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12v automotive relay current consumption, HOT

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by RaG, Mar 16, 2005.

  1. RaG

    RaG New Member

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    I'm using a 12volt automotive style relay for a custom project.

    The coil resistance of the relay is 80ohms, this generates a decent amount of heat and may reduce the life of the relay?

    The relay will stay on for long durations. How can I reduce the current to the coil and keep the voltage @ 12 volts?

    The coil will switch on with as little as 9v direct from the power source, So i know it doesn't absolutely need 150ma to keep the coil activated.

    The only relays I can find suitable to my price range of a max of $5 a relay have coil resistances of 155 and 80ohms. Needs to be rated for automotive use, @ 20-30amps.

    Any ideas?
    Should I be using something other than a relay? High power Transistor?
     
  2. stevez

    stevez Active Member

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    The short answer to the current reduction question - no, the current drawn is the result of the voltage applied to the coil - it is what it is - Ohm's law governs that.

    While you could do a bunch of things to reduce the heat only the manufacturer has the knowledge and experience on how their relay was designed. I might be reasonable to expect that the relay will run cooler if you reduced the coil voltage by some means but if the contacts, not the coil are what fails then it doesn't matter much. If you lower the voltage you might have a problem keeping the contacts pulled in at all times - it all depends on what is going on in your application.

    It sounds like you are trying to make a low cost item do more than it was designed to do - but we all do that. You might contact the manufacturer to see what they can tell you - maybe what you think is hot is really just cruising along nicely. Unfortunately my experience suggests that it will be hard to find good data on a low cost item.

    Alternatives could be solid state switching or simply a relay rated for your application. Cost and complexity are likely to be issues. If the load can tolerate the performance of the solid state relay it might be a good solution.

    Sorry I don't have the answer - lots of choices and issues.
     
  3. gerty

    gerty Member

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    One thing to make sure of, is that the relay is rated for continous duty, not intermittant duty..
     
  4. dave

    Dave New Member

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  5. David Bridgen

    David Bridgen Member

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    It might seem an obvious thing to say but why not use the 155 ohm relays?
     
  6. David Bridgen

    David Bridgen Member

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    It might seem an obvious thing to say but why not use the 155 ohm relays?
     
  7. Russlk

    Russlk New Member

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    An FET at 30 amps will dissapate a lot of power, so the mechanical relay is probably best. You can get a 20amp relay from Mouser, SPDT, for$2.73. Max coil power is 2.8 watts, shouldn't even get warm. !! I just relized your 80 ohm relay only disapates 1.8 watts; if it is getting hot there is something wrong.
     
  8. k7elp60

    k7elp60 Active Member

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    If the relay coil is getting hot I would think that the voltage is greater than 12V. It has been my experience that a 12VDC coil relay is designed for a maximum of 12 volts. In an automotive application the voltage can be higher than 14 volts. I have also found that most DC relays once energized the coil voltage can be reduced to about half of normal and not affect the holding power of the contacts, and thus the contact resistance.

    A number of years ago before power mosfet's with a low on resistance were readily available, I developed a circuit with a 12 volt power relay that consumed less power than the on resistance of a power mosfet. The trick was to turn on the relay and a few seconds reduce the coil voltage. For one relay it is very practical. Since the circuit used a LM723 regulator the applied voltage could exceed the maximum coil voltage, yet the voltage appied to the coil was 6V after a few seconds.
    If anyone is interested, I will be happy to post the circuit.
     
  9. Rescue1

    Rescue1 Member

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    I use those relays all the time in all my automotive installs.They do get warm even if the coil is energized and no current in flowing through the switched side.It is normal for them and I don't change them on a regular basis.Maybe I find a bad one a few times a year....... :p
     

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