hi.
finally got round to 'finishing' this pick up /drop project, works well apart from the fact that the electromagnet does not release when the power is turned off due to residual magnetism.
i have got it to release by desensitising the poles with a bit of insulting tape but this is not entirely ideal.
i am thinking of reversing the coil polarity at release somehow?
any suggestions appreciated.
thank you
dougal

 

Post the schematic of the coil driver.

 

hi
i have revised slightly, the coil is now at the collector,the base resistor has been replaced with a 1ok pot.
IC2 is an op amp.
thnk you
doug

 

supply voltage is 4.5 volts, 9volt was found to be too high causing overheating
L1 THE ELECTROMAGNET IS LABELLED as 1 henry, this is a typo.
the transistor is a TIP 31
current around 500 ma

 

Try a 100 uF capacitor across the electromagnet. When the transistor cuts off, the resultant oscillation of the capacitor-inductance will demagnetise the electromagnet.

 

hi.
i am thinking of reversing the coil polarity at release somehow?
dougal

This is how it is done in most commercial lift magnets, a short reverse shot releases any load that remains attached after a lift.
Max.

 

thank you
i think this is solved in a roundabout way, i am using this to pick up a small nut and bolt -the holding force was fairly close to the residual force, by turning up the holding force and using a heavier nut and bolt it works perfectly, so i guess im swamping the residual force.

 

You are probably swamping the residual force by using a reverse polarity, but you may reach the point where the residual is now reversed which then returns to the same problem?
Max.