I agree with comments of Spehro Pefhany.
Just wondering, what did that 5V in the subject line come from?, you use 12V in the text.

Even simpler, cheaper and better if you aready have a microcontroller: PWM.

I have been driving magnetic valves by using PWM to get very high efficiency.  Use schottky diode parallel to the coil, and the coil to act as the PWM inductor itself.  Set the frequency to slightly above hearing range...  Start with a few ms full, on, then PWM.

IIRC, 2V RMS was enough to reliably enough hold the 12V valve open in the actual machinery, including vibration.  Orientation matters, BTW!  (Gravity on armature)

Mechanically bistable arangements is not a solution when you want it to fall reliably at power failure, such as an oil furnace valve.

/Morgan
--
Morgan Olsson, Kivik, Sweden

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