Convert LED lights to use a dimmable driver - Page 1
Forum section: electronics / projects

author: Garywoo
author_rank: Contributor
timestamp: May 01, 2020, 09:50:01 pm
content: I have bought 4 outdoor LED wall lights to light up the side passageway of my house. They each contain a small constant current LED driver to power the small COB LED array in each light. I'd like convert these to allow for dimming and automation through a single Philips Hue compatible Zigbee dimmer, and could use some advice on the power configuration and LED driver selection.The existing LED driver output spec in each light is currently DC 12-24V - 260mA. I live in the UK so my input voltage is 230v.I think the most cost effective plan is to remove the individual internal power supplies from each light, and run low voltage to each LED from a single separate driver and dimmer. These can be mounted inside so weatherproofing isn't a concern.Firstly, I'm not sure whether I should be thinking about wiring the 4 LED arrays together in series or parallel. Looking at the available compatible dimmers, I've found 3 main types:Constant voltageLeading edgeTrailing edgeI have a basic understanding of these dimmers. I know that the CV one could only be used in a parallel wiring circuit due to the input voltage limitation, and that the leading/trailing edge dimmers have to be used with compatible dimming drivers.For the former, I'd need to drive the LED arrays at their nominal forward voltage, which must be somewhere between 12-24v. I suppose I could cut the wires and measure this. I imagine the maximum current required would be around 1200mW, accounting for the overhead of the dimmer.For the latter, would I be better off with a series or parallel circuit, and what kind of LED driver would be suitable?All advice is appreciated!

author: John B
author_rank: Frequent Contributor
timestamp: May 01, 2020, 10:15:17 pm
content: I'd pass on the mains trailing/leading edge dimming methods. There are much better ways now. Many AC/DC LED power supplies have dimming input controls, either by PWM, analogue voltage or variable resistance.If you want all the lights to be controlled at once, the cheapest method would be a single driver running all the lights in series. Just run 2 core wire from the LED chips back to a single point where your driver is, then wire them in series there. It may use more wire that way, but it allows for future modifications. Maybe earth the metal light case for good measure. Then it's easy to interface the AC/DC supply with either a simple dimming controller, or a larger digital system.Have a look on digikey or element14 etc. Meanwell probably has the largest selection of drivers, but many brands will have models that work.

author: John B
author_rank: Frequent Contributor
timestamp: May 01, 2020, 10:38:25 pm
content: Take something like this for example:https://au.element14.com/mean-well/elg-150-c1750b/led-driver-psu-ac-dc-86v-1-75a/dp/2943970With most of the meanwell stuff, take note of the "B" at the end of the model. If you look at the datasheet you'll see it designates the models with dimming inputs. Also you'll need to double check the actual voltage range of the LEDs you're using, as the varying output currents of different models in the same series also changes the output voltage range they are capable of. Let's say in the worst case, the LEDs are 12V nominal and run at 10V when dimmed, or they're 24V chips. Multiply by 4 and the needed voltage could be as low as 40V or as high as 96V.

author: Garywoo
author_rank: Contributor
timestamp: May 02, 2020, 09:40:20 pm
content: Thanks for the advice John.Firstly, I'm not quite sure how I'd go about controlling such a dimmable driver with the Hue/Zigbee compatible options available to me. What would you suggest?I measured the LED array voltage and it's around 9.1v initially, then drops to around 9.07v. There appears to be 6 diodes in the COB array.I'm thinking a cheap and easy solution would be to wire them up in series/parallel like shown below, then use the constant voltage dimmer with a 18v power supply.What do you think?

author: John B
author_rank: Frequent Contributor
timestamp: May 02, 2020, 10:46:55 pm
content: I don't know much about the Hue or Zigbee systems, but it looks like the first one you listed, the 12/24V DC Zigbee LED controller would be able to interface with a proper driver. Since they were nice enough to include a photo of the PCB, it looks like a simple PWM controller. Instead of using it to power the lights directly, I'd try using the output PWM waveform to control the dimming input of one of the proper constant current meanwell drivers.You'll also need to measure the current of the LED chips. 9.1V was lower than I was expecting, so are they high current? Trying to balance parallel high current chips with resistors will waste power. Luckily if the required output voltage is less than 40V when they are all in series, there's going to be a larger selection of drivers you can use.I never dim LEDs by PWMing them directly any more, especially when they are high current. It generates a lot of EMI.

author: Garywoo
author_rank: Contributor
timestamp: May 02, 2020, 11:32:14 pm
content: I don't think the constant voltage dimmer I posted would work for controlling a dimmable driver with a PWM output, as the Meanwell datasheet specifies a 10V PWM signal. The dimmer operates on a minimum of 12V. Supposing it did operate as low as 10V, I'd then also need a secondary 10V power supply for the dimmer, which is a little inconvenient.I measured the current, and they draw 230mA.If I used the 2x2 series/parallel arrangement, the resistors would only be dissipating about 53 mW which I don't think is too bad. I'm not too worried about EMI in this case as it'll be operating inside a garage. Do you think it would be sensible to go this route?

author: John B
author_rank: Frequent Contributor
timestamp: May 02, 2020, 11:41:14 pm
content: The PWM voltage level is easily shifted, but fair enough it sounds like your power requirements are a lot lower than I was thinking. Sounds like they are only 2W LEDs, and so your solution should work fine.