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Forum Index : Solar : [REQ] reco, cheap NO, 5Vdc 20-25A relays
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| john61ct Newbie  Joined: 07/02/2019 Location: United StatesPosts: 27 |
Looking for recommendations, ideally with purchase links, for an inexpensive Relay (or Contactor), cheap but "good enough" of course! Normally Open, 5V and 20-25A, is all I know for specs, but I'm told that is "too general", maybe hundreds would fit the bill? Problem is, I don't know enough myself to give any more requirements. What sort of questions would help narrow things down? Note, for background info on the overall project, details and "big picture" discussion, please use this thread https://www.thebackshed.com/forum/forum_posts.asp?TID=11115 to keep this one about the relay / contactor functionality only, please. So, it seems "electromechanical relay" is what I'm looking for here, apparently that means a "coil" is involved. Is "metallic contact" an equivalent term? Other "more electronic" options may exist, but off-topic here, other than general short suggestions, so as not to derail. Contactor is an alternative term, but apparently implies higher currents. Are there other differences? How does "SSR" fit in? ________ The power circuit (charge buss) being open/closed will be in the voltage range 2.0-4Vdc. Does that mean a relay rated for 20A @24Vdc would withstand higher currents than 20A because of the lower voltage? Since the project may ultimately involve hundreds of these relays, price is a very important consideration, I doubt we'll be considering the likes of Blue Sea, Eaton, Schneider or ABB here. I am concerned about the lack of precise data sheets for the cheap-Chinese stuff, and apparently overly optimistic ampacity ratings. To start with, looking for the ability to remain reliable over a long lifespan, ideally thousands of cycles opening/closing while withstanding an actual current of 20-25A but to get really cheap units, willing to start prototyping at 6-10A Future versions should be able to go much higher, say 100+Ah, but obviously things then get very pricey. _______ Normally Open is critical, so no (or miniscule) current draw in that state. A relatively high draw while Closed is not a big downside. I guess that means a "latching" type relay is not required? The ideal would be an assurance that device Failure would result in an Open state, but apparently that is difficult if not impossible to find in inexpensive relays. Is a "disconnect relay" a thing? In this case the charging buss will remain powered, the relay's job being isolating and connecting the load from/to that buss. ________ The trigger voltage should be in the 5V range, a microcontroller like Arduino will be sending the open / close signals. Other suggestions welcome on that, ideally one "μc" board or PLC will control up to say 16 of these relays. Obviously low-jitter / realtime response is critical. Maybe the option of using circuit breakers remotely controllable should also be included here? That would include a high-current protection function as well, but the need to manually reset rather than via the μc seems a "safest but inconvenient" trade-off. Thanks in advance for any and all suggestions and constructive feedback. |
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LadyN Guru  Joined: 26/01/2019 Location: United StatesPosts: 408 |
Good thread John! I am hoping the Gurus pitch in but here are a few quick wins: Most "electromechanical relay" use "metallic contact". It's a solenoid that activates a "metallic contact" to make/break a connection. A SSR helps when either the freq. or the physical restrictions of the electromechanical relay would cause problems If they design an electromechanical relay that works 20A, find it and post it here as it's a good candidate! Short answer: NO might be, but we should write down the pricing with a link to the datasheet or product page nonetheless That's ok. We will use those as a price ceiling OK, but what's the price? we should write down the pricing with a link to the datasheet or product page nonetheless I don't think the trigger voltage is of any concern so don't constrain yourself on that. A few cents level translator will take care of it yes, we will need to design a safety backup that's independent of the μC |
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| john61ct Newbie Joined: 07/02/2019 Location: United StatesPosts: 27 |
Starting with this search string 5V-Relay Arduino Beefcake Relay Control Kit can see the relay component itself is: JQX-15F / 005-1Z1 spec sheet the 005 means: Rated Voltage: 5V Coil Resistance: 27Ω ±10% Pick-up Voltage: 3.75V Drop-out Voltage: 0.5V 0.9W required @5V coil dissipation, ~200mA to power the relay Form A 1H contact (SPST-NO) rated for 30A at 240Vac and 28Vdc |
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| john61ct Newbie Joined: 07/02/2019 Location: United StatesPosts: 27 |
I started at Mouser and Digikey, but man just overwhelming. I may come across pricey gear with special characteristics worth noting here, but really, the more **limiting** factors to narrow choices down the better. |
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| hotwater Senior Member  Joined: 29/08/2017 Location: United StatesPosts: 120 |
Why would anyone in the world drive a 20A relay with 5V? |
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