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Funky relay ratings - beware!

Discussion in 'The Hokey Ass Message Board' started by patman, Apr 2, 2009.

  1. patman
    Joined: Apr 30, 2007
    Posts: 561

    patman
    Member

    I installed an electric fan. The fan draws more current than I'd like to see without a relay (somewhere between 10A and 20A) so I picked up a relay while I was out today. I wandered in the store for a while looking for a generic relay, no luck. I bit the bullet and (gasp!) asked a clerk for help. Turns out he actually sort-of knew what he was doing, which was a pleasant surprise.

    I need a 20A relay...got anything generic? He replies "I don't think we have 20's, but I am pretty sure we have 30's." Sure, that will work I say, and he shows me to the right spot in the store with boxes with big lettering "40 Amp Relay" I figure 40A is *really* bigger than I need, but...for $2.99, that will work.

    It's a blister package, so I squint at the printing on the relay itself...30A in the open position, and only 20A in closed. Hmm...so basically the packaging was *a complete lie*, and it's really a 20A relay. It turned out to be exactly what I was looking for in the first place, but I wonder how many electrical meltdowns and fires were caused by this mislabeling.

    Moral of the story: READ THE FINE PRINT!

    [​IMG]
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  2. RichG
    Joined: Dec 8, 2008
    Posts: 3,919

    RichG
    Member

  3. Butch11443
    Joined: Mar 26, 2003
    Posts: 353

    Butch11443
    Member

    The rating is exactly what the package says. Using it as a fan relay you will get a 30 amp rating, which is a usable 24 aqmp under full load..
    Butch
     
  4. patman
    Joined: Apr 30, 2007
    Posts: 561

    patman
    Member

    OK my bad, I swapped the NC and NO definitions, so in a 'close when coil energized' application, it's rated at 30A.

    I agree I'm fine for what I need, but...what about someone who expected to be able to switch 40A? 30A is not 40A.

    Misleading at best...
     

  5. Shifty Shifterton
    Joined: Oct 1, 2006
    Posts: 4,964

    Shifty Shifterton
    Member

    If you actually need to switch 40 amp, you need something like the 2 inch cube sized relays as seen on OEM heavy duty cooling fans. Not those little cheapie $5 "bosch style" units.

    When it comes to switching current size matters. You need physically bigger contacts and stronger springs and coils in the relay. And you need a larger case to dissipate heat and resist the higher forces generated by the stronger switch. That clicking sound is the mechanism inside. Despite being electronic, they're also mechanical. Just like anything mechanical, ask it to do more, and you need bigger, stronger stuff

    good luck
     

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