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paralleling relay coils

Discussion in 'Electronic Design' started by [email protected], Aug 16, 2013.

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  1. Guest

    I am connecting five 12V relay coils in parallel to switch five different loads. Is it necessary to have fly-back diode for each relay coil or just one 1N400x diode in parallel is sufficient? Each coil is 300 ohms.

  2. Uwe Hercksen

    Uwe Hercksen Guest


    the current through one coil is 40 mA, the five coils take 200 mA. A
    1N400x is good for 1 A. If these five relays are not far away from each
    other, only diode will do in my opinion. The current flowing through the
    diode should not be much larger than the current flowing through all
    coils together.

  3. Guest

    The relays stacked together and trace length is approx 70 mm from 1st relay coil to the 5th relay coil. So, do you think it would be a better option to place the diode across the coil of the middle (3rd) relay?
  4. Guest

    Thanks. Would it be better to use 1A schottky than normal diodes like 1N400x?
  5. Guest

    BCX56 is the transistor. Relay coils are connected between emitter and ground. 12V is applied at the collector.
  6. Nico Coesel

    Nico Coesel Guest

    I always use a transistor or MOSFET with internal diode. No need for
    an external diode.
  7. bud--

    bud-- Guest

  8. Guest

    Here is the schematic I quickly drafted. All the 5 coils and fly back diode are in parallel and will be connected to 12V_RLY_COIL.
  9. I was thinking the same thing with a NPN and the collector tied to the
    12v rail,
    you'd need to take the Base above the 12v rail to turn it on.

  10. Guest

  11. Harry is right.. there are multiple issues.

    1) Can we assume you're trying to drive the relays with a
    3.3V CMOS output, and 3.3V in should turn the relays "on"?

    1a) If so, is the common for the 3.3V supply same as the ground

    2) Any particular reason you want a high-side driver
    (so one side of the relays is grounded)?

    3) Is this an automotive 12V system with the associated
    transient issues?

    4) What's with the 100uF cap across the relays, and the
    0.1uF cap, for that matter?

  12. amdx

    amdx Guest

    Why is it that nobody helps the guy, rather than just letting him
    know you're all smarter than him.

    The more common method to drive a relay is;
    +12v to one side of relay, other side of relay goes to the collector of
    an NPN transistor, emitter of transistor goes to ground. Put a 1k
    resistor in series with base of transistor and apply your 3.3v the 1K
    Install diode across relay.
    Is there a reason you don't want to do it that way?


    *1K value can be adjusted.
  13. Guest

    Could be automotive relays with one terminal at frame ground, then he needs a high side switch.
  14. Guest

    Not at max operating temperature...and depending on relay design, you could be looking at 40oC core temp rise in steady state.
  15. He could use this as a $2 one-chip solution: TL751M12QKVURQ1
    (active low input).

    It's possible to modify the given circuit a bit and make it work okay
    too.. eliminate the caps, connect R68 to the input voltage (and make
    it 10K), reduce R67 to 330 ohms, add Q1 same type as Q3, E to ground,
    C to base of Q3, base to input through 2K. It will have a higher
    dropout voltage than the regulator solution so examine carefully the
    high ambient temperature (at the relay)/low input voltage condition.
    (this is also active low input).

    Best regards,
    Spehro Pefhany
  16. Guest

    Thanks for taking time to draw them. Yes, as you said I am trying to regulate as well as switch the power to the coils. In the fig-1, why is that pnp needed instead of resisting to limit current?
  17. Hi, Phil:-

    Consider a relay such as this one:- Sheets/Tyco Electronics P B PDFs/ORWH.pdf

    Pull in voltage is rated at 8.4V for the 12V nominal relay. That's
    specified "without pre-energization at ambient temperature 23°C" (fine

    If you require the type F insulation (155°C) they offer, then the coil
    resistance could then be more than 50% higher than at 23°C, so the
    guaranteed minimum pull-in voltage would actually be more than 12VDC!

    Somewhat less than nominal voltage is normally okay if the input
    voltage is regulated and/or the environment is fairly benign, but
    that's not always true. Probably sluggish turn-on doesn't help relay
    life either. I don't like unnecessarily to give away voltage in a
    relay driver (eg. by using darlingtons).

    Best regards,
    Spehro Pefhany
  18. Possible pattern-sensitive relay operation frightens and confuses me.

    Best regards,
    Spehro Pefhany
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