Limiting Inrush Current on LED driver - Page 1
Forum section: electronics / beginners

author: andycsmith
author_rank: Contributor
timestamp: January 20, 2021, 07:40:01 pm
content: Hi,I have purchased one of thesehttps://www.tagra-lighting.co.uk/product/24v-premium-tagra-professional-ip67-encased-led-driver-100w-4-16a/LED drivers for a friend to drive some LED Strips and have just recently discovered that the inrush current could be at least 70A, I'm looking to wiring this into the existing lighting circuit, which in the UK would have a 6A fuse so I need to limit the Inrush Current to this to avoid fuses tripping,.Is it as simple as putting a NTC in line, if so can someone recommend one that would do the job or would it be better to use an active current limiter. Any advice would be appreciated.Thanks,Andy

author: TimNJ
author_rank: Super Contributor
timestamp: January 20, 2021, 07:48:28 pm
content: I would not expect a single 70A inrush peak will blow any fuses, unless you have fast acting type installed in your fusebox. 70A is not uncommon for modern switchmode converters in my experience.

author: ledtester
author_rank: Super Contributor
timestamp: January 21, 2021, 09:04:19 am
content: I would just get in touch with the manufacturer:https://www.tagra-lighting.co.uk/contact/They should be able to tell you if the inrush current will be a problem and how to mitigate it -- after all, it's in their interest to help customers install and use their product.

author: Ice-Tea
author_rank: Super Contributor
timestamp: January 21, 2021, 10:04:29 am
content: Not as uncommon as you might thinkResidential fuses are curve C I think:So, even a 10A breaker should easily support that inrush.

author: Jwillis
author_rank: Super Contributor
timestamp: January 21, 2021, 10:42:04 am
content: If your still concerned about inrush current you can put a soft start module between the source and LED driver. A proper soft start doesn't limit the voltage but slows the current until the capacitors charge then a relay closes the circuit for direct power from the source . Much more efficient than tossing in a bunch of NTC's and hoping for the best.Single phase soft starts are fairly inexpensive and can relieve the extra stress on rectifiers from excessive inrush current.

author: Zero999
author_rank: Super Contributor
timestamp: January 21, 2021, 12:16:04 pm
content: When I had a problem with LED drivers blowing the plug-top fuse, I used a simple series resistor to limit the current surge. The voltage drop was low enough, that it didn't heat up much.

author: BrokenYugo
author_rank: Super Contributor
timestamp: January 21, 2021, 05:33:58 pm
content: Nasty inrush current is nothing new in lighting (go measure a cold 100 watt light bulb sometime), I'd try it before assuming things will need modification.

author: ocset
author_rank: Super Contributor
timestamp: January 23, 2021, 08:25:57 pm
content: Quote..you ask an excellent question. But you cannot select a fuse which will avoid nuisance tripping from inrush pulses......because eventually, after so many inrush current pulses, your poor fuse will die. Its just a matter of how many inrush pulses you want to withstand before it dies...and you select the fuse on that basis. But yes, inrush kills fuses...eventually...its just a matter of how long it holds out.Pages 6 and 7 show you how to select a fuse for this...based on the inrush current pulse shape etchttps://www.littelfuse.com/~/media/electronics/product_catalogs/littelfuse_fuseology_selection_guide.pdf.pdfWhat you usually do is calculate the "pulse I^2.t". (using the chart shown in the littelfuse link.)Then, for example, to survive 100000 inrush pulses, you would need a fuse with an I^2.t" value of (100/22) * "pulse I^2.t".Might as well take the worst case pulse at "on at mains peak".And yes, their are piles upon piles of scrapped electronics products , all over the world, which have been scrapped for no other reason than the fuse died after so many inrush pulses......and all because the nice manufacturer (or rather the millionaire middle man who used the manufacturer) didnt want the added cost of an inrush limiter....which could have been as cheap and simple as an "on at zero cross" circuit.In truth, to be kind to the environment, all offline power supplies should start up through a massive inrush resistor, which allows through just enough current to power a mains zero crossing detector". Then, a little micro should switch on a relay or back-to-back fet etc which shorts out the big inrush resistor....and this being done at the zero cross.............____________------------So from the inrush pulse example, (LTspice sim attached, LTspice is free) the I^2.t = 0.5 * 284^2 * 241us = 9.72(as per the littelfuse fuseology example)So, to get a fuse that can withstand 100000 of these pulses, it must have a melting i^2.t of (100/22) * 9.72 = 44.2 A^2.s…So as you can see, the 3.15A version of this (0034.3122) is good for 100000 pulses of inrush at mains peak, at high mains of 265VAC.https://us.schurter.com/bundles/snceschurter/epim/_ProdPool_/newDS/en/typ_FST_5x20.pdf

author: andycsmith
author_rank: Contributor
timestamp: January 26, 2021, 02:40:46 pm
content: Thanks for the responses, I contacted the manufacturer and they advised that if there was only one drive on a circuit then there will not be a problem.

author: MathWizard
author_rank: Super Contributor
timestamp: January 26, 2021, 09:50:32 pm
content: Quote from: Ice-Tea on January 21, 2021, 10:04:29 amPeople/scientists label graphs/etc lots of ways? I'm just wondering if in Germany, or other countires, which axis do they normally have time on (when not doing Cosmology or GR lol) ?

author: ve7xen
author_rank: Super Contributor
timestamp: January 26, 2021, 10:11:39 pm
content: Quote from: MathWizard on January 26, 2021, 09:50:32 pmIndependent variable on x, dependent variable(s) on y. In this case, the time is the dependent variable, so it goes on y axis. This is the same everywhere.

author: hsn93
author_rank: Regular Contributor
timestamp: January 27, 2021, 09:07:53 am
content: Quote from: Zero999 on January 21, 2021, 12:16:04 pma question .. why you didnt use NTC for the inrush .. is there practical reason .. or you just didnt have it on hand ..

author: Zero999
author_rank: Super Contributor
timestamp: January 27, 2021, 10:26:56 am
content: Quote from: hsn93 on January 27, 2021, 09:07:53 amThe running current was so low, NTC wouldn't have made much difference, since it didn't heat up enough.