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Relay control

Discussion in 'Electronic Design' started by [email protected], Aug 8, 2013.

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  1. Guest

    Hello,

    I need to control four 12V (each 360 ohm coil) relays with a 3.3V IO micro.Due to possibility over voltage on the 12V line I am supplying the relays with 12V 300ma LDO regulator (250ma drop out) which has 3.3V compatible enable pin. Since I need to ON or OFF all four relays in parallal, can I use the regulator's enable pin to switch the 12V power ON or OFF instead of driving the relays through a transistor or FET or a relay driver? Are there anypotential issues with this as most of the relay driving schematics I have seen use drivers?

    Instead of LDO, Is it fine to use DC-DC boost converter like TPS61040/1 andstep up from 3.3V to 12V? I read somewhere that switching converters cannot be used for inductive loads.

    -sridch
     
  2. Guest

    Can go as high as 16V. I am looking at the options whether to use driver transistor / FET is required or control relays using regulator's enable pin.
     
  3. Syd Rumpo

    Syd Rumpo Guest

    There's no reason why you shouldn't use the regulator's enable pin to
    switch power to the relays. Be sure to protect the regulator from back
    EMF from the relays as they switch off.

    Cheers
     
  4. Guest

    Thanks. Each relay has diode for protection from back emf. On the other side, any issue if I use 3.3V to 12V boost converter to drive inductive loads like relays?
     

  5. A diode across the relay actuator coil. Or a transzorb, actually might
    be better.
     
  6. Syd Rumpo

    Syd Rumpo Guest

    I don't see why not in principle, using the switcher's enable pin to
    switch the relays. Four 360ohm relays at 12V is 1.6W and with SMPS
    (in-)efficiency probably around 600mA from 3.3V, so the TPS61040/1 may
    not be man enough.

    As Jan said, you could maybe use the micro as a switcher if you have a
    spare ADC or comparator input, but I wouldn't bother unless you're
    making enough of these (thousands?) such that the component saving
    covers the development costs. Unless you just want to do it for fun, of
    course.

    Cheers
     
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