High amps and lots of very bright LEDs - Page 1
Forum section: electronics / projects

author: NickS
author_rank: Unknown
timestamp: December 12, 2013, 04:54:02 am
content: I've been working on a LED Christmas tree using WS2812 strips and have severely under estimating the power needs.It is supposed to have 12 strands, but those 5 strands are chewing somewhere between 50 - 75 watts which I was not betting on.I am using an ATX power supply but since it has to be under cover (8m away) I am using both 5v and 12v from it and 'assisting' the voltage drop from the 5v with small cheap nasty 5v 5A regulators from DealExtreme.I've had a problem with the regulators fighting with each other. I match their voltages closely via the trimpot, switch both on and they sit there powering up and instantly resetting. I know having two regulators connected together can fight when their voltages are different but I don't quite understand when the voltage is within 0.01v or so.I am intending to do a LED dining table next with the same strips and I really need to figure out the power issues.It will be easier since the PSU's 5v is closer, but there will still be significant voltage drop.Is there any tricks to make regulators play nicely together or do I simply have to keep them separated e.g. powering the ends of separate strips so the resistance of the strip prevents interference? Or is it best just to get one really big regulator and run many wires from it?

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: December 12, 2013, 05:12:30 am
content: My first thought is somehow get rid of the voltage regulators.My next thought is 2 in series.Also you better post some data sheets.And how many ATX power supplies do you have lying around.It might be better to have a local weatherproof enclosure to save a bit of transmission loss.Maybe worth doing a cost benefit analysis to see if moving to a more efficient psu.The I would imagine the standard ATX psu are tuned to low manufacturing cost rather than efficiency. You only have a short time to Christmas but what about next year.

author: NickS
author_rank: Unknown
timestamp: December 12, 2013, 05:50:38 am
content: Quote from: HackedFridgeMagnet on December 12, 2013, 05:12:30 amUnfortunately with 3 meter strips, there will always be massive transmission loss.The 8m power cables from under cover don't help, but these strips need power from both ends otherwise they hit 3v midway down the strip and the rest do funky things.Quote from: HackedFridgeMagnet on December 12, 2013, 05:12:30 amThe ATX actually seems to be running really nicely.The regulators I'm using are these:http://dx.com/p/kim-055l-dc-12v-to-dc-5v-5a-buck-converter-step-down-module-green-227205Don't hate me for using them - they are $3.90 for 5A! And they do actually deliver it in under 1% voltage tolerance even when overloaded - right up to the point they hit about 100C and the thermal kick in stops them.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: December 12, 2013, 06:07:19 am
content: Yeah but "the strips need power at both ends"?Hopefully the strips have a datasheet too.Hopefully they are not massively parallel with one pull up resistor like the Christmas lights I bought once. Where did they go? Oh that's right, in the garbage bin.Quoteno hate here, hey it's Christmas after all.Quoteoh. maybe outside is best.

author: calexanian
author_rank: Super Contributor
timestamp: December 12, 2013, 06:53:52 am
content: If I may comment.... Clark Griswald would be very proud, and envious..... O0

author: MrsR
author_rank: Regular Contributor
timestamp: December 12, 2013, 06:59:41 am
content: Quote from: NickS on December 12, 2013, 04:54:02 amCould you post the specs on the strips you are using.I have a Santa made up of 500 3v 35mA LEDS. Input voltage is 12v.It is made up of 4 LEDS in Series =1 array and a total of 125 arrays in Parallel. This is center fed.A power meter shows a current draw of 4400mA. It should be less than this but I probably have some resistance in the solder joins and leads.If you can run it at 12v that would avoid using the Regs. and save a lot of fiddling.Have FunRachael

author: IanB
author_rank: Super Contributor
timestamp: December 12, 2013, 08:21:43 am
content: Quote from: NickS on December 12, 2013, 04:54:02 amI believe that putting two (or more) voltage regulators in parallel with closely matched voltages is the worst thing you can do. Power supplies have the opposite connection rule from loads: voltage regulated supplies go in series, current regulated supplies go in parallel.If you want to put voltage regulated supplies in parallel, this is what you do. Set all but one of the supplies to a higher voltage than the load needs, say 6 or 7 V. Then set the last supply to the regulated voltage, e.g. 5 V. Then this is what happens: the 6 V supply will deliver as much current as it can in an attempt to reach 6 V. Of course it won't reach 6 V because the load is too heavy for the supply, so it will become current limited. Now the second supply, the 5 V regulated one, will deliver additional current until the load voltage gets up to 5 V. Once this happens the supply will start regulating and keep the voltage at 5 V. So one supply will be in a current limited state delivering maximum current and the other will be in voltage regulation.If you set both supplies to 5 V, then they will be unable to decide which one is taking the constant current role and which one is doing the voltage regulation and they will fight and go unstable.If you need current balancing between the supplies then you need to put a balancing resistor on the output of each of them to separate the voltage regulators.

author: NickS
author_rank: Unknown
timestamp: December 12, 2013, 09:58:04 am
content: Quote from: HackedFridgeMagnet on December 12, 2013, 06:07:19 amCheck the video. These are really nice WS2812 LEDs. They combine a RGB led with a daisy chainable 3x current source. You give it power and RGB data and it handles the rest with zero extra components. They are not plain RGB LEDs.Quote from: IanB on December 12, 2013, 08:21:43 amCan a diode also be used on the output of them? I know they are often used for solar panels.I haven't tried one at 5v and one at 5.5v or something like that. I will see if they run better that way.I don't seem to be able to make their voltage drop however. They are rock solid on regulation up to about 8 - 10A when they just kill the output and start oscillating at about 50 Hz.

author: Psi
author_rank: Super Contributor
timestamp: December 12, 2013, 10:01:10 am
content: Quote from: IanB on December 12, 2013, 08:21:43 amhehe, thats a neat trick.but it assumes the supply cant provide the required current at 7V and will go into C_limit. If it can provide the current it will do and push the voltage up to 7V

author: macboy
author_rank: Super Contributor
timestamp: December 12, 2013, 02:25:57 pm
content: Quote from: IanB on December 12, 2013, 08:21:43 amThe above will work if the supply has good current regulation. It works very well with lab supplies.Another alternative is to have some series resistance on each supply's output. This is not ideal, but since the OP said his supplies are 8m from the load, he could use a separate 8m cable for each supply, and use the resistance of the cable as the ballast resistor. The voltage of each supply can be trimmed to allow you to split the current between them any way you want.

author: macboy
author_rank: Super Contributor
timestamp: December 12, 2013, 02:32:21 pm
content: Here's another alternative: consider using one of those little 5V/5A regulators for every one or two LED strips. Place the regulators close to the load ("POL" or Point Of Load) so that the low-voltage, high-current wiring is limited in length. Run only the 12V from the ATX over the 8m to the vicinity of the tree/regulators. The current over the 12V run will be much less than at 5V, so the voltage drop will be less too.

author: NickS
author_rank: Unknown
timestamp: December 12, 2013, 09:18:43 pm
content: Quote from: macboy on December 12, 2013, 02:25:57 pmThe ATX power supply is 8m away, I have 2x of the smaller regulators at the bottom and 2x at the top of the tree its self.I just can't use all 4, 1 at the top and 1 at the bottom works fine.I might split the bottom two apart so they power different strips. That should assist with the overheating problem.

author: dannyf
author_rank: Super Contributor
timestamp: December 12, 2013, 09:35:17 pm
content: Using ATX is actually a pretty good idea in my view: it is cheap and efficient.But it is bulky and that can be an issue. In those cases, you can use numerous small switching mode regulators - mode them from constant voltage to constant current and run individual strips off of those guys.

author: David_AVD
author_rank: Super Contributor
timestamp: December 13, 2013, 08:12:45 am
content: All you need a single decent 5V power supply and power injection at the start end end of every strip. Strips like these are used by the Christmas light community.The 5V ones pull 50-60mA per RGB LED (pixel). The 12V versions have 3 LEDs in series and pull 50-60mA per 3 LEDs.Expect to have to use a "null" pixel ever few metres of data line to avoid corruption.