Meanwell MDR 20-15 (20W) says AC Current (typ) is 0.35A at 230V.  What means? - Page 1
Forum section: electronics / general

author: davros1973
author_rank: Newbie
timestamp: August 04, 2022, 08:12:52 am
content: I'm only an occasional electronics enthusiast. Just a programmer for a day-job. Please be gentle.I'm confused! Meanwell DIN power supply ... MDR-20-15 (20W, 15V output). I wanted to estimate if it'd be OK turning it on/off with an Omron G3MB-202P based "solid state relay" ... a photo triac with built in snubber.The Meanwell spec sheet talks about 40A inrush. I was expecting an 80% ish efficient SMPS at 20W full output load to require approx. 25W ... so a steady RMS current of about 0.11A ish or something. I measured 0.13 but the spec says AC Current (typ) 0.35A at 230V.If I can't understand the spec. on this point, maybe I'm not understanding the 40A inrush current either. I'll check with a scope for a 2-channel diff across a series resistor eventually, but just using a 55,000 count multimeter Max function I couldn't see higher than 1.48A RMS. That 40A ... if it exists, must surely last for only a tiny fraction of a cycle even.I'm hoping someone can explain what typical current means in this context. Why is it 0.35A at 230V? Where does that figure come from?

author: Faringdon
author_rank: Super Contributor
timestamp: August 04, 2022, 02:26:46 pm
content: datasheethttps://www.meanwell-web.com/content/files/pdfs/productPdfs/MW/Mdr-20/MDR-20-spec.pdf..the inrush will be the peak current when switched on at mains peak, so its the peak of the current that surges into the input electrolytic cap. 40A isnt unusual, as they often skimp on inrush resistance.Not surprising, because an inrush resistor that can handle regular , enormous short duration surges like inrush is often quite expensive......and very often, customers might only switch the product on twice a year or so anyway.It probably contains an NTC for inrush, so if you test for inrush, then be sure the NTC is cold first....otherwise the inrush will be overly high.The 0.35A of "AC current", must be the "reactive current" into the product.Sub 75w SMPS have no PFC stage, so they have terrible low power factor, and the AC current is therefore high.

author: davros1973
author_rank: Newbie
timestamp: August 25, 2022, 10:53:36 pm
content: Hi Faringdon ... sorry for not acknowledging your helpful reply ... I must have somehow missed notification of it and have only just now noticed revisiting some open tabs in my browser (even I don't keep my browsers open that long but it must have restored or something).What you say makes a lot of sense - thanks for making it clearer for me.I've been terribly busy but I do plan on doing some better experiments to determine exactly what's going on regarding the inrush stuff soon.I was planning on a series resistor and using my oscilloscope but I then realised my brand new oscilloscope is earthed and that it might be a bit ... risky.I tried using a current sense coil thingy (forget name of these things) rated at 5A and I did see interesting traces but I do think the core of the current transformer was saturating and in any case I don't think it was suitable for such transient traces.I did try a series resistor idea as a proof of concept - using an old multimeter just for it's internal current measuring resistance, and using a DC to AC invertor as an earth-isolated source. The idea being with just one channel I could see the trace for normal running current, and then see the peak single-shot event trace for inrush and get a multiplier from that. From memory I can't remember details now, but I think I estimated over 170A of inrush for that ... bearing in mind a short cable from the inverter and I assume some capacitive element to the inverter output? I might be wrong any how and I might be over-looking some other phenomena in my ignorance. I forget the time for that but it was nanoseconds. A tiny tiny tiny fraction of a 50Hz cycle.Since then I've actually bought another oscilloscope - battery-powered this time, and a clamp meter ... and I did try some quick tests running off proper-mains and I was seeing a multiplier between "normal" and "inrush" over a bit of a longer time but still ... I think maybe a millisecond or something, that could potentially fit a 40A ish sort of range. But I do need to do these tests somewhat more rigorously. Screenshots and notes will help lol.I will still try using one of these solid state relays even if initial tests suggest the peak I measured might be out of spec. because maybe the built-in snub circuit will make a difference to the peaks I'm seeing ... especially as I was seeing very transient peaks that might average out a bit put through the solid state relay, and if it works without destroying it after initial testing then I want to rig-up a bank of 8 so I can trial a test algorithm from a Pi just using some I2C expander or something to see if I can break anything with different timings. In practice I want to stagger 8 of these coming on to keep inrush currents manageable. I know modern contact relays are a lot more reliable now - if you can afford them, but, on a more modest budget, I hoped solid state relays might prove reliable enough. Hopefully my experiments will inform me of the plausibility of my plans with the parts I currently possess.Also I'll try setting-up a couple of channels to maybe get a better idea of the power factor in action.--Yeah - I'm using 8 of these SMPS to charge 8x LiFePO4 12V approx. 100Wh batteries that comprise 2x pairs and 1x set of 4 used in parallel via ideal diodes to power a couple of Frost Canyon nucs (2 batteries each), and 4-batterys powering a 300W inverter for my "workstation" set-up (laptop + small back-up computer and 4x monitors between them plus various accessories etc., and a 70TB back-up NAS in the attic) plus a couple of Pi's (pi-kvm etc.) and micro-controllers etc.Ideal diodes seemed the easiest way to implement a double conversion approach with the batteries and chunkier SMPS's. And just having separate AC supplies for the DC isolation for charging the batteries seemed easiest and cheapest way of doing things (via cheap buck/boost converters for CC and CV but with separate DIY microcontroller - control ... actually I want to achieve fractional charging ... e.g. charge to 100% and control partial discharge to hopefully utilise more of the potential calendar life of the batteries).These 8x SMPS were so low-power I just assumed they wouldn't present a problem for a solid state relay with nearly 20x the rated current for normal use although I realised later that wasn't quite true either even disregarding the inrush. Oh well. Best laid plans and all that. This has been a huge time/money project for me in cramped conditions and competing for time with my day-job and sometimes shortcuts don't work out.Nothing's "easy" (and cheap). With in my limited resources I'm trying to set something up that I can depend on with high observability and that I can hopefully repair as things might fail over the next 10 to 20 years, and that's fairly fixed infrastructure and amenable to energy management so I can have a shutdown/startup button for example that'll handle all my VM's and Containers, distributed hardware and network connections and IoT 868MHz channels etc. and so on. I want something that'll scoff at blackouts/brownouts in an intelligent way and ultimately falling back to microcontroller sleep states, and wouldn't bat an electronic eye lid whether I'm home or not as it goes about it's business. Time is money and my skills in this stuff is limited and I don't have much money ... it's hard to get the balance right. I'm so limited in space I have to put all this stuff on a wall. I'm investing in electronics test equipment because there's a lot more stuff I want to build once I've got myself sorted out with these main objectives.I probably sound mad.Ta again for your help.