12v SPDT relay Works only on 12Volts?

Discussion in 'General Electronics Chat' started by Tajiknomi, Apr 26, 2013.

  1. Tajiknomi

    Tajiknomi Thread Starter New Member

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    So i have buy 12V SPDT(5 pins) Relay, & when i Supply 7V to its Coil, it begins to Operate.
    so my question is ....
    1) Is 12v the MAX volts it can handle without damage Or its the Operating volts ?
    2)And what about the MAX current it can handle ?
  2. KMoffett

    KMoffett Senior Member

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    Relays "can" operate at higher voltages than rated...but they will have to dissipate more heat. 12V automotive relays operate up to the maximum voltage of a fully charged lead-acid battery, about 15V. The current capacity depends on the particular relay's contacts. Please post the make and model of the relay.

    Ken
  3. #12

    #12 AAC Fanatic!

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    The last one I looked up would work to 150% of the label voltage.
  4. crutschow

    crutschow AAC Fanatic!

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    The coil voltage rating is the nominal value.

    The max. contact current rating is given in the relay spec.
  5. Mike33

    Mike33 Senior Member

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    Your relay MAY work on 7v, or may not. Likely, it will chatter a little and be unreliable. I'd be surprised if it wasn't.

    Being a mechanical system, they build in a little 'more' than they need to assure that reliability. So staying close to 12V and the rated current is a good idea :eek:)
  6. Tajiknomi

    Tajiknomi Thread Starter New Member

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    Thanks to all, but i need to know whether my relay isn't working correctly COZ it begins to allow current on (NOT Closed) pin at 7v, and on its top, 12v is being printed ,so i want to know how it begins to operate on 7V ?
  7. Mike33

    Mike33 Senior Member

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    Sounds like it IS closing at around 7v. They will do that. Hook up an LED, resistor (330 ohms or so) and a 9V battery to the switch contacts.

    "Bring up" the voltage from 4 or 5V to 10 or 12V if you can, with a potentiometer, and watch for when the LED goes on.

    Just because it allows some current to flow at 7V, doesn't mean it would stay on at that voltage. It may go on and drop out, faster than you can see. If you want to use a lower voltage, you can get a 5V relay, etc.

    Are you trying to make something that has 'precision' switching? You could use a transistor or other device to make sure the relay goes from nearly zero volts on its coil to 10 or 12 V with some certainty, using more of a 'logic level', if you will. If you're using this in a project, post the schematic or state your idea so people can help you do what you're trying to do, the right way :)
  8. Mike33

    Mike33 Senior Member

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    PS - relays are NOT precision devices, they work on an electromagnet...when the coil has induced enough energy to build a magnetic field strong enough to pull the armature down and make contact, it does. That can happen at a lower voltage than you expect. You should hear it when it does this (makes contact) and when it drops out, too.
    I suspect you'll want something to give you more control over your circuit....
  9. #12

    #12 AAC Fanatic!

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    Manufacturers do not build relays that are guaranteed not to work until they get 12.00 volts and melt at 12.01 volts They always have a "range of voltage" in which they will work. It's usually about 70% or 80%, up to 150% of labeled voltage.

    The current handling ability of the contacts is a very separate issue.
  10. strantor

    strantor Well-Known Member

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    on the inside, a relay is like a spring loaded light switch with an electromagnet attached to it. as you increase the voltage to the coil, the current in the electromagnet increases and the magnetic field gets stronger, which brings the contacts closer to touching eachother. At some point (7V apparently) the electromagnetism has increased sufficiently to juuuust barely make the contacts start to touch eachother, and you start to detect current flowing through the contacts. This is not a great place to stop. you need to continue all the way to 12V. You need to have a strong magnetism to hold the contacts firmly together. If you do not (if you leave it at 7V) then you will have sparking and arcing inside between the contacts which will cause them to wear prematurely or possibly even weld themselves together if your load draws enough current.

    You can observe this with a household light switch. If you try to hold the switch in the center position and move it back and forth in tiny increments you can hear the arcing inside; if you remove the cover, on some switches, you may even be able to see the sparks.
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  11. k7elp60

    k7elp60 Active Member

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    I agree with strantor. If the relay doesn't have the normal operating voltage on the coil the contacts are not fully making contact and the contact resistance can be affected. This could cause more voltage drop across the contacts than anticipated.
  12. ErnieM

    ErnieM AAC Fanatic!

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    A relay coil has various voltage ratings:

    Code:
     - Nominal voltage, or the "design" midpoint. 
        5V, 9V, 12V, 15V, 24V are commonly found.
    
     - maximum voltage (so you don't burn up the coil)
     
     - pick up (pull in) voltage: the minimum guaranteed voltage to 
        make the contact close. Obviously, it is designed to work at a 
        lower voltage to insure making the spec.
     
     - release voltage: the maximum voltage a closed contact will release.  
       Again, it is designed to release at a voltage greater 
       to meet this spec.
    I've seen engineers select a 15V relay for a 12V system where they are depending on the maximum pull in voltage to be met by their driver, and also are using the higher coil resistance of the higher voltage rated part to lower the current consumption.

    "Nominal voltage" is only a suggestion. It is the other numbers that make or break the design.
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  13. THE_RB

    THE_RB AAC Fanatic!

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    I'll agree with ErnieM and disagree with you two. ;)

    With relays the voltage is needed primarily for pull-in, because the least efficient magnetic path for the armature is at the start.

    Once the relay pulls-in, the metals parts of the amrature are touching, or very close to touching, and the magnetic path is so much more efficient they only need a fraction of the current to remain closed and keep the contacts closed.

    So operating a relay from a reduced voltage would first start to give the symptom of failing to close altogether, long before you got the symptom of "increased contact resistance".

    Example; 12v relay;
    12v = 100mA coil current
    pull-in current might be 70mA
    hold current might be 30mA
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  14. strantor

    strantor Well-Known Member

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    I'll disagree with myself, and agree with you. At least on the theory part about the efficiency of the magnetic path and actuation current vs. holding current. But I stick to my story about using the proper voltage. In practice, I have witness the things that I described (chatter, arcing, welded contacts) and correlated them with low supply voltage. If I could go back in time I might find though, that the problems were caused by fluctuating supply voltage and not just low supply voltage.
  15. Tajiknomi

    Tajiknomi Thread Starter New Member

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    @Strantor : I got that point now, Coz i have tried another Relay , Which is 6Volts but it active on 3.5V (According to my DMM) But as you have said ,it isn't firm Contact.. So one should make a relay working on its Described voltage, not below that voltage Coz of Arcing..
    Thanks to all for your kind and helpful replies :)
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