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I want a relay that can be controlled by a Raspberry Pi and can handle a load up to 42V and 3A DC. I was looking at this: relay which is rated at DC 30V/10A. Although, 30V is less than 42V but I was wondering if this could still work as I found a website that explains how to select a relay:here. According to this, power rating of the relay is 30V x 10A= 300W whereas power rating of the load is 42V x 3A=126 W which is within the limit of the relay. Based on this, does it mean that the relay I am looking at would work?

Ahmed
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    A 30V DC rating is not a 42V DC rating. 48V DC is kind of a telecom standard so you may be able to search for 48V DC relay with appropriate current ratings. Edit : just looked at that second website. YIKES! "a 5 Amp relay rated at 125VAC can also switch 2.5 Amps at 250VAC." Not safely... –  Aug 03 '20 at 17:01
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    To add to Brian's comment: Tempting as it is to think that "power handling" is the key to selecting a relay, it's not how solid state matter physics works nor how mechanical systems depending on solid matter can work. A very thin wire can handle megawatts of power given sufficient voltage. A much thicker wire can only handle a few watts given sufficiently low voltage. It will be arcing and other details that will wear the contacts here. And 42 V isn't as easily handled as 30 V. Focus on exceeding the DC voltage rating (ignore AC ratings) and ensure that it handles at least your current needs. – jonk Aug 03 '20 at 17:06
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    The "Telecom" suggestion by Brian is excellent, by the way!! They may be older, though. Just bear it in mind. We used to use battery packs rated for 24 V and 48 V in telecom and I used to get "tossed" battery packs from the garbage bin behind telecom offices. It's almost certain in my mind that there are still relays rated to switch 48 VDC that should be relatively common and inexpensive, globally. – jonk Aug 03 '20 at 17:09
  • What's your load? If it's inductive, then it's going to give your relay a much more difficult job at turn off, and that's when you'll need your voltage rating. If it's filament lamps, it's gong to be rough on it at turn on. – Neil_UK Aug 03 '20 at 20:54

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The article you linked really neglects some stuff.

The contact voltage rating of a relay is effectively how far apart are the contacts. This limit keeps the relay from spontaneously conducting because the voltage can arc from one contact to the other. Do not exceed the contact voltage rating. The coil has its own separate rating.

The current rating of a relay typically has 3 ratings, sometimes 4. The AC current, the DC continious current, and the DC breaking current. Sometimes the AC breaking current is added. With AC the current constantly switches back and forth. This guarantees that the current will pass zero. When this happes, even if the contact are activly arcing, the current will stop. Typically the AC current rating is the same for continuous current and breaking current. DC is a whole other story. Relay tend to have high DC continuous current but low breaking current. Because DC flows constantly, if an arc forms in the contacts as they open, it may not extinguish. The lower breaking current is to make sure that the arc extinguishes.

vini_i
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    Hi vini_i, you are indeed a relay expert. Now I understand why the spec says two current limits, AC and DC. I never knew about the "breaking current thing. I need to google further. Thank you very much for telling us the interesting relay story. Cheers. – tlfong01 Aug 04 '20 at 01:17
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If in the specs they rate their relay for 30V, there must be a reason. If there is not a reason, then they are stupid because they are publishing lower good characteristics for their product. And this last, would be a good reason to not chose their products :-).

Especially in electrical and electronics fields, specs are a thing to consider well: they are well measured and tested.

If you have 42Vdc, search for a relay rated at least 42 volts (but I would choose something more, expecting perhaps uncontrolled and unwanted spikes).

If you have 3A of current, chose a rating of at least 3A but, again, take some more for safety.

Finally, read the datasheet: you can discover that there are situations where, even if not trespassing a single limit, you are anyway at risk. I don't think this is the case for a (relatively) small DC relay, but who knows?

I took a look at the website you cite that explains how to choose a relay. Nice but, I think, a little incomplete. I didn't try to search but, again: it could be possible to find a relay rated for 48V, 3A, so you think you are ok, and then discover that the maximum switchable power is not 48*3=142 watts but only 100, and that perhaps would not fit your application. Or you can discover that those specs refer to a certain temperature range, while your application goes outside that range...

Relays are quite complicated pieces of hardware; given that they are not very expensive, related to the work they do, you can think to spend something more to stay in the safe side. Don't forget to consider also the coil - bigger relays have bigger coils to drive.

linuxfan says Reinstate Monica
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  • Well, eBay guys come and go, so I don't trust what the eBay guys say. I have been reading Amazon and Aliexpress shops selling 5V relays. Usually they say 110/220VAC 10A, for example, actually they means 10A continuous 24x7. I usually buy from reputable TaoBao (sort of AliExpress) shops. Some of them actually remind their customers that the max is not 10A, but around 6 to 8A, – tlfong01 Aug 04 '20 at 01:13