RGB Strip has 600 5050 SMD on it, but comes with a 12V 5A PSU? - Page 1
Forum section: electronics / projects

author: DW1961
author_rank: Frequent Contributor
timestamp: July 26, 2020, 05:31:43 am
content: I've posted the question for the merchant, but I'm wondering how you can power two 5 meter 5050 RGB light strips with a 12V 5A supply?The max power for the 5050 is .21 watts, and 600 of them would be 10 A at 12V!!The resisters are 151s on each green and blue diode and 331 on the red. Are hey reducing the power of the 5050s by half+?

author: sleemanj
author_rank: Super Contributor
timestamp: July 26, 2020, 05:49:49 am
content: There are 3 leds in series.Green and Blue we can stab at about 3v VFwd, and that gives 20mA on 150 Ohm Resistor ((12 - (3*3)) / 150)Red we can stab at about 2v VFwd, and that gives about 18mA on the 330 Ohm Resistor, let's round up to 20.So running fully blast all colours at once each set of 3 will require 20+20+20 mA = 60mA600 Divided by 3 is 200200 * 60mA is 12AYeah, you're not going to be running all the leds at once off that power supply.It might be passable to run one of those 5meter strips off the power supply, given you're probably not going to run all the colours at once.

author: MarginallyStable
author_rank: Regular Contributor
timestamp: July 26, 2020, 06:07:06 am
content: If my assumptions are correct from the picture... Looks like three leds in each section are wired in series. A quick look at a datasheets shows typical forward voltages:R = 2.1v G = 3.2v B = 3.2v. So the current per three 5050s would be:(12-(2.1*3))/331 + (12-(3.2*3))/151 + (12-(3.2*3))/151 = 0.049a0.049a * 200 = 9.8a. Yes the power supply is under powered if these are to be powered at 100% duty cycle. Do these have some controller? Maybe there is some multiplexing where they cycle each color one at a time or PWM?

author: DW1961
author_rank: Frequent Contributor
timestamp: July 26, 2020, 06:29:14 am
content: Quote from: sleemanj on July 26, 2020, 05:49:49 amYou know what's really strange? I just did plug both of them into that power supply and controller and they all did actually work, all the way to the end of both strips. WTF? lol, secret Chinese engineering?I measured the voltage of the PSU and it was dead on 12.5v

author: DW1961
author_rank: Frequent Contributor
timestamp: July 26, 2020, 06:31:29 am
content: Quote from: MarginallyStable on July 26, 2020, 06:07:06 amIf it is a PWM, wouldn't that just make the light more dim by decreasing power? I plugged them all in. They actually work. All 600 of them. I didn't do a current test, but they aren't dropping off at the end of the strip. I may do a current test on them. See my next new thread?EDIT TO INCLUDE CORRECT PICTURE!There is a controller, but it's secret what is inside. lol

author: MK14
author_rank: Super Contributor
timestamp: July 26, 2020, 06:46:33 am
content: Quote from: sleemanj on July 26, 2020, 05:49:49 amYour answers technically/theoretically, good and right!But in practice, the long chains of Leds (can) suffer significant, voltage drop (due to current on the PCB tracks etc), as you head towards the end of the chain. Hence the actual currents will tend to be lower, overall.Practical experiment, more explanation and results here:

author: tooki
author_rank: Super Contributor
timestamp: July 26, 2020, 05:48:28 pm
content: Quote from: DW1961 on July 26, 2020, 05:31:43 amThe dirty secret is the really high resistance of the power traces in LED strips. Take one of the strips and connect it to the power supply and turn it on. Measure the voltage at the beginning of the strip, and then at the end. You’ll find a massive voltage drop. This drop, in turn, massively reduces the current drawn by the LEDs. (I’ve seen it go to 2-2.5A instead of the theoretical 5-6A!) To actually get the full current draw of the whole strip, you need fat power wires, feeding into the strip every meter or so. (Even with a single meter, there’s a visible difference in brightness when powered from one end vs both ends!!)Now consider that if the second strip is connected in series, it’s only beginning with the much lower voltage. So its LEDs will draw even less current. (In essence, every trio of LEDs in the whole chain of strips is getting less voltage, and thus drawing less current, than the one before.)

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2020, 05:53:48 pm
content: Can you just measure the actual current draw with a meter?

author: DW1961
author_rank: Frequent Contributor
timestamp: July 26, 2020, 07:36:06 pm
content: I can current test it. and voltage test it too.I have another thread opened asking the best way to test the current from the driver to the controller. I'll post both relevant images.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 27, 2020, 04:31:13 am
content: I voltage tested these tonight. Damn if they aren't 5 VOLTS all the way to the very last set of SMDs. They are running 600 5050 SMDs full voltage with a 12v 5A power supply. WTF?Something not seen in the pictures is that the controller has two connections coming from it, once for each strip. The strips are very bright, but I did not do a current test yet.Typo ed! It was 5V's all the way down the strip.

author: MK14
author_rank: Super Contributor
timestamp: July 27, 2020, 04:44:06 am
content: It's the voltage which will drop, because of losses, in the PCB (flexi) strips, and any wiring you have. So, what is the initial voltage ?What is the voltage, as you move further and further away, from the first led triple. Even a small drop, will somewhat dramatically lower the average current for that set (of 3) leds. Because the leds will subtract their Vf (forward voltages), from the voltage. So, not that much, will remain on the current limiting resistor, e.g. 150 Ohms, which might only have half its usual voltage across it, which would halve the current, for that set of 3 leds.

author: sleemanj
author_rank: Super Contributor
timestamp: July 27, 2020, 05:55:25 am
content: Measure the voltage drop across the "Blue" (151) resistor nearest the power supply, and for the one furthest from the power supply.Do the same for the "Red" (331).Do the same for the "Green" (151).Now you can calculate the maximum current each set of 3 is getting, and the minimum current each set of 3 is gettingMaxI = ( NearestRed / 330) + (NearestBlue / 150) + (NearestGreen / 150)MinI = ( FurthestRed / 330) + (FurthestBlue / 150) + (FurthestGreen / 150)

author: DW1961
author_rank: Frequent Contributor
timestamp: July 27, 2020, 06:08:36 pm
content: I made a typo. It was 5V all the way down both the strips. Each strip is 5 meters.I didn't do a current test. I tested all colors and then set them to WHITE at 100%. Same deal. 5V all the way down, no drop at all. It was actually a little over 5 volts.The two light strips are split at the controller. Both (I assume) receive 5 volts, which is 10 volts. A 20% headroom would be 12V, right? If so, they did it perfectly.I got a reply back from the vendor and he said the resistors in the tape limit overall wattage to 55 watts for both strips."Dear CustomerWe installed a resistor on the light strip to control the overall current of the light strip,So the actual power of our 10m light strip is about 50-55W."Also, just to be clear, each SMD diode has its own resistor. That is, 3 resistors per SMD.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 30, 2020, 04:52:02 am
content: I received a reply back from teh vendeor:"The resisters are 151 and 331 series. We have adjusted the current to optimize the brightness of the lights Limiting the power will not reduce the brightness of the lights."I emailed back asking if they did that in the strip itself or the power supply, but didn't get a satisfactory answer. It seems there is a language barrier, so I'm not going to chase it anymore.What do you all think? Did they do the power reduction in the controller? It must be. And, what do you think about the statement that the brightness won't be affected?I can see what they mean, fro example, if you only have one color on, since that is only one diode in the SMD. I can also understand if you have all three colors on, but they are not calling for maximum power to render the specific color they are asking to render. But with them on all white, the brightness would have to be affected vs having enough amps to drive them to their maximum input?

author: ozcar
author_rank: Frequent Contributor
timestamp: July 30, 2020, 06:51:46 am
content: Quote from: DW1961 on July 27, 2020, 06:08:36 pmIt’s not clear to me where you are measuring the voltage. If you measure it from the common +12V conductor, to one of the G, R, or B conductors, then you would expect to see close to 12V when the strip is on at maximum brightness white.Of course, the controller can reduce the power to the LEDs, but it would probably achieve that by means of PWM. That is, rapidly turning the LEDs on and off (too fast to seem them flicker), and varying the amount of time they are on, vs the time that they are off. Trying measure a voltage in that situation can get a bit tricky ( eg seehttps://www.eevblog.com/forum/testgear/how-a-multimeter-measures-pwm-voltage/).It is possible that the controller does not allow the LEDs to reach their true maximum brightness. You could do a test without the controller, either using another 12V power supply, or with the one you got with the LEDs - you are not going to overload it if you test one channel at a time.Otherwise, what test equipment do you have other than a DMM or two?

author: DW1961
author_rank: Frequent Contributor
timestamp: July 30, 2020, 07:12:48 am
content: Quote from: ozcar on July 30, 2020, 06:51:46 amAll I have is a DMM. I measured the voltage at the end of the strip, both 12+ and the cathode sides of all LEDS when the strip was on 100% and White.Why couldn't they just reduce power to LEDs suing resistors in the controller, and avoid possible RF interference from PWM?

author: ozcar
author_rank: Frequent Contributor
timestamp: July 30, 2020, 07:51:01 am
content: Quote from: DW1961 on July 30, 2020, 07:12:48 amIf that is where you measured 5V, then it does seem that the controller limits the maximum power to the strips.Quote from: DW1961 on July 30, 2020, 07:12:48 amThat can waste a lot of power, and cause heating of the controller. It can also cause colour shift as brightness is varied, but maybe you would not notice that.A lot of DMMs can measure frequency, which would allow you to confirm that PWM is being used, and some DMMs can also measure “duty cycle”, which is what varies as the brightness changes.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 30, 2020, 07:11:21 pm
content: Quote from: ozcar on July 30, 2020, 07:51:01 amSadly, none of mine read Hz or duty.

author: ozcar
author_rank: Frequent Contributor
timestamp: July 30, 2020, 10:06:26 pm
content: Quote from: DW1961 on July 27, 2020, 06:08:36 pmI think you have already all but proved that the controller must be using PWM.The two strips would be connected in parallel - connecting two 12V strips in series would require a 24V power supply, and would go wrong if the two strips were not identical (voltage would not distribute equally across the two strips - your strips may be the same now, but say you decided one was too long, and cut some of it off).If you were to connect those strips (all three colours, and without the controller) to a variable power supply (or, as you more-or-less suggested, to a fixed voltage power supply, in series with a variable resistor), and started with the voltage across the strip at 12V. The strip would be at maximum brightness, and with all three colours on, white. If you were to gradually reduce the voltage, then as you would expect, the brightness would reduce. So how bright would it be at 6V? And, would it still be white? There are some clues further back in this thread.Without a variable power supply, cut one section (three LEDs) off of one of the strips and connect it to one of the 5V power supplies that you probably have lying around.Another thing you could do is simply switch your meter to measure AC volts instead of DC, and repeat your measurement. While you are about it, try measuring the output voltage of the power supply by itself with the meter set to AC and DC, and see if you can understand what you find (clues this time in the thread I pointed you to before).