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I'm using this SMPS (12 VDC, 12.5 A model) and in my application in steady state the SMPS draws much less than one ampere from the AC line. But as you see its datasheet mentions its cold-start (inrush) current as 45 A. And my relay's max contact rating is 5 A; so without a limiter it might weld.

I will use this relay module:
https://download.mikroe.com/documents/add-on-boards/click/relay/
And that module uses this relay:
https://docs.rs-online.com/356b/0900766b80660fe5.pdf

At this site I came across the following scheme for SMPS inrush:

enter image description here

So I'm planning to use a NTC thermistor for inrush current limiting. I could find this one where one needs to decide its resistance.

In my case V = 230 Vrms and the inrush of the SMPS is 45 A.

What should be the resistance of the NTC limiter in this case? And is there way to interpret from the plots would that limit the current enough?

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  • \$\begingroup\$ You gave power supply datasheet, what about relay? \$\endgroup\$ Jun 1, 2022 at 8:44
  • \$\begingroup\$ If you're not planning on bridging over it with a relay after soft start is done, it's a tradeoff between inrush current and steady-state losses. 5 A won't be trivial. 230/5 = 46 ohm cold resistance. You want to select one small enough that your 150 W load (0.65 A) heats up the NTC significantly. Perhaps a 47 ohm 1 A rated one? \$\endgroup\$
    – winny
    Jun 1, 2022 at 8:47
  • \$\begingroup\$ @winny 5A is the max realy contact current. The steady state SMPS AC current is 0.7A. \$\endgroup\$
    – cm64
    Jun 1, 2022 at 8:51
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    \$\begingroup\$ @TimWilliams I will use this relay modue download.mikroe.com/documents/add-on-boards/click/relay/… And that module uses this relay docs.rs-online.com/356b/0900766b80660fe5.pdf I need to limit inrush current. \$\endgroup\$
    – cm64
    Jun 1, 2022 at 8:52
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    \$\begingroup\$ Yes, I'm aware. And that ~7 times difference won't be trivial to achieve. Get a better relay? \$\endgroup\$
    – winny
    Jun 1, 2022 at 8:53

1 Answer 1

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Evidently the contact material is suitable for higher inrush currents while being less susceptible to welding: https://www.electronicspecifier.com/products/power/relay-design-evolves-to-address-new-applications

Though I'd like a better reference than this. (If nothing else, you can contact Omron directly and see what they have to say. Or check if they have references regarding this.) [Edit: the OP did contact them, but did not quote their response, or alternative suggestions.]

Relays don't specify inrush, or any time-current curves or anything like that, unfortunately. But they may assume typical -- quite nasty -- mains circuit conditions, including closing on capacitive loads or motor inrush, and inductive turn-off arcing. 5A is only the nominal load current, which you're well within.

Edit: from https://www.fa.omron.com.cn/upload/doc/qa/20080708084507281.pdf we have,

  1. OMRON has also prepared the above [G2R series] relays with AgSnIn contacts, which are more tolerant of large inrush currents and physical movement compared with relays with standard contacts. When ordering, add "-ASI" to the model number. Example: G2R-1A-ASI
  2. Standard, NO contact type relays are TV-3 class products in accordance with the TV standards of the UL/CSA. Models with AgSnIn contacts are TV-5 class products. Example: G2R-1A-ASI

No definition of what "TV-" ratings are.

It seems it's sometimes called a "tungsten" rating, because incandescent lamps have considerable inrush:
https://components.omron.com/us-en/products/basic-knowledge/relays/applications

I also see a reference suggesting they were developed for literal TVs (televisions), which used direct rectified mains power for a long time (i.e., hot chassis, mains rectified into large capacitor, huge inrush capacity), so inrush would be a big problem. I don't know that a lot of TVs were relay-operated though. Maybe relevant to early remote-controlled units (e.g. IR or ultrasonic activated relays)?

Possibly, VDE0435, UL508, CSA22.2, VDE0700, VDE0110, IEC 61810, etc. are the relevant standards. None of which are searchable on the internet at present so I'm afraid you'll have to consult someone with access to them to determine from where this rating and terminology actually comes from, and what precisely it entails.

In any case, the type G6R does not specify a "TV" rating, so likely does not handle exceptional inrush; but what exactly the baseline value is, is not apparent without access to these standards.


In absence of exact standards to draw from -- and in absence of a requirement for UL rating of the finished project -- I would be more than inclined to simply use it as-is.

If you must, additional inrush protection can be added, but what size of NTC is required, and what value is acceptable given the supply's ratings, isn't apparent from the datasheet. Replacing the relay with a higher rated one is the more reliable option.

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    \$\begingroup\$ Interesting. This relay has AgSnIn contacts. And this document fa.omron.com.cn/upload/doc/qa/20080708084507281.pdf says: "Models with AgSnIn contacts are TV-5 class products" And it seems TV-5 relays are rated for 5A steady state current and 78A inrush. \$\endgroup\$
    – cm64
    Jun 1, 2022 at 9:16
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    \$\begingroup\$ So seems an NTC not necessary? \$\endgroup\$
    – cm64
    Jun 1, 2022 at 9:17
  • \$\begingroup\$ Also, it already has inrush limiting -- EN61000-4-11 is listed, and they likely use an NTC to do it (else, why note inrush from "COLD START"?). @cm64 that link isn't loading for me right now, but sounds like exactly the kind of reference needed! \$\endgroup\$ Jun 1, 2022 at 10:50
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    \$\begingroup\$ @cm64 Yes, every time; in contrast to, probably some 100s of A when hot, or even a few more if it had no limiting at all. \$\endgroup\$ Jun 1, 2022 at 17:37
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    \$\begingroup\$ I actually contacted the company and recently they replied and told me this cannot handle 45A inrush. \$\endgroup\$
    – cm64
    Jun 13, 2022 at 18:32

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