Circuit for controlling a solenoid (cutting off the power supply when extended)

[JBurnie]

Hi all,

I'm currently trying to design a circuit to control a solenoid which will be used in a locking system.

I have an existing circuit running a 12V motor. When this circuit is powered on the solenoid must be in the extended (locked) position. When the motor circuit is powered off the solenoid must retract to the "unlocked" position.

The obvious thing to do here would be to use a 12V push solenoid with a spring return (or similar) and link it directly into the motor circuit. However I'm struggling to find a solenoid which is capable of remaining in the extended position, with power running to it, for the length of time the "lock" is required.

I need to design a circuit which will provide a blip of power to the solenoid to extend it when the motor circuit is turned on, but then cut the power (I'll use a solenoid without a spring return so the solenoid remains in the extended position when the power is stopped). The circuit should then supply a blip of power to the solenoid in reverse polarity when power in the motor circuit is stopped (to retract the solenoid).

Has anyone got any ideas / thoughts on this design? Is there an easier way of doing it?

Thanks in advance,

Joe.

 

[Mr RB]

Reversing solenoids are rare, especially ones that take "reverse polarity". That would require a permenent magnet at the solenoid core.

Normal solenoids only pull, and use a spring to push (ie extend) the armature back out.

Have you looked at door locking mechanicsms as used on most cars? They use a small motor and rack to pull/push the door lock and can run both ways, and also use no power when stationary.

 

[adamey]

What is the problem with the solenoid staying extended? Is it overheating from being on too long?

Some automotive fuel injectors have a low impedance and require high currents to open, but require less current to stay open. Injectors are nothing more than solenoids. Perhaps a circuit to reduce the current a short period of time after the solenoid is energized will help.