2835 Led strip from Aliexpress rated for 2A but only drawing 0.7A? - Page 1
Forum section: electronics / beginners

author: nulogic
author_rank: Newbie
timestamp: June 14, 2021, 03:47:06 pm
content: I bought a few led strips from Aliexpress, advertised as 2835s (by the looks of the chips it seems right but I'm a noob).They're 3 chips and a 150ohm resistor per segment, the strip has 100 segments or 300 leds total for 5m length and take 12V dc input. I've looked online and these chips are supposed to be 20mA per segment so a strip should draw 2A but I've measured it and for me, it's only drawing 0.7A. My power supply is rated at 12V 8A output so that's not the limiting factor. What else could it be? Is the multimeter wrong (it's a cheap multimeter from Amazon) or am I doing something wrong?

author: Benta
author_rank: Super Contributor
timestamp: June 14, 2021, 04:06:26 pm
content: I don't think so, your logic is wrong. Each 150 ohm resistor limits three LEDsin series. This fits well to your 0.7 A (2 A / 3).

author: nulogic
author_rank: Newbie
timestamp: June 14, 2021, 04:27:23 pm
content: Oh, I was expecting the multimeter in series with the whole strip to measure the total current drawn by all LEDs combined, is this not the case? If it is then what is the 2A rating for the LED strip really for? I'm mostly just trying to determine wire thickness necessary to drive all the LEDs I need.

author: Manul
author_rank: Super Contributor
timestamp: June 14, 2021, 04:46:11 pm
content: Quote from: Benta on June 14, 2021, 04:06:26 pmI'm interested, how you got this number? OP writes, that there are 100 segments, each with 3 series LEDs and a 150R resistor. So for me it looks like this: lets say LED forward voltage 3V, so LED current and also current of one segment is (12V - 3 * 3V) / 150R = 0.02A (seems about right), and now there are 100 of these segments in parallel, so 0.02A * 100 = 2A. Or I'm somehow blind too?

author: tunk
author_rank: Super Contributor
timestamp: June 14, 2021, 05:01:54 pm
content: A more accurate way to measure the current is to measurethe voltage across the resistor. There's also some voltagedrop along the strip - what's the voltage at the end of thestrip?

author: richard.cs
author_rank: Super Contributor
timestamp: June 14, 2021, 05:03:05 pm
content: Often the voltage drop along the strip can be significant. I have a lot of strips where they don't come anywhere near their rated current if a long section is fed from one end (in my case 5 m of 1.2 A/m rated strip drawing about 2 A rather than 6 A), but if you cut just a short piece it draws the expected current.Try measuring the voltage across one of the 150R resistors near the feedpoint, that will give current for that group of LEDs, and then another one near the other end. Worth also measuring the voltage across the far end and seeing how much less it is than at the source.Possible mitigations include feeding from the middle, feeding from multiple points, but the best one is to feed from opposite ends with a constant current supply (positive in one end, negative in the other, the supply can then wind the voltage up to overcome the drop without any groups of LEDs getting overrun).

author: nulogic
author_rank: Newbie
timestamp: June 15, 2021, 11:15:24 am
content: I took some measurements today and there's a fairly significant voltage drop by the end of the strip, from the start being almost exactly 12V to about 9.5V at the very end. If you look closely you can indeed see a drop in brightness, although it's not huge.So this means the actual power usage is 0.7A? This means, if I use 3 of these, all I need is a wire capable of driving 2.1, i.e. AWG 24 is good enough?

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2021, 04:27:13 pm
content: Quote from: nulogic on June 15, 2021, 11:15:24 amFeeding from the middle will help to reduce the voltage drop.24AWG is the bare minimimum for 2.1A. Whether it's acceptable or not, depends on the cable length. A two core 24AWG cable, carrying 2.1A will drop 353mV/m or 108mV/foot. The current will obviously drop, as the cable gets longer.

author: tszaboo
author_rank: Super Contributor
timestamp: June 15, 2021, 04:38:40 pm
content: Chinese mA is smaller than English mA, and hour is also shorter. Its because they are closer to the equator, so time goes faster there.That's why the rated W, mA, mAh of devices is smaller than what the seller claims it to be.The other possibility is that they are lie about the parameters, but that's impossible now, is it?

author: nulogic
author_rank: Newbie
timestamp: June 15, 2021, 04:48:25 pm
content: Quote from: Zero999 on June 15, 2021, 04:27:13 pmOnce I design my circuit, I will break up the strips into rows and I will feed them all individually from the same end (rows will be 50 cm long) so this will also reduce voltage drop, correct? Although this means I will have to measure the true amperage once I cut the strips to length.And I meant one 24AWG for power and one for ground, so that should be safe?

author: richard.cs
author_rank: Super Contributor
timestamp: June 15, 2021, 05:09:22 pm
content: Quote from: tszaboo on June 15, 2021, 04:38:40 pmLike I said, I've found them to be in-spec for short lengths where voltage drop along the strip isn't significant.

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2021, 05:26:07 pm
content: Quote from: nulogic on June 15, 2021, 04:48:25 pmYes, that should be much better.QuoteIt will be safe, but whether the voltage drop is too high, depends on the length of the cable. The numbers I gave were for two cores: one for +V and another for 0V. You probably want to aim for <500mV of volt drop, so it doesn't affect the brightness too much.

author: Manul
author_rank: Super Contributor
timestamp: June 15, 2021, 05:37:31 pm
content: Quote from: nulogic on June 15, 2021, 04:48:25 pmI see no reason to use the thinnest wire possible. Some (espeacialy no-name) manufacturers are doing that, because they save a few cents from manufacturing cost, for DIY projects does not make much sense. Option one: see how much amps you need, double or triple them, pick the wire, end of story. Option two: decide how much voltage drop is acceptable, select wire gauge based on that.

author: richard.cs
author_rank: Super Contributor
timestamp: June 15, 2021, 05:42:54 pm
content: Quote from: nulogic on June 15, 2021, 04:48:25 pmIt should be fine in 50cm lengths however it is fed, but for reference feeding from one end gives you no voltage drop at that end and max voltage drop the other. If you start to see a brightness gradient then feeding opposite ends fixes it because each LED group sees the same total voltage drop. I wouldn't bother for 50 cm though.

author: nulogic
author_rank: Newbie
timestamp: June 15, 2021, 06:20:53 pm
content: Thanks for all the information/tips.Voltage drop over the wire shouldn't be a problem because the wires are quite short (less than 1m) as it's for an indoor greenhouse, not for ambient lighting. And yes, I was going to buy AWG 20 to be sure but I have plenty of 24 and 22 lying around which means I don't have to bother buying any if they're good enough. The 22 I got off Aliexpress though and I'm not sure if it's actually 22 which is why I wanted to make sure 24 could do it safely.

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2021, 08:17:53 pm
content: Quote from: nulogic on June 15, 2021, 06:20:53 pmThe danger is if the wire is a different metal than copper. If it's made of steel, which is much cheaper, it will have a higher resistance, drop more voltage and could overheat, especially if it has rubbish quality insulation.

author: nulogic
author_rank: Newbie
timestamp: June 15, 2021, 08:36:03 pm
content: Quote from: Zero999 on June 15, 2021, 08:17:53 pmNo it's copper I'm pretty sure.

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2021, 09:13:15 pm
content: Quote from: nulogic on June 15, 2021, 08:36:03 pmHow do you know? Don't go by the colour. It might be copper plate.

author: nulogic
author_rank: Newbie
timestamp: June 16, 2021, 01:09:02 am
content: Quote from: Zero999 on June 15, 2021, 09:13:15 pmI got the 24 AWG off of Farnell, I'd hope they wouldn't lie about the material used.

author: Zero999
author_rank: Super Contributor
timestamp: June 16, 2021, 02:44:17 pm
content: Quote from: nulogic on June 16, 2021, 01:09:02 amI hope so, but be careful. We ordered some 1mm2cable at work, from RS Components and we got 18AWG, which is 0.82mm2.