Solenoids

[MickP]

Ok, this is actually a solenoid for a LPG system on a boat, but i figured the principles should be the same wherever...

 

The solenoid on the propane system on a boat i just bought didn't work. I took the solenoid out of the system and all worked fine. I also checked i was getting a voltage at the solenoid terminals and i was.

 

I had a closer look at the (removed) solenoid, and realised that the plunger to close the valve was pushed harder shut when a voltage was applied. Turning the coil round (head over heels) meant it worked as expected ie open when a voltage was applied, pushed shut by a spring when there is no current through the coil. So it looks like someone had put the coil on the wrong way round. Which seems bizarre, but is there any other explanation?

 

I tried reversing the current, but the plunger moved in the same direction wrt the coil whichever side i applied the +ve voltage to. Is this what would be expected?

 

Thanks

[Jonathan Kay]

Quoting MickP: 

I tried reversing the current, but the plunger moved in the same direction wrt the coil whichever side i applied the +ve voltage to. Is this what would be expected?

Yes

 

Jonathan

[MickP]

Thanks Jonathan.

So what determines (without getting too technical) the direction in which the plunger moves? Not the direction of the current, clearly. Is it the direction the coil is wrapped? - in which case i don't see how changing the direction of the wrapping is not the same as changing the direction of the current.

[Jonathan Kay]

If the moving bit were a permanent magnet then reversing the current would reverse the throw. But it isn't, it's a bit of iron which the current makes magnetic. When you reverse the current the force is reversed, but so is the polarity of the magnet produced by the current.

 

Jonathan

[MickP]

Thanks again Jonathan

So the coil must have been installed incorrectly. Can't think how anyone could do this