Wiring Solenoids in Series for voltage drop

Discussion in 'General Electronics Chat' started by masa6614, Mar 16, 2016.

  1. masa6614

    Thread Starter Active Member

    Nov 9, 2008
    49
    0
    Hello,
    I'm wondering if it's an acceptable practice to wire up two solenoids in series to drop the voltage across each?
    I've got an application where I need to control two solenoids, however, they are each only rated for 12VDC continuous, and I've only got 24V available. Each solenoid requires 10W, so instead of complicating the design with a separate DC/DC or using a string of high power resistors to burn away the remainder, I was wondering if I can just wire them up in series and use a single control, since they can both be activated/deactivated at the same time anyways. I'll be sure to use a flyback diode across each. See attached sketch for an idea of what I'd like to do, I'm wondering if I may be missing something or if this is an acceptable practice. Thanks!


    SERIES_SOLENOIDS.png
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Not usual practice but should work in a pinch as long as the coils are identical.
    Max.
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,974
    3,220
    You only need one diode across both of the coils.
     
  4. mcasale

    Member

    Jul 18, 2011
    210
    12
    As Max said above, this will work as long as there's not too much variation between the coils.

    You should check the spec sheet of the coil to figure the largest DC resistance variations, calculate worst case voltage drop (lowest), and verify that it is within spec for coil activation.

    How fast are you switching the solenoids on & off? If it's slow (once in a blue moon), you don't have to worry about inductance effects.

    Is this a hobby project, or are you designing a product for sale?
     
  5. masa6614

    Thread Starter Active Member

    Nov 9, 2008
    49
    0
    Thank you guys,
    This is for a product for sale, which is kind of the reason I didnt want to add another power supply (may effect EMI for CE/FCC compliance) .
    Maybe I can look into adding a smaller resistor in series to account for variations between the two. The solenoids are the same part number from the same vendor, so shouldn't be too much variation.

    The switching duty cycle is very low. The solenoids control stowing pins of a rotating system, so when no power is applied, the system is locked (stowed) and cannot move. When operation is needed, the solenoids are energized which unlocks the stow pins allowing movement.
     
  6. mcasale

    Member

    Jul 18, 2011
    210
    12
    Even though they're the same part number, there can still be variations.

    The job of the designer is to ensure it all works properly under worst case conditions, including minimum supply voltage and maximum coil variations.

    Never ASSume.
     
  7. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    Since this is for a commercial product, why can't you use 24 V coils? It's one thing if that's all you have handy for a one-off item, but for a product....?

    Also be aware of the failure modes -- if one coil fails open then the other coil won't energize. Maybe that's a problem or maybe that's a good thing.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
    2,364
    There is no need to add a resistor, there should be enough hysteresis in the pull in and release point, (easy to test) to allow for minor differences.
    Max.
     
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