Need help for value of resistor

[Harald Kapp]

I simulated this:

V(1) is the flyback voltage without diode.
V(2) is the flyback voltage clamped by a 1n4148, turn off time of the transistor base drive is 5 ns.
V(3) is the flyback voltage clamped by a 1n4148, turn off time of the transistor base drive is 0.05 ns.
V(4) is the flyback voltage clamped by a slow rectifier diode (no 1N400x available), turn off time of the transistor base drive is 5 ns.
Regardless of turn-off time and diode type, clamping is effective in all sub-circuits 2...4. Only sub-circuit 1 shows almost 300 V flyback voltage!

 

[AnalogKid]

In the discussion about how high the flyback voltage might get before the suppression diode "kicks in", something important has been left out, something that HK covered by inference. The risetime and size of the flyback spike are critically dependent on the turn-off time of the switch device (in this case, a 2N2222). Part of the reason the super slow 1n400x diode can capture the spike is that the risetime of the spike is not infinite. It is limited by the turn-off time of the switch and the parasitic capacitance of the inductor. These combine to slow the edge to the point that the diode can clip it.

or something like that

ak