problems or questions feel free to ask.Sent from my iPhone using Tapatalk

author: rs20
author_rank: Super Contributor
timestamp: July 11, 2017, 05:06:32 am
content: Quote from: welpester on July 11, 2017, 03:55:54 amJust a nomenclature correction: what you're describing here is something like this:Arduino PWM out -> RC smoothing filter -> OpAmp buffer -> MOSFET operating in linear mode -> LED stripIn this scenario, the LED strip has a fairly constant current flow through it, and the MOSFET is dissipating potentially quite a lot of heat, and will probably need heatsinking. More to the point, if someone says "PWM it", the arrangement above is absolutely NOT what is generally meant.This is what "PWMing it" looks like:Arduino PWM out -> (sometimes optional) MOSFET driver chip -> MOSFET operating in switching mode -> LED stripIn this case the MOSFET is almost ways fully on or fully off, and thus wastes very little power as heat, and will generally not require heatsinking. The downside is that this approach is potentially much more flickery, especially when digital cameras try to look at it.So yeah, these are two very different options; the first step to helping the OP choose which approach to take is to not horribly conflate the two.

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2017, 07:37:06 am
content: As mentioned above, a house lamp dimmer is more than just a potentiometer. It will most likely be a phase controller, which works by turning on a TRIAC at different parts of the AC waveform and will be completely unsuitable for use with a 12VDC supply.To PWM your LED strip, you could use the potentiometer from the mains lamp dimmer and replace the rest of the circuit with the arduinio.

author: strawberry
author_rank: Super Contributor
timestamp: July 11, 2017, 07:50:57 am
content: How about 555 linear PWM board?eBay auction: #302375968418

author: robotic
author_rank: Contributor
timestamp: July 11, 2017, 09:33:08 pm
content: hmm never thought of just buying a module. So can any Motor Speed Variable pwm regulator switch work?would thismodulework?

author: Zero999
author_rank: Super Contributor
timestamp: July 11, 2017, 10:21:51 pm
content: Quote from: robotic on July 11, 2017, 09:33:08 pmIt will probably work but is quite cheap, so don't expect it to be able to control the rated current. I don't know whether you have enough headroom as your LEDs are 3A and it's supposed rating is 5A: give it a go.

author: strawberry
author_rank: Super Contributor
timestamp: July 11, 2017, 11:25:08 pm
content: @Hero999 Have you tested this board? What control chip on it?

author: tooki
author_rank: Super Contributor
timestamp: July 11, 2017, 11:46:55 pm
content: Search eBay for "LED Dimmer" and similar. You can find all manner of LED dimmers (all PWM based), pushbutton or knob, for $2 or less. The kind that are a pretty inline knob are generally based on a 555 timer, LM339 comparator, and some MOSFET, and handle several amps without breaking a sweat. (They're between 500-1000Hz PWM frequency by default, but I modded some to 2-3KHz to reduce flicker.)

author: tooki
author_rank: Super Contributor
timestamp: July 11, 2017, 11:59:49 pm
content: This is the kind I was referring to, as far as the internals:http://www.ebay.com/itm/LED-Dimmer-Controller-Single-Color-Bright-Adjust-12-24V-For-LED-Light-Strip-/262532980588?hash=item3d202f5f6c:g:UCgAAOSwbsBXjXWcBut these are basically the same circuit, just in a different enclosure with different connectors:http://www.ebay.com/itm/12-24V-8A-LED-Light-Protect-Strip-Dimmer-Adjustable-Brightness-Controller-NEW-TA-/292140683120?epid=1465117143&hash=item4404f10f70:g:v3EAAOSwRoxXmpEQNote that I just picked the first results that look like the ones I've bought; I'm sure you can find them for cheaper.

author: Zero999
author_rank: Super Contributor
timestamp: July 12, 2017, 07:56:47 am
content: Quote from: strawberry on July 11, 2017, 11:25:08 pmNo, I've never purchased anything like that off ebay. The last time I used a PWM module like that, I made it myself.It probably uses the dual comparator IC. I doubt it's a 555 timer because the voltage rating is 3V to 35V, yet a 555 is only rated to 4.5V to 15V. I wouldn't advocate running at 35V continuously though, as the electrolytic capacitor is rated to 35V.http://www.ti.com/lit/ds/symlink/lm393-n.pdfIt has a resettable PTC resistor fuse for over current protection which is an indicator they've thought about the design.

author: robotic
author_rank: Contributor
timestamp: July 14, 2017, 05:38:17 pm
content: Quote from: tooki on July 11, 2017, 11:59:49 pmFirst one i think is limited to 2A. Second one looks promising. A lot cheaper/less time required than me creating one. Cheapest is either bid or 1.69USD which is nice.Didn't think about searching LED dimmer on ebay -_-. Not very smart of me lol.

author: tooki
author_rank: Super Contributor
timestamp: July 14, 2017, 08:18:10 pm
content: Quote from: robotic on July 14, 2017, 05:38:17 pmI don't rememeber off the top of my head what MOSFET was in my examples of the first type, but I seem to vaguely recall that it could handle quite a bit. I've got one of those dimmers on a 12V, 3 or 4A LED circuit and it's had no trouble. My hunch is that, amazingly, this is an item where the Chinese have under-specced it. The other factor, I suppose, is the DC power plugs and the wire used.I'm out of town at the moment, but when I get home in a week, i could open one up and see what MOSFET is inside.The second type is uglier, but probably better in the sense of letting you use better wire.FYI, I've been working on Arduino-controlled dimming at high PWM frequencies to prevent flicker (e.g. 32KHz), using totem pole BJTs to drive the MOSFET harder so that it switches faster and thus heats up less. Results are promising, so if you want a super fancypants dimmer, I'd be happy to share my learnings.

author: tablatronix
author_rank: Regular Contributor
timestamp: October 07, 2017, 03:38:17 pm
content: Quote from: tooki on July 14, 2017, 08:18:10 pmshare away, very interested in some numbers compared to just using an opamp driver

author: Richard Crowley
author_rank: Super Contributor
timestamp: October 07, 2017, 04:47:18 pm
content: Two options:https://www.eevblog.com/forum/blog/eevblog-392-555-led-pwm-hack/http://www.mpja.com/LED-Strip-Dimmer-12-24VDC/productinfo/32760+MI

author: David Hess
author_rank: Super Contributor
timestamp: October 08, 2017, 05:17:30 pm
content: Quote from: rs20 on July 11, 2017, 05:06:32 amIt also generates a rather large amount of EMI. Flicker would depend on the switching frequency.Both problems may be solved by including an LC filter at the output of the MOSFET which may be desirable anyway in one form or another if the LEDs include their own decoupling. A slight modification to the standard buck converter allows for a constant current output and nearly ideal drive for bare or decoupled LEDs.