Hi Peter, this may be of use to you, it is a P&B app note on "...coil
supression with DC relays".

http://relays.tycoelectronics.com/app_pdfs/13c3311.pdf

-Mark

>
> 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
>
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