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I need a fix for sticky solenoids

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Hank Fletcher

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I have some small solenoids that activate (push) at 12V and I'm guessing about 1/3 an amp. I'm using an elastic band to retract them again when they're not energized. They work great if they're only energized for about 1 second or so, but any longer than that and they tend to stay activated (magnetized in pushed position) for about one or two seconds after the power is disconnected. That's unacceptable for my application, and I was wondering what a fix for this might be. If I reversed the polarity on the solenoid for a couple milliseconds, will that de-magnetize it and allow it to retract quickly? Thanks.
 
Hank Fletcher said:
I have some small solenoids that activate (push) at 12V and I'm guessing about 1/3 an amp. I'm using an elastic band to retract them again when they're not energized. They work great if they're only energized for about 1 second or so, but any longer than that and they tend to stay activated (magnetized in pushed position) for about one or two seconds after the power is disconnected. That's unacceptable for my application, and I was wondering what a fix for this might be. If I reversed the polarity on the solenoid for a couple milliseconds, will that de-magnetize it and allow it to retract quickly? Thanks.

Maybe it is back EMF? Have you got a diode connected in parallel with the coil?
 
Hi Hank.

Yes, rubber bands would not be an acceptable solution since they tend to rot quickly.

I've had experience with sticky solenoids in early Sony industrial videotape units. They were the kind with the solid cylindrical plunger. The plungers (piston) were manufactured with a thin invisible coating of teflon for lubrication. After a lot of use, striations would wear into the plungers and the cylinder walls, causing the plunger to stick in the deactivated position.

Other than replacing the entire solenoid, a medium term solution was to mechanically disconnect the plunger and rotate it 180 degrees in the cylinder, then reassemble. That way the worn striations in the plunger would no longer line up with the striations in the cylinder and the plunger would move freely.

Later solenoids in battery operated industrial Beta Sony equipment had a built-in magnet in the capstan solenoid so that the plunger would purposely stick in the activated position under reduced current, thereby saving battery capacity. A quick momentary reverse polarity was necessary to positively disengage this type of solenoid; all timed by the on board system control microprocessor

There ya go with some ideas. I don't know what type of solenoid you actually have, but there are several possibilities.
 
The solenoids were probably designed for activation from AC, not DC. Try using AC then they won't be magnetised as much.
 
A non-magnetic shim of 1 or 2 mils thickness in the magnetic path will release the solinoid fast.
Ofcourse one has to also imagine whether the sticking is not due to dust or any such matrial in the hinges of moving parts.
 
The solenoids were probably designed for activation from AC, not DC. Try using AC then they won't be magnetised as much.
This seems like the most likely cause of the problem, but how can I do this? If I connect each solenoid to an h-bridge, can I just alternate the current with the h-bridge (I'm guessing at 60Hz)? Should the voltage be 12VDC in either direction, or less? Sorry, I don't know much about AC! Perhaps there's a better way to do this?
 
Try PWM on the solenoids, start with full power 100% then reduce to 30% after a fraction of a second. This should keep everything nice and cool and may solve the sticking problem.
 
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