Do I need inrush current protection? - Page 1
Forum section: electronics / projects

author: crgarcia
author_rank: Regular Contributor
timestamp: March 20, 2022, 06:36:59 pm
content: I am a beginner and self-taught in electronics. I am building my first PCB to control my home's lights using ESPHome and Home Assistant:link to schematics, diagrams, pictures and moreThe board has 4 relays connected to mains (230VAC) that are then driving a good-brand DIN latching relay. The DIN latching relay is the one that will drive the load (lights or plugs).I don't care at all if my board stop working or if it breaks. I really care if it catches fire, or if it becomes dangerous to people. For that reason I added a 400V varistor and a 1A slow blowing fuse at the mains input.Since this relays activate other latching relays, during normal operation there should not be big current loads. In the worse case I expect around ~70mA x 4 for around 500ms. That is what it takes to activate each of the 4 latching relay.I don't have a lot of space in the PCB, since it should fit in a DIN enclosure. But safety is first!I will build 2/3 of this boards, so component price is not a problem. I want to best and safer instead of the cheapestDo I need an inrush current protection?Is there anything else I am missing?Can I use an NTC Inrush Current Limiter like this one:datasheet:https://www.mouser.ch/datasheet/2/18/AAS-920-325D-Thermometrics-NTC-Inrush-031814-web-1315885.pdfmouser:https://www.mouser.ch/ProductDetail/Amphenol-Advanced-Sensors/CL-90?qs=K5ta8V%252BWhtYTYrFAkIfaLQ%3D%3D

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2022, 06:38:17 am
content: Inrush current is specified.https://recom-power.com/pdf/Powerline_AC-DC/RAC05-K_277.pdfInternally fused too.More important is emissions. Those tiny modules don't have room for a proper common mode filter. See Note 11. They're tested with a floating load resistor, no ground reference. The output will be bouncing along at some voltage, at the internal switching frequency. So if you have any wire length attached to it, or a ground connection, the emissions in both directions (up the mains, and to whatever attached) will be nasty. Something like this will help:https://www.digikey.com/en/products/detail/sumida-america-components-inc/UU9LFNP-B103/813993and probably some 1nF Y1 caps around it, in the usual arrangement. Note that these are safety rated components: the CMC for line-to-line voltage, the Y1 caps for line-to-ground voltage (namely, handling surges of some kV without failure).Tim

author: crgarcia
author_rank: Regular Contributor
timestamp: March 21, 2022, 09:26:24 am
content: Quote from: T3sl4co1l on March 21, 2022, 06:38:17 amThanks Tim for the tips!!Two questions:1) Do you know any better module that have emissions filtering or any other missing safety feature?2) My relays are wet contacts (I think). The output is mains, not the output of the AC/DC convertor - Even if I know that this will be connected to another relay, do I need to limit inrush? (my concern is not if I blow the fuse, but if something can catch fire)Thank you!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 21, 2022, 09:53:42 am
content: I don't know where you got the idea that inrush and fire go together?I think, if anything, it would be a fire hazard only due to thermal cycling of already-marginal contacts, causing a spark on internal wiring that then burns things down? And if the wiring can't handle typical inrush limits (up to 100A or so?), it was probably going to happen sooner or later, and your tiny load isn't going to be any kind of cause. It's just so far down the list, compared to say, shock hazard from insufficient clearance, or fire hazard from insufficiently fused circuits, etc.Inrush is more just a nuisance thing: causes lights to dim, reduces power line capacity, nuisance trips (fuse/breaker), etc.As for others, like the uh, I've tested XP's ECL05 series before; they have onboard filtering, though not enough of it, so expect to need an external CMC or two, and Y caps. IIRC I found good results with a 220uH on the output side; Y cap from AC_N, to DC GND after the CMC; and, I forget if the 10mH on the primary side was optional or required for that one.Relay contacts are just that, contacts. If a load decides to blow up, there's nothing you can reasonably expect to do on this side to prevent it. Wire up well behaved, approved loads and you're fine.Tim