Testing LED light strip - Page 1
Forum section: electronics / projects

author: DW1961
author_rank: Frequent Contributor
timestamp: August 08, 2020, 06:37:21 pm
content: I posted this in another thread, and have another question. I was going to post it in that thread, and now I can't remove this, so well, here it is.When I test the strips I want to build before installing them in my computer (plugged into the RGB 12V riser), I want to test the strips, of course, for any shorts. But I also want to test it for maximum current draw. So I'm going to just bypass any controllers that come with the strips I am cutting up.One of you explained how to do it for each color on the strip. That was well taken.However, if I want to test the strip for maximum current with all RGB diodes on max, should I create a circuit with my MM, and then connect the positive side to the 12v supply, and then tie the RGB cathode sides together on teh neg side to get a maximum current reading? That way the RGB will be operating at maximum current, and I'll know exactly how many amps so as not to let the smoke out of my motherboard's 2A RGB header.So something like :driver 12V+ -->MM--> to 12V+ side of stripdriver neg to all three RGB leads on strip??

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2020, 07:41:57 pm
content: The current meter should be in series with the power supply, connected to the LED strip, with the red, green and blue sections connected in parallel. It doesn't matter whether the meter is on the positive, or negative side of the power supply.

author: DW1961
author_rank: Frequent Contributor
timestamp: August 08, 2020, 09:13:36 pm
content: Quote from: Zero999 on August 08, 2020, 07:41:57 pmSo, just like I said? And thanks for the meter positioning, as that does make perfect sense.Can you think of anything else I may want to check on the strip to prevent frying my Motherboard fets?

author: ozcar
author_rank: Frequent Contributor
timestamp: August 08, 2020, 10:35:19 pm
content: Quote from: DW1961 on August 08, 2020, 09:13:36 pmIn one of your other threads somebody already suggested you could include a fuse in your wiring.And, a reminder of something else also mentioned in that thread: When measuring current with a DMM, start with the highest range on the meter, particularly if you suspect there could be some problem.Of course, if you had a bench power supply as was discussed in another of your threads, to test the strip all you would have to do is set the voltage to 12V, set a reasonable current limit, connect the strip and then check the display on the power supply.

author: RJSV
author_rank: Super Contributor
timestamp: August 08, 2020, 10:54:07 pm
content: Hello, I methodically avoid reading current (but who cares...) So, a heavy duty resister, ceramic 0.47 ohms and the like keeps me out of the 'mistake - trouble happened' syndrome. At a half amp, there's only about 0. 235 or two thirty five milli volts of drop.I got impatient, over the years, intolerant of (usually my own) dumb mistakes.I also own a 'shunt' : that's a 1 ohm, 100 watt resistor, soldered with a strip of metal:. like 5 mm by 4 cm (quarter inch wide), with value of 0.012 Ohms !

author: ozcar
author_rank: Frequent Contributor
timestamp: August 08, 2020, 11:07:25 pm
content: Quote from: RJHayward on August 08, 2020, 10:54:07 pmThe first DMM I owned (made by NLS = Non-Linear Systems, an odd name for a maker of hopefully linear-reading meters), could only measure current by using supplied plug-in shunts. That DMM is long deceased, but I'm pretty sure the shunts are still lurking in a dark corner here.

author: DW1961
author_rank: Frequent Contributor
timestamp: August 09, 2020, 01:09:55 am
content: Quote from: ozcar on August 08, 2020, 10:35:19 pmYeah after talking it over in depth, a fuse won't work. There was a specific reason, but I can't remember now what the limitation was. I think it was because the fuse was after the fets and the fets would fry even if the fuse blew.I didn't mean what I can do after installation, but before, to make sure the strip isn't going to let the smoke out for some now unknown to me problem. I'm just trying to stay ahead of the failure curve. I already had a strip take out my RGB 12V fets, or it could have been the motherboard, but I don't want to repeat that. UPS has since lost my motherboard, and I sent it off for repair over a month ago. Hopefully they can't find it and will just reimburse me.Oh yeeeeeah! The bench power supply would be sooooo nice. If you have time, is there a cheap on you would recommend off Amazon? When choosing one, should I favor amps over voltage or voltage over amps? For instance, I see two right now and one is 60V and 5A and one is 30V and 10A.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2020, 10:20:57 am
content: Quote from: DW1961 on August 09, 2020, 01:09:55 amA fuse won't protect the MOSFETs. It should be there to protect the cable, which can cause a bigger mess and fire, than the MOSFET blowing up.Measure the current like this.

author: DW1961
author_rank: Frequent Contributor
timestamp: August 09, 2020, 08:58:10 pm
content: Quote from: Zero999 on August 09, 2020, 10:20:57 amThe strip has 12+ and three LED colors (channels). \12+ R G BSo I have to measure the RGB all together to get max amps. Otherwise, it would just be one color at a time. I was asking if I was doing it correctly, as yuo diagram shows, teh MM is in the circuit, but the RGB circuits have to be tied together.12+ RGB strip"+ driver---DMM--12+ strip --->- driver<-- (together, with 3 wires)--RGB side of strip<--

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2020, 09:21:49 pm
content: It doesn't matter whether you measure the current of all of them separately and add the currents together, or all simultaneously.

author: ozcar
author_rank: Frequent Contributor
timestamp: August 09, 2020, 10:06:05 pm
content: You have mentioned "driver" several times in this thread. I hope you mean "12V power supply", and not something that can flash or dim the LEDs.Quote from: DW1961 on August 09, 2020, 08:58:10 pmThe manual I could find for your motherboard simply says "2A". If I had written something like that, I would have made it 100% clear whether that was 2A in total, or 2A per channel. In absence of something more definitive, I would take it as meaning 2/3A max for each individual channel (which is more strict than saying 2A in total).So, while you are about it, check the total and each individual channel. Not that I really expect there to be a big difference between the channels, but a little bit of sanity checking is not a bad thing. The total should be the sum of the three individual readings (provided your power supply keeps the voltage constant).At one point you mentioned using only 2.5ft of strip, which you said was 36 LEDs. That does not add up, as the strip you got has 60 LEDs per meter, so 2.5ft of it would be 45 LEDs, but either way that should be less than 1A total.Quote from: DW1961 on August 09, 2020, 08:58:10 pm?To get the LEDs to light up, the positive side of the power supply has to go to the common 12+ on the strip, and the negative side of the power supply has to go to the R, G, and B lines on the strip (clearly, if you want only one colour to light up, only connect that one line, if you want all three to light up, connect all three of them together and to the power supply negative.Once you can do that, then all you have to do is break one of the connections to the power supply, as Zero999 said either the negative or the positive, and insert your meter in series in that wire.

author: DW1961
author_rank: Frequent Contributor
timestamp: August 09, 2020, 11:06:21 pm
content: Quote from: Zero999 on August 09, 2020, 09:21:49 pmIt would matter for total current because if I measured, for instance, 12+ and the green only circuit, that would not draw full power. They would all (RG&B) have to be on. At least, that's how I understand it.

author: DW1961
author_rank: Frequent Contributor
timestamp: August 09, 2020, 11:17:28 pm
content: Quote from: ozcar on August 09, 2020, 10:06:05 pmYes Driver = 12V PSUIt's 2Amps total. I contact Gigabyte.It was more like 2' but maximum I was going to use was 2.5'. So, between 36 and 45 5050 SMDs.Yep, got that on the current test. Probably going to set that all up tonight and then start cutting and soldering. My motherboard was lost by UPS in transit back to the repair facility and they are now trying too find it. Hopefully they can't find it and will just cut me a check so I can get my F-ing rig back together. In the mean time, I might as well make my strips again, and test them before installation.

author: ozcar
author_rank: Frequent Contributor
timestamp: August 09, 2020, 11:33:02 pm
content: Quote from: DW1961 on August 09, 2020, 11:17:28 pmSo it is OK to draw 2A from the first channel, as long as you draw 0A from the other two?I know that is not your intention - I'm just saying that "2A total" still leaves some doubt as to exactly what it means.

author: DW1961
author_rank: Frequent Contributor
timestamp: August 10, 2020, 01:18:26 am
content: Quote from: ozcar on August 09, 2020, 11:33:02 pmPoint well taken. lol. That's probably why it has a length restriction too. I'm also assuming that the 12v+ side is out toward the edge of the board, but they don't specify that.The length restriction is "maximum of 2 meters." Using a 5050 watts of .21 each max and 300 5050SMDs per 5 meters, 2 meters is right at 2 Amps. So, if you ran all blue, it would be farr below 2 amps for that one channel. I think B and G take less at full power, and R takes a little more more power.