In article 1173503476.441339@ftpsrv1,
Terry Given wrote:

jasen wrote:
huh I'm getting 42/0.7 (which is over 50 times slower)

are you assuming a 5V vcc? OP claims 42V.

no, the original voltage across the coil during turn-off is Vz -
Vcc = 47 - 42 = 5V. When a freewheeling diode is used, the
voltage across the coil is 0.7V.

You blokes have forgotten R and L, and L/R. :-)

I couldn't be bothered to do the sums so just LTspice'd
a quick 42V supply, 100mH and 42 ohm coil, switched by
a MOSFET and clamped by a Schottky diode to a variable
voltage.

The current Risetime at switchon, from 0.1A to 1A was
about 5.5mS, as per the L/R exponential sum.

Below is a little table of LTspice current Falltimes.

Vclamp. Falltime (1A to 0.1A).

42 5.3mS -- nearly equal to the L/R Risetime.
47 3.9 -- only 1.3x 5.3mS.
57 2.6
84 1.4 -- Changing from an L/R sum to
mainly a V = L.dI/dT sum.

--
Tony Williams.