5v to 12v 30A relay help

Discussion in 'The Projects Forum' started by remout, Apr 26, 2011.

  1. remout

    Thread Starter New Member

    Apr 18, 2011
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    I want to control a 12VDC up to 30Amp circuit with a 5v pin off of a microcontroller. My thoughts are to use a relay.

    I know how to code the micro controller to set a pin HIGH. Is a relay the right approach? If so, where could I get such a relay?

    Thanks!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    A logic-level MOSFET would also work, and could be a better choice if you have frequent switching. If you're well below 1 Hz, a relay is a reasonable choice. Relays and MOSFETs can be scavenged from old electronics, or bought new from any parts supplier such as Mouser, Digikey, and many others. Even the Shack might have what you need.
     
  3. mcgyvr

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    Oct 15, 2009
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  4. remout

    Thread Starter New Member

    Apr 18, 2011
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    Okay, thanks! My circuit is low frequency, like at the push of a button.

    I now understand why it's not wise to drive the relay off the micro controller. I'll use a transistor instead.

    What would this type of relay be called or described as?

    Thanks!
     
  5. mcgyvr

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  6. remout

    Thread Starter New Member

    Apr 18, 2011
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    Thanks!

    So my micro controller sends 5v to the transistor. How do I get the transistor to send 12v to the above relay (in the datasheet it reads 7.8v minimum)?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    You can probably use the same power supply that you are switching, the 12v. Connect one end of the relay coil directly to that supply, and the other to the drain of a FET (or collector of a BJT). The gate of the FET goes to your controller, and the source pin goes to ground. You could use a low power FET to power the relay, but you'd need a "big" (high current) one if you decide to skip the relay altogether.
     
  8. remout

    Thread Starter New Member

    Apr 18, 2011
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    Ah the light finally come on.... I get it, Thanks!

    http://www.sparkfun.com/products/10213

    I really appreciate you guys walking me through this :)
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    be aware that stated current ratings on Mosfets are really based on having the perfect heatsink.. Which you are unlikely to replicate...
    30 Amps through that sparkfun mosfet you just posted will destroy that thing faster than you can blink.
     
  10. wayneh

    Expert

    Sep 9, 2010
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    Just to elaborate, Rds = 0.047 per the datasheet. At 30A, power dissipation is I^2•R = 900•0.047 = 42.3W

    So yes, it'll toast quickly without a LOT of sinking. May not be practical.

    A IRFS4010 MOSFET has an Rds of 0.0039Ω, knocking the heat dissipation down more than 10X. Of course it may actually cost more than a relay. (And it's not logic-level.)
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    Personally, since you seem to be relatively green on the vine it might be wise for you to go the relay and relay driver rout. Far less chance of you making smoke! It will also eliminate the next issue that would have been high side or low side switching. ;)
     
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