>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 14 March 2005 13:27
>To: Microcontroller discussion list - Public.
>Subject: [EE] Clamp diode slows down solenoid switch off.
>
>
>Hi,
>
>I'm trying to use a PIC to control an old paper tape reader 
>like this one...
>
>http://members.ozemail.com.au/~jacksmth/images/e803ptr.jpg
>
>The problem I'm having is that after turning off the tape 
>break solenoid it seems that it takes ~15mS before the tape 
>starts to move (as seem by the sproket hole output going low).
>
>I'm using a "matching unit" between the coil and the switching FET
>(IRF630) as detailed in the original circuit.  This is a 18ohm 
>resistor and a 50uF capacitor in parallel.
>
>I think the problem could be caused by the protection diode 
>I've put across the coil winding.  It seems that when the FET 
>turns off, the current that was flowing through the coil into 
>the transistor now loops back via the diode.  This looks like 
>a short across the coil and thus prolongs the decay of the 
>current and hence maintains the magnetism in the coil long 
>after the drive has been removed.
>
>Does this make sense ?  If so, any suggestions how to improve 
>the situation ?  I've tried the circuit without the diode, but 
>the large voltage spike that resulted seem to get into the 
>photocell amplifiers and cause false detection of the sproket holes.
>
>Peter

A diode to clamp the flyback voltage certainly will slow down the
collapse of the magnetic field by allowing a current to continue
flowing.  In place of a simple diode you could use a zener which allows
the voltage to increase to some level below the breakdown of the
transistor.  Ensure you calculate the power dissiptaion in the zener
(could be surprisingly high) and rate it accordingly.

Regards

Mike

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