Current measurement for my new LED strip light doesn't look correct - Page 1
Forum section: electronics / projects

author: DW1961
author_rank: Frequent Contributor
timestamp: September 20, 2020, 06:12:13 am
content: 12V RGB strip lights-----------I finally finished my custom length and sectioned LED strip light but soldering some cuts together.Before I did that, I tested a couple feet of a strip that I was using to create my own, and when I did the current test, the reading in Amps was exactly like the calculation I did using .21-.24 watts for a single 5050 SMD wide open max wattage.After I finished soldering my custom strip together, I tested in for amps wide open and then did the theoretical calculations. They are now off by quite a lot.New strip is 39 5050 SMDs. There are 7 solder points connecting the RGB +12V wires. The entire length is 39" and has about 12" of new 22 AWG wire in it for the connections.Math:39 SMDs x .225 watts = 8.77 watts= .73 A @ 12VWhat I got current testing was:.82 amps.So now I'm a little worried I've done something wrong.

author: ledtester
author_rank: Super Contributor
timestamp: September 20, 2020, 07:15:19 am
content: Well - it's not like it's way off: .82A * 12V / 39 = .252W per LED.What RGB LED strips are we talking about? Even a link to a product page would help.QuoteWhat exactly was the test set up in these cases and what did you measure?

author: DW1961
author_rank: Frequent Contributor
timestamp: September 20, 2020, 05:59:08 pm
content: Quote from: ledtester on September 20, 2020, 07:15:19 amWhen I did the original calcs, I uses .21 watts per SMD and the amps when tested were spot on. I have only found one source saying .24 watts for each SMD. The others said .21. However, from .21 to .25 is an increase of 19%. That's pretty significant.I measured current with a power supply, no controller between the power supply and strip for both measurements, and the LEDs were all 100%--all three RGB lights on 100%.Here is picture of the strip. They are just Chinese LED strips. I have no idea who the manufacturer is.

author: Ian.M
author_rank: Super Contributor
timestamp: September 20, 2020, 08:21:06 pm
content: I would suspect you are feeding power to your assembly of custom length LED strip sections at more than one point hence the discrepency.The copper tracks in the original un-cut strip aren't very heavy so there is considerable voltage drop along the length of it if you only feed it from one end. This results in LEDs at the far end drawing less current.If you aren't going to be using it in this configuration, feed the strip from both ends when testing it. That eliminates 3/4 of the voltage drop, (half the length per feed point so half the resistance, and also half the current) and will get you much closer to the current per LED when using short lengths.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 21, 2020, 12:54:02 am
content: Quote from: Ian.M on September 20, 2020, 08:21:06 pmOn both measurements, I just hooked up the meter at the beginning of the strip at one point only. The measurement difference was that I now have cut the section into multiple smaller sections, and soldered them together with 22 AWG wire.I can test a non-sectioned piece with the same number of 5050 SMDs and see how that measures.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 25, 2020, 07:15:02 am
content: Quote from: ledtester on September 20, 2020, 07:15:19 amOk, so I finally got around to testing again. I did the same test, but this time on a fresh strip with no solder joints. It is just one short strip with 39 5050 SMD on it. It's the same number of SMDs my custom strip has, but no solder joints except where I soldered on 3" wires to the pads to do the current test.Using .21 Amps per SMD(1) Math calculation for 39 SMDs: .68A(2) Custom strip, 39 SMDs: .82 Amps(3) Section of new strip, 39 SMDs: .78 AmpsWhere is the resistance coming from? I only have about 12" of new AWG 22 wire in the custom strip, plus the solder joints. I'm going to bed. I'll think on this some more before posting again. I'm starting to think the strip itself has different resistances due to quality issues. Easy to test for that. I have enough to make a few cuts and test them for current.

author: DW1961
author_rank: Frequent Contributor
timestamp: September 26, 2020, 07:40:15 am
content: So, tonight I decided to test the current on another section of the same strip with the same SMD number as above, which is 39. However, after soldering the wires to the pads, I was getting a short between the 12+ pad and one of the other pads that I soldered wire on, two pads away from the 12+. I took a magnifying glass and looked and looked and could not see any bridges or stray pieces of wire or anything. I cleaned it again with 99% iso Alcohol and still couldn't see anything.What must have happened is the pad somehow made contact with the other pads underneath the solder joint, but I don't know how. I had to kinda push down on it, but it did have a short.Thinking aback, my new strip did that on one pad, and I made sure there were no stray tiny wires or anything else between any of the pads, and tested again. I didn't get any continuity and so I triple checked it and pricked all over the joints, but not continuity where it should not have been. I called it good. But now I'm doubting the new strip is sound, even though I ran it with current and had no problems, except for why this thread was started (higer than calculated amps). After tonight, I'm scared to use it. I'm doubting all of my work on this new strip now.I read somewhere that when you cut an LED strip, sometimes the cutting alone can compress the strip pads, and they can short that way. Can anyone explain how or what may have happened tonight?Anyway, after checking and double checking I pulled up on the 12+ wire, and part of the pad and the wire came off. It was like it was just delaminated. The other connections I also tried to pull them part, trying to understand where the weak link was, but they held firmly until the strip itself started to tear.What the hell am I doing wrong?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: September 29, 2020, 01:06:58 pm
content: I don't think there is anything going wrong.The differences you see look normal to me.Such LED strips usually have 3 or 4 LED's in series, and a small series resistor for current balancing and limiting. The LED's have a relatively constant voltage of around 10V or so, and the resistor just 2V or less. Dissipation on the resistors is wasted energy.If you connect the LED's to a lab power supply you will see there is no Light from the LED's below this knee voltage of around 10V, and above 10V current and brightness increases rapidly. It could well be that the current already goes below your expected current if you lower the voltage to just 11.8V.