Ron,
The P Fet is IRF4905 55V-74A-20mΩ, and I killed it, the M4 BS170, 60 v 500 ma has survived fine, the power supply is a Lambda 20 v, 8 amp with volt and amp limit, and you caused me to look at something that hit nerve & I lost my concentration for a while, The sim is for use in a vehicle and the .1 ohm is in place of motor that stalls at about 50 amps.
Yes the power supply does drop and that is probably why I killed the P Fet.
As I started working on the board today, a crack in it became a break, so I'm rebuilding it.
The problem, as I see it , is with the SCR having the cathode grounded, although via NFet BS170, it seemed the other devices were tripping it with their normal bias voltage and to combat that, I did what you did, but with a diode, which raised it normal bais voltage to the next device. I ended up using an LED in the gate so the normal bias voltage was well below what it took to turn on the device. Another item is the VGS(th) Gate Threshold Voltage VDS = VGS, ID = 1mA All 0.8 2.1 3 V for the BS170, the Vak of the MCR100 is almost there so I wanted some room for what ever.
Working on the new board, I'll let you know , I'm doing the end of travel part first and when I'm happy with that, I'll do the rest.
Jeff