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Simplest electromechanical relay circuit

Discussion in 'Electronic Design' started by Don Kuenz, Dec 22, 2013.

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  1. Don Kuenz

    Don Kuenz Guest

    I construct electronics circuits in the same manner that I write
    software. Start with a simple circuit. Debug the circuit, add a wrinkle,
    and then rinse and repeat until a full blown solution emerges.

    For my purposes, the simplest electromechanical relay circuit uses a
    push button switch to energize the coil and close the contacts. It seems
    intuitively wrong to simply connect 24VDC to the coil. It seems that one
    needs to insert at least a resistor in series to keep the coil from
    burning up. Apparently a diode across the coil helps attenuate voltage

    So, is that the simplest circuit? A resistor in series with the coil and
    a diode across the coil?

    You might think that a simple Internet search ought to provide the
    answer. It turns out that my question's too elementary or something.

    Matter of fact, Internet searches provide less useful information with
    each passing day. Search engines routinely ignore the very keywords
    included in a query to winnow the results down. Search engines tend to
    return the same useless information (sans keyword) repackaged by
    websites that want to play the oracle and become everybody's universal
    "go to" page it seems.

    __/ \
    / \__/
    \__/ Don Kuenz
    / \__
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  2. If the relay coil is rated for the voltage applied to it, there should
    be no reason for a series resistor. If the relay coil is rated for less
    than the voltage of your power supply, then choose a series resistor
    such that the voltage applied to the relay coil is within its specified
  3. Phil Hobbs

    Phil Hobbs Guest

    Which will usually resemble a camel. ;)


    Phil Hobbs

    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510

    hobbs at electrooptical dot net
  4. Don Kuenz

    Don Kuenz Guest

    Thank you for your prompt, verified correct, answer. Although I half
    suspected as much, given the length of the coil winding, I was unwilling
    to gamble with one of my four relays (all of which are needed for my
    project) to find out.

    My bench power supply only goes up to 15 VDC. But, my intuition told me
    that 15 VDC might just be enough to energize the coil to the point of
    contact. This time my intuition was correct. 11 VDC is enough to close
    the contacts, which then exhibit hysteresis and remain closed until the
    voltage falls to 3.7 VDC.

    My intuition also tells me that, for safety's sake, to place the relay
    contacts into the (white) neutral side of the AC circuit instead of the
    (black) hot side of the AC circuit.

    __/ \
    / \__/
    \__/ Don Kuenz
    / \__
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  5. Don Y

    Don Y Guest

    Hi Don,

    Chances are, you have a 12V coil.
    Um, no. You want to switch the *hot* side, not the neutral.
    Ditto with fuse placement, "power switch", etc. Always interrupt
    the *hot*, not the neutral.

  6. Don is correct. *Always* switch the hot leg; never the neutral leg.
  7. piglet

    piglet Guest

    burning up. Apparently a diode across the coil helps attenuate voltage
    Or you could use a resistor instead of a diode across the coil. This would limit the transient peak voltage to the relay coil current (as it was just before turn off) multiplied by the resistor value plus of course the supplyvoltage. The disadvantage is the added power dissipation during the relay on time but the technique can be useful if polarity cannot be assured or for AC relay coils. For further refinement add a capacitor in series with theresistor, creating a classic RC snubber, then steady state relay on loss is reduced but the transient is still caught at the instant of turn-off.

    Plenty of other solutions exist too.
  8. Don Kuenz

    Don Kuenz Guest

    The relay case clearly says 24V. It surprises me that a mere 11 VDC
    energizes it.
    Of course! That rule of thumb is second nature whenever I replace wall
    switches and outlets. AC power always spooks me. Something got lost (the
    potential) when I downshifted back home to the low voltage electronics
    domain where things are infinitely safer (for me). :)

    Thank you for all of your great advice. It allows to start my project on
    a sure foot without the nagging doubt of overlooking something basic.

    __/ \
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    \__/ Don Kuenz
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  9. I should qualify that statement. Always switch at least the hot leg. If
    you have a double-pole relay, switch both hot and neutral.
  10. Don Kuenz

    Don Kuenz Guest

    Thank you for your practical advice. One or more of _The Art of Electronics_,
    _The ARRL Handbook_, and _The Digital I/O Handbook_ (co-written by Jon
    Titus if that name rings a bell) says as much. But, none of those good books
    really get into the elementary aspects of relays.

    __/ \
    / \__/
    \__/ Don Kuenz
    / \__
    \__/ \
  11. hamilton

    hamilton Guest

    Why are you re-engineering what has already been engineered ?

    OK, I take it your new to electronics.

    If you want to question the engineering of parts that have been on the
    market for years, get an engineering degree and challenge those existing
    products with your own designs.

    IF your designs are better, you will take over the market.

    It is good to verify those parts that may be questionable, but a relay
    has been on the market for over 120 years.

    You'd think they would have got it right by now.

    On the other hand, people how don't read the specifications will have
    Or those that do not have a reasonable engineering understanding, will
    also have problems.
    ( does not necessary mean a degree, just a good/reasonable understanding )

    Good Luck on your future projects.

  12. hamilton

    hamilton Guest

    This is where a basic/reasonable understand of electronic components
    would be nice.

    Then you would not have to ask such a simple question.

    A relay do not just have contact parameters. ( Contact Voltage/Current )

    There are also timing parameters.

    The speed of contact closure is a parameter that is based on the voltage
    of the coil.

    Think old time relay dialing circuits.

  13. Robert Baer

    Robert Baer Guest

    * One CANNOT "burnup" a relay unless one applies at least twice voltage
    rating for a goodly period of time.
    24V across the coil of a 24V relay can be applied "forever".
  14. Guest

    Good observation, most relay circuits consume far too much power in
    the steady state.

    With a suitable circuit, 24 V industrial relays can be reliably
    operated from a 12 V car battery, provided that the switching cycle is
    not too frequent to allow the capacitors in the circuit to recharge.
  15. Don Kuenz

    Don Kuenz Guest

    You lost me.
    You might want to re-think your feelings. :) I earned an electronics
    merit badge in Boy Scouts over fourty-five years ago. I started
    designing my own electronic circuits in junior high school. I started
    taking EE classes after high school and my first job was with a
    company that manufactured electronic medical devices (brain scanners).

    Now, don't you feel silly? (You ought to even if you don't.)

    The only reason I'm troubling myself to school you is that you offered
    up a good suggestion in another followup. You more or less said told me
    to look at the datasheet. Excellent advice!

    Too bad that this datasheet lacks an example circuit, ergo my post.

    I'm not afraid to ask questions. The only stupid question is the one
    that remains unasked out of fear.

    __/ \
    / \__/
    \__/ Don Kuenz
    / \__
    \__/ \
  16. A reply like that may likely result in no future help from this group.
    An example circuit for an electro-mechanical relay? Shirley, you jest.
  17. Don Kuenz

    Don Kuenz Guest

    Well, that'd be a pity because your earlier answer proved most useful.
    Allow me to apologize for my hasty words. What can I say? hamilton
    "got my goat." It happens.

    __/ \
    / \__/
    \__/ Don Kuenz
    / \__
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  18. Smart move. Good luck with your project.
  19. Don Y

    Don Y Guest

    Hi Tim,

    Find an old electromechanical pinball machine and have a look at
    what switching coils via relay contacts does to the contacts over
    time! You'll be amazed at how much material migrates! And,
    what a PITA it is to service! :<
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