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solenoid valve....switching problem?

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jit_singh_tara

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Hello friends ,

i have a relay (driven at 5 v) ,driving a solenoid valve at 12 volts .
The problem is that it is in standing position and a metallic rod is inserted in it.when the relay is switched on it switches on the solenoid valve and the shaft is pushed up due to magnetic field, but when the relay is switched off the shaft doesnt comes back quickly , there is some delay in it .this is due to the fact that the current in solenoid is not decaying quickly on being switched off . Please suggest a solution for this problem ,i tried to put 1n4007 in reverse polarity across its terminals but not much use .

i am attaching the pics of the circuit as well as solenoid....

Any other practical ideas are welcome .

Thanks and regards
 

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emontllo

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Hello,

these valves are usually slow... I would have suggested the anti-parallel diode. If this is not enough you may need a demagnetizing circuit. You can drive the solenoid with an H bridge and apply a short negative voltage pulse to accelerate the current decay when turning it off.

Ernest
 

FvM

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There seems to be a misunderstanding of inductive circuit operation. The free wheeling diode actually slows down the current decay, also any parallel load to the coil. The inductor time constant is L/R, reducing the load resistor increases the time constant. Omitting any voltage clamp or load gives the fastest possible decay. To limit the inductive voltage to a reasonable value, you can place a varistor or a high voltage z-diode back-to-back with the free wheeling diode.

P.S.: Magnetic remanence may be an additional reason for slow release of the magnet. It can be in fact reset by an exactly adjusted opposite current pulse in the coil. Finally, simple inertia will slow down the magnet action. You may want to add a spring to accelerate it.
 
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FvM

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There are some points, that can't be clearly seen from your post:
- is the relay contact rating sufficient for the solenoid current?
- can you determine if the relay is acting without any delay

Assuming that the relay is working correctly, your original setup without a diode will result in fast current decay, although some contact arcing occurs, which can reduce the contact lifetime. Placing a diode accross the solenoid coil suppresses arcing but increases the current decay time slightly. The fact that you don't observe a difference between both cases suggests, that the problem hasn't to do with current decay. Nevertheless, fast current decay without strong current arcing would be achieved by allowing a inductive kick-back voltage of the coil of a few 10 volts, using a varistor or a z-diode + rectifier diode combination, as already suggested.

To check, if the solenoid itself is acting fast, you can also check the behaviour when disconnecting a wire.

If curent decay isn't the problem, I mainly suspect inertia of your mechanical setup. Besides changing the design, adding a return spring may be helpful.

I also mentioned magnetic remanence as a possible problem. The solenoid yoke can be expected to be made from magnetically soft iron and not affected by remanence. Using hard steel for the pallet (the coil core) wouldn't be optimal, not necessarily cause a slow release. Applying an oppsosite current pulse of reduced magnitude to reset remanence requires a reverse of polarity and can't be assured to be effective. A return spring would also help in this case.
 

cubanflyer

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A return spring would also help in this case.
has Fvm says a spring would return the shaft back faster, most commercial soleniods I have seen, use this method.
 

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