Voltage Divide vs Buck - Page 1
Forum section: electronics / beginners

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 01, 2017, 08:51:27 pm
content: I have the need to power 3 LED strips. Each strip requires 12 volts to operate.I have a 36V dc supply which I would like to use (...actually a main's step_down transformer which goes through a full-wave bridge and some filter caps).My question is:Since I expect to have a fairly constant/consistent xx mAmp draw (from the 3 strips of LED's), should I :Option 1) plug each strip to the 36V dc supply IN series (assuming they will have the same resistance). This assumes they will "divide" the 36v evenly/nicely to 12v per LED_strip each.Option 2) get a switch-mode buck regulator/converter to reduce the 36V down to 12V, and then connect the 3 LED strips in parallel?orshould I be planning some OTHER method to power the three LED strips?

author: Ian.M
author_rank: Super Contributor
timestamp: February 01, 2017, 09:15:50 pm
content: If they are identical strips of the same make + use resistors to limit the LED current, rather than a constant current LED driver chip or circuit, they can safely be wired in series. However your unregulated 36V supply may not be sufficiently stable for good results. N.B. you would not be able to switch them on and off individually.If they use a constant current LED driver, or are not identical or if you need to switch them individually, or your 36V supply output varies too much with load, you will need a regulated output buck converter that can supply the maximum total current of all the strips in parallel. Beware of cheap Chinese buck module current ratings - derate by at least 50%!

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 01, 2017, 10:27:38 pm
content: hmmm, it sounds like the buck converter to parallel LED_Strips might be abetteroption.I have one of these:https://www.amazon.com/CJRSLRB%C2%AE-Adjustable-Regulator-Converter-Voltmeter/dp/B0142CPRUO/ref=sr_1_10?ie=UTF8&qid=1483111656&sr=8-10&keywords=LM2596which are based on theLM2596.Would this be a good choice for such a task?

author: Ian.M
author_rank: Super Contributor
timestamp: February 01, 2017, 10:47:13 pm
content: 40V in doesn't give you much margin above your nominal 36V unregulated supply voltage - any small surge on the line and it will probably blow. It may not even survive being initially connected if the unloaded voltage of your 36V supply is too high. A *GENUINE* LM2596 will usually survive up to 45V in for a short time, however the odds of getting one with a genuine chip when the description is written in 'Chinglish' aren't that good. It claims to be capable of outputting 2A, but I wouldn't trust it over 1A. Also, you don't actually need a built-in meter display.I'd start by checking my 36V dc supply's unloaded voltage, then add 10% allowance for surges. Also check its voltage under load - use a series string of three identical car bulbs if you don't have any other loads that can take 36V at a reasonabe current.Once you have that data you may well find it may not be cost-effective to buy a buck converter that's suitable for it compared to buying a dedicated 12V LED PSU.

author: Seekonk
author_rank: Super Contributor
timestamp: February 01, 2017, 10:48:55 pm
content: Do you have an idea of how much current each of these strips takes. Love those 2596 buck converters, I buy them 10mat a time. I would never use them at at near 2A and certainly not with a 36V transformer. I get skittish when they get above 24V. I'd run it off the raw 36V supply in series with an extra resistor and never at the strips rated current.

author: james_s
author_rank: Super Contributor
timestamp: February 01, 2017, 11:51:39 pm
content: Do the LED strips have any active electronics or are they just strings of 3 LEDs each? If the latter then wiring them in series ought to work, give it a try. Otherwise a buck regulator is the way to go.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 02, 2017, 12:51:07 am
content: Here is the LED striphttps://www.amazon.com/Bright-Aquarium-300LEDs-Waterproof-String/dp/B01K1DAZH8/ref=cm_cd_al_qh_dp_i300 LED's total.I did not see anywhere how many mAmps per LED or a total number of Amps for the entire strip.I do see a comment on ANOTHER/DIFFERENT but similar strip, which also has 300 LED'shttps://www.amazon.com/WenTop-Waterproof-Flexible-Lighting-Bathroom/dp/B01GJ3O0J8/ref=sr_1_1?ie=UTF8&qid=1485996717&sr=8-1-spons&keywords=smd+led&psc=1Someone measured .72 Amps for the entire 300 LED's, however that sounds very low to me, even for small smd led's.

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2017, 01:12:23 am
content: Oh I thought you meant little LED strips with just one series string. Yeah for those big ones I'd say go with the buck regulator to get 12V.

author: Rick Law
author_rank: Super Contributor
timestamp: February 02, 2017, 04:05:24 am
content: Quote from: mikeinfodoc on February 02, 2017, 12:51:07 amre: " .72 Amps for the entire 300 LED's "A close inspection of the photo confirms a reasonable-guess of their organization (for both the SMD5050 and SMD3528):N_Parallel of (3_Serial_LED + Ballast).300 LED strip would be 100_Parallel of (3_Serial_LED + Ballast)That works out to be7.2mA going through each LED. That actually is not too low for ultra-bright LED. The ultra-bright LEDs can generate a fair amount of light at even a couple of mA. When I used ultra-bright for error indicator light (projected to ceiling), I often ballast it down to just a couple of mA or less. Even a a couple of mA, it would be bright enough that I can't look at it direct for long.You actually can run the three strips in series if they are close to exactly the same. But when chips start to fail, it will reach a point that it wont stop until the whole strip is dead. So, I amdefinitely in agreement with run the three strips in parallel(at constant voltage, not constant current).0.72Amp each means = 2.16A for three strings! You need some extra head room also since 0.72A is reasonable but may not be right. Most of the LM2596 boards (real TI or not) are not equipped with good heat sink. That is the most important. Even real TI would not be able to handle the 3A of the chip's max without good heat sink. So hunt for onewith good heat sinkplus perhaps asmall PC fan!I would also suggest looking at the buck with the XL-Semi's XL4015 (5A). I found more of the XL4015 buck boards with better heat-sink provision since it is a 5A chip. I have two XL4015 boards that I run up to 2.7A continuous for days at a time (charging my SLA's) and it is cool enough to touch/hold - with merely a single 1 inch (for rack mount PC or drive-bay) fan ~ 6 inch away. 2.7A just happens to be selected limit. I have not try higher.They look like this:(oh, if you look at the original post with the picture, I actually wrote it down wrong at the time as XL4051 instead of XL4015. Trouble with not being able to read the damn number once the heat sink is on. So when my initial note was wrong, I stayed wrong for a while.)[EDIT... silly, I know. I went back to correct the two year old post and corrected the transposed number, as if any cares... At least, now I have one less thing I must confess to.]

author: Ian.M
author_rank: Super Contributor
timestamp: February 02, 2017, 04:28:41 am
content: If the buck converter fails, there is a high probability the output voltage will rise steeply and take out the LED strips.Unfortunately theXL4015has a lower input voltage rating than a LM2596 - 36V nominal, 40V Abs. Max. so its unlikely to survive being run from an unregulated 36V linear PSU.An individual 2A buck module for each strip may be a better option, *IF* ones with an adequate voltage rating can be found. Alternatively forget about using the 36V supply and scrounge up a used Laptop PSU >60W with an output in the 18V to 24V range to power the buck module(s).Or, much simpler. and probably cheaper, if you don't need voltage control to trim the brightness, get a regulated 5A 12V SMPSU, e.g.https://www.amazon.com/100-240V-Switching-Supply-Adapter-Flexible/dp/B015G8E3X0or similar.

author: Rick Law
author_rank: Super Contributor
timestamp: February 02, 2017, 08:46:24 am
content: Quote from: Ian.M on February 02, 2017, 04:28:41 am+1Individual bucks will be the best to go.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 02, 2017, 11:27:57 pm
content: I was given a box of step down transformers. They have several wires on the primary side, to accommodate various mains voltages, however only the 2 wires on the secondary side, which outputs 24V.After the full wave bridge and cap filters, it raises the DC output voltage. A similar sim screenshot is attached.I was hoping to learn good ways to use these transformers, such as for these LED 12v projects.I wonder if I used my LM2596 buck to push 12v in parallel to the LED strips. I will plan to cut the strips up, so I will likely only use 50 LED's per strip.What if I put a resister between the unfiltered power source and the LM2596 circuit, in order to bring the voltage down a bit.Would that be a decent option?

author: Rick Law
author_rank: Super Contributor
timestamp: February 02, 2017, 11:53:10 pm
content: Quote from: mikeinfodoc on February 02, 2017, 11:27:57 pmre: "What if I put a resister between the unfiltered power source and the LM2596 circuit, in order to bring the voltage down a bit."First, that resister would do nothing as your LM2596 should be adjusted to 12Vout, but that is not the issue at all. If you cut the strip at the indicated cut lines so as not to damage the strip,you ended up with a shorter strip that still requires 12V to operate. Cutting does not change the voltage but the shorter strip will draw less amps.If you cut it down to a point where the strip is worth less than the buck converter for that strip, there may be no sense in that. You may be spending $10 to safeguard $5 (just picking "random" $ for illustration.) Consider using a single stronger buck converter, I would say.

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2017, 11:57:29 pm
content: No, a resistor won't work, if there is ever no load on it, such as a wire comes loose from the output, regulator overheats, etc the voltage will rise up to the full value with no drop across the resistor. A 24VAC secondary should result in about 34VDC once rectified and filtered which is cutting it a bit close for a regulator with a 35V max input but if the absolute max is listed at 40V that ought to still be ok. The regulator boards are so cheap that if it does fail you're not really out much and nobody is going to die.What mains voltages can be selected? If you can set it to 240V and feed it from a 120V line then the output voltage will be half of what it is on the rated voltage. If you have a 240V supply there is probably not much you can do though unless you want to add another transformer to buck the voltage down, if you do so on the primary side the buck transformer can be much smaller.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 03, 2017, 01:03:45 am
content: Excellent. I was wondering about that possibility.Yes, I have standard US mains of 120V in my home.So, using the 240 wire on the primary side sounds like a great option. (instead of using the 120V wire)I will test to verify the output voltage and let you know.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 03, 2017, 02:45:19 am
content: Ok, The primary side has a common white wire. And has 4 colored wires for the following input voltages:120V, 208V, 240V and 480VI connected my mains to white_common and the 240V_orange,and the secondary side was still fed through the FWB and cap filter,and the output was 15.5V.Therefore, I suspect the LM2596 board becomes a viable option again, yes?.This assumes I would keep the LED count low enough to keep the TOTAL amperage through the LM2596 board WELL BELOW the 2A max.Thoughts?

author: Ian.M
author_rank: Super Contributor
timestamp: February 03, 2017, 03:52:32 am
content: That's a bit low, as the voltage will certainly drop under load, and if that voltage was measured no-load, the average input voltage at full load would be 36% lower. To produce a regulated output, the troughs of the ripple need to stay above the desired output voltage with a bit of a margin.TheLM2596 datasheetindicates a dropout voltage of 1V @1A load or 1.5V @3A load, decreasing with temperature. You'd only have 2V headroom for ripple at 14.5V Vin, and your totally FUBARed if 14.5V was the no load voltage, as an average of 9.2V just wont cut it.Try the 208V tap, which should get you about 26V. It also utilises the transformer more efficiently, as, to maintain the same copper losses, you need to derate the transformer's VA rating by the ratio of the nominal input tap voltage to the actual supply voltage. (for Vin<Vnom. If Vin>Vnom you run into problems with core saturation, usually there's a 10% safety margin, but going above Vnom is a *REALLY* bad idea.)

author: james_s
author_rank: Super Contributor
timestamp: February 03, 2017, 04:27:45 am
content: You know, an even better idea there's the LM2596HV which is good for 60V input:http://www.ebay.com/itm/DC-DC-LM2596-HV-S-60V-3A-Buck-Constant-Current-Voltage-CC-CV-Step-Down-Module-/252674671018?If you need more current there are these:http://www.ebay.com/itm/200W-15A-Voltage-Power-Buck-Converter-Step-Down-Module-DC-DC-8-60V-TO-1-36V-ZP-/122087936223?

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: February 03, 2017, 11:45:42 pm
content: Just a quick follow up. The 15V_in to 12V_out through my LM2596 board is working great.I am keeping the amp draw low, by limiting the number of led'sThanks for all the feedback everyone.